Carburetor EGR port question

aarcuda69062 wrote:
> In article > ,
> Simpson > wrote:
>
>> This would explain the low vacuum at the EGR port. The
>> lack of EGR function, plus the vacuum leak, which would cause a lean
>> condition, would combine to create a higher than normal temperature in
>> the combustion chamber resulting in the abnormally high NO readings.
>
> Not to nit pick but lean air fuel mixtures don't burn hotter.
> They burn slower which causes associated component temperatures
> to rise. Lean doesn't cause higher NOx, the extra O2 in the
> exhaust from a lean mixture makes the reduction bed of a 3 way
> catalyst less efficient resulting in higher NOx at the tail pipe.


Nit picking appreciated! It's always valuable to learn more precisely
what is going on. It makes perfect sense to me the way that you
explained it. Perhaps you could enlighten me on the effect of ignition
timing on emissions.

I just got back from passing the smog test after initially failing it.
The failure was caused by improper vacuum to the EGR valve deom the
carburetor and a minor manifold vacuum leak. The vacuum being supplied
to the EGR valve by the Holley 2280 carburetor was insufficient to open
the EGR valve, even at over 2000rpm. This non-functioning EGR valve and
the minor manifold vacuum leak caused a measurement of over 3600 PPM of
NOx. Anything over 1195 PPM is a failure. Below is the failed test results.

--------------------
Percent of CO2

15mph 1333rpm - 10.8

25mph 1309rpm - 10.7
--------------------
Percent of O2

15mph 1333rpm - 5.9

25mph 1309rpm - 6.0
-------------------
HC PPM

15mph 1333rpm - 34 - PASS (maximum allowable is 134)

25mph 1309rpm - 26 - PASS (maximum allowable is 106)
-------------------------
Percent of CO

15mph 1333rpm - 0.01 - PASS (maximum allowable is 0.90%)

25mph 1309rpm - 0.01 - PASS (maximum allowable is 1.14%)
--------------------------------------------------------
NOx PPM

15mph 1333rpm - 3641 - GROSS POLLUTER (maximum allowable is 1095)

25mph 1309rpm - 3225 - GROSS POLLUTER (maximum allowable is 1140)
-----------------------------------------------------------------

After failing the test, I checked the action of the EGR valve and found
the condition I described above. After correcting the condition I took
the truck back and passed the test with the following results.

--------------------
Percent of CO2

15mph 1333rpm - 10.4

25mph 1309rpm - 10.7
--------------------
Percent of O2

15mph 1333rpm - 6.7

25mph 1309rpm - 6.1
-------------------
HC PPM

*15mph 1333rpm - 132 - PASS (maximum allowable is 134)*

25mph 1309rpm - 65 - PASS (maximum allowable is 106)
-------------------------
Percent of CO

15mph 1333rpm - 0.01 - PASS (maximum allowable is 0.90%)

25mph 1309rpm - 0.04 - PASS (maximum allowable is 1.14%)
--------------------------------------------------------
NOx PPM

15mph 1333rpm - 626 - PASS (maximum allowable is 1095)

25mph 1309rpm - 606 - PASS (maximum allowable is 1140)
------------------------------------------------------

I'm a bit concerned that the truck passed the 15mph test for HC by only
2 PPM. I would like to try to lower the HCs from the tailpipe. The
catalytic converter is a brand new replacement. Would changing the
ignition timing lower the HC reading? Or changing the air fuel ratio
through either resizing the main jets or adjusting the float level? The
ignition timing is currently set at 7 degrees BTDC with the vacuum line
disconnected from the transducer at the computer, as per the shop manual.

The carb is a new (not rebuilt) Holley 2280. It is the non-feedback
version of the Holley 6280 that came stock with this 87 Dakota V6. The
mixture control solenoid for the Holley 6280 is no longer available as a
replacement part so I tracked down a new 2280 on eBay and bolted it on.
This configuration no longer benefits from the input from the O2 sensor,
but it was the best I could do on short notice.

The way I see it I have three main ways to lower HCs:

1. Advance or retard the ignition timing

2. Enrich or lean out the fuel mixture by increasing or decreasing the
size of the main jets in the carb.

3. Enrich or lean out the fuel mixture by raising or lowering the level
of fuel in the bowl via the float setting, which is currently stock.

From the test results, I have more room to increase NOx than HCs so if
lowering HCs cause NOX to rise, there is some headroom there.

Please feel free to nitpick away. It would go unappreciated.

Jack

"Simpson" > wrote in message
...
> aarcuda69062 wrote:
>> In article > ,
>> Simpson > wrote:
>>
>>> This would explain the low vacuum at the EGR port. The lack of EGR
>>> function, plus the vacuum leak, which would cause a lean condition, would
>>> combine to create a higher than normal temperature in the combustion
>>> chamber resulting in the abnormally high NO readings.
>>
>> Not to nit pick but lean air fuel mixtures don't burn hotter. They burn
>> slower which causes associated component temperatures to rise. Lean
>> doesn't cause higher NOx, the extra O2 in the exhaust from a lean mixture
>> makes the reduction bed of a 3 way catalyst less efficient resulting in
>> higher NOx at the tail pipe.
>
>
> Nit picking appreciated! It's always valuable to learn more precisely what
> is going on. It makes perfect sense to me the way that you explained it.
> Perhaps you could enlighten me on the effect of ignition timing on
> emissions.
>
> I just got back from passing the smog test after initially failing it. The
> failure was caused by improper vacuum to the EGR valve deom the carburetor
> and a minor manifold vacuum leak. The vacuum being supplied to the EGR valve
> by the Holley 2280 carburetor was insufficient to open the EGR valve, even
> at over 2000rpm. This non-functioning EGR valve and the minor manifold
> vacuum leak caused a measurement of over 3600 PPM of NOx. Anything over 1195
> PPM is a failure. Below is the failed test results.
>
> --------------------
> Percent of CO2
>
> 15mph 1333rpm - 10.8
>
> 25mph 1309rpm - 10.7
> --------------------
> Percent of O2
>
> 15mph 1333rpm - 5.9
>
> 25mph 1309rpm - 6.0
> -------------------
> HC PPM
>
> 15mph 1333rpm - 34 - PASS (maximum allowable is 134)
>
> 25mph 1309rpm - 26 - PASS (maximum allowable is 106)
> -------------------------
> Percent of CO
>
> 15mph 1333rpm - 0.01 - PASS (maximum allowable is 0.90%)
>
> 25mph 1309rpm - 0.01 - PASS (maximum allowable is 1.14%)
> --------------------------------------------------------
> NOx PPM
>
> 15mph 1333rpm - 3641 - GROSS POLLUTER (maximum allowable is 1095)
>
> 25mph 1309rpm - 3225 - GROSS POLLUTER (maximum allowable is 1140)
> -----------------------------------------------------------------
>
> After failing the test, I checked the action of the EGR valve and found the
> condition I described above. After correcting the condition I took the truck
> back and passed the test with the following results.
>
> --------------------
> Percent of CO2
>
> 15mph 1333rpm - 10.4
>
> 25mph 1309rpm - 10.7
> --------------------
> Percent of O2
>
> 15mph 1333rpm - 6.7
>
> 25mph 1309rpm - 6.1
> -------------------
> HC PPM
>
> *15mph 1333rpm - 132 - PASS (maximum allowable is 134)*
>
> 25mph 1309rpm - 65 - PASS (maximum allowable is 106)
> -------------------------
> Percent of CO
>
> 15mph 1333rpm - 0.01 - PASS (maximum allowable is 0.90%)
>
> 25mph 1309rpm - 0.04 - PASS (maximum allowable is 1.14%)
> --------------------------------------------------------
> NOx PPM
>
> 15mph 1333rpm - 626 - PASS (maximum allowable is 1095)
>
> 25mph 1309rpm - 606 - PASS (maximum allowable is 1140)
> ------------------------------------------------------
>
> I'm a bit concerned that the truck passed the 15mph test for HC by only 2
> PPM. I would like to try to lower the HCs from the tailpipe. The catalytic
> converter is a brand new replacement. Would changing the ignition timing
> lower the HC reading? Or changing the air fuel ratio through either resizing
> the main jets or adjusting the float level? The ignition timing is currently
> set at 7 degrees BTDC with the vacuum line disconnected from the transducer
> at the computer, as per the shop manual.
>
> The carb is a new (not rebuilt) Holley 2280. It is the non-feedback version
> of the Holley 6280 that came stock with this 87 Dakota V6. The mixture
> control solenoid for the Holley 6280 is no longer available as a replacement
> part so I tracked down a new 2280 on eBay and bolted it on. This
> configuration no longer benefits from the input from the O2 sensor, but it
> was the best I could do on short notice.
>
> The way I see it I have three main ways to lower HCs:
>
> 1. Advance or retard the ignition timing
>
> 2. Enrich or lean out the fuel mixture by increasing or decreasing the size
> of the main jets in the carb.
>
> 3. Enrich or lean out the fuel mixture by raising or lowering the level of
> fuel in the bowl via the float setting, which is currently stock.
>
> From the test results, I have more room to increase NOx than HCs so if
> lowering HCs cause NOX to rise, there is some headroom there.
>
> Please feel free to nitpick away. It would go unappreciated.
>
> Jack

I don't know why your HC's went up just from fixing an inop EGR valve ? High
HC is from a rich mixture. Did you also mess with the charcoal canister vent
line ? I would first try to lower the HC reading by leaning out the idle
mixture screws, if that doesn't work then try the float level before you try
switching main jets.

One other question, was the engine and converter fully warmed up before the
test ?

In article >,
Simpson > wrote:

> aarcuda69062 wrote:
> > In article > ,
> > Simpson > wrote:
> >
> >> This would explain the low vacuum at the EGR port. The
> >> lack of EGR function, plus the vacuum leak, which would cause a lean
> >> condition, would combine to create a higher than normal temperature in
> >> the combustion chamber resulting in the abnormally high NO readings.
> >
> > Not to nit pick but lean air fuel mixtures don't burn hotter.
> > They burn slower which causes associated component temperatures
> > to rise. Lean doesn't cause higher NOx, the extra O2 in the
> > exhaust from a lean mixture makes the reduction bed of a 3 way
> > catalyst less efficient resulting in higher NOx at the tail pipe.
>
>
> Nit picking appreciated! It's always valuable to learn more precisely
> what is going on. It makes perfect sense to me the way that you
> explained it. Perhaps you could enlighten me on the effect of ignition
> timing on emissions.
>
> I just got back from passing the smog test after initially failing it.
> The failure was caused by improper vacuum to the EGR valve deom the
> carburetor and a minor manifold vacuum leak. The vacuum being supplied
> to the EGR valve by the Holley 2280 carburetor was insufficient to open
> the EGR valve, even at over 2000rpm. This non-functioning EGR valve and
> the minor manifold vacuum leak caused a measurement of over 3600 PPM of
> NOx. Anything over 1195 PPM is a failure. Below is the failed test results.
>
> --------------------
> Percent of CO2
>
> 15mph 1333rpm - 10.8
>
> 25mph 1309rpm - 10.7
> --------------------
> Percent of O2
>
> 15mph 1333rpm - 5.9
>
> 25mph 1309rpm - 6.0
> -------------------
> HC PPM
>
> 15mph 1333rpm - 34 - PASS (maximum allowable is 134)
>
> 25mph 1309rpm - 26 - PASS (maximum allowable is 106)
> -------------------------
> Percent of CO
>
> 15mph 1333rpm - 0.01 - PASS (maximum allowable is 0.90%)
>
> 25mph 1309rpm - 0.01 - PASS (maximum allowable is 1.14%)
> --------------------------------------------------------
> NOx PPM
>
> 15mph 1333rpm - 3641 - GROSS POLLUTER (maximum allowable is 1095)
>
> 25mph 1309rpm - 3225 - GROSS POLLUTER (maximum allowable is 1140)
> -----------------------------------------------------------------
>
> After failing the test, I checked the action of the EGR valve and found
> the condition I described above. After correcting the condition I took
> the truck back and passed the test with the following results.
>
> --------------------
> Percent of CO2
>
> 15mph 1333rpm - 10.4
>
> 25mph 1309rpm - 10.7
> --------------------
> Percent of O2
>
> 15mph 1333rpm - 6.7
>
> 25mph 1309rpm - 6.1
> -------------------
> HC PPM
>
> *15mph 1333rpm - 132 - PASS (maximum allowable is 134)*
>
> 25mph 1309rpm - 65 - PASS (maximum allowable is 106)
> -------------------------
> Percent of CO
>
> 15mph 1333rpm - 0.01 - PASS (maximum allowable is 0.90%)
>
> 25mph 1309rpm - 0.04 - PASS (maximum allowable is 1.14%)
> --------------------------------------------------------
> NOx PPM
>
> 15mph 1333rpm - 626 - PASS (maximum allowable is 1095)
>
> 25mph 1309rpm - 606 - PASS (maximum allowable is 1140)
> ------------------------------------------------------
>
> I'm a bit concerned that the truck passed the 15mph test for HC by only
> 2 PPM. I would like to try to lower the HCs from the tailpipe. The
> catalytic converter is a brand new replacement. Would changing the
> ignition timing lower the HC reading? Or changing the air fuel ratio
> through either resizing the main jets or adjusting the float level? The
> ignition timing is currently set at 7 degrees BTDC with the vacuum line
> disconnected from the transducer at the computer, as per the shop manual.
>
> The carb is a new (not rebuilt) Holley 2280. It is the non-feedback
> version of the Holley 6280 that came stock with this 87 Dakota V6. The
> mixture control solenoid for the Holley 6280 is no longer available as a
> replacement part so I tracked down a new 2280 on eBay and bolted it on.
> This configuration no longer benefits from the input from the O2 sensor,
> but it was the best I could do on short notice.
>
> The way I see it I have three main ways to lower HCs:
>
> 1. Advance or retard the ignition timing
>
> 2. Enrich or lean out the fuel mixture by increasing or decreasing the
> size of the main jets in the carb.
>
> 3. Enrich or lean out the fuel mixture by raising or lowering the level
> of fuel in the bowl via the float setting, which is currently stock.
>
> From the test results, I have more room to increase NOx than HCs so if
> lowering HCs cause NOX to rise, there is some headroom there.
>
> Please feel free to nitpick away. It would go unappreciated.
>
> Jack

Hi Jack,
Does this truck have an air pump? The reason I ask is because
there is an awful lot of O2 in the exhaust sample.
If the truck has an air pump, that would account for it, but
unfortunately the dilution from the air pump makes analyzing the
gas samples difficult.
If there is NO air pump, here;s what I think; too much O2, too
much HC, not enough CO2 and not enough CO. That is a lean
mixture. Richening it up a bit will drop the HCs, lower the O2
and increase the CO2. A richer mixture doesn't contribute to more
NOx typically as long as the other NOx treatments are functioning
as they should.

Advancing ignition timing hurts NOx, it tends to hurt HC and CO
also, the exception being if the ignition system is marginal and
HCs are caused by an ignition misfire, this is because it takes
less voltage to ionize the plug gap the farther the piston is
away from TDC (advanced). Late ignition timing tends to help HC
and CO for the exact reasons given in my first post, hotter
combustion chamber wall, hotter exhaust valve and port, but late
timing can reveal marginal ignition components.

CO2 is the best indicator of combustion efficiency, higher is
better so anything you do that raises CO2 shows you're headed in
the right direction. I mention this because your CO2 reading are
low, by about 3 percent or more.

An efficient catalytic converter lowers CO and HC and raises CO2
by its very nature, OEM catalysts are usually 90%+ efficient,
aftermarket catalysts are usually only 40%-50% efficient.
The only way to tell if the tailpipe readings are due to an
inefficient catalyst is to take pre-cat and post cat gas samples
and calculate the efficiency.

Without an air pump diluting the sample a stoichiometric air/fuel
ratio would read .5% CO and .5% O2 give or take .1% either way.

Mike wrote:
> "Simpson" > wrote in message
> ...
>> aarcuda69062 wrote:
>>> In article > ,
>>> Simpson > wrote:
>>>
>>>> This would explain the low vacuum at the EGR port. The lack of EGR
>>>> function, plus the vacuum leak, which would cause a lean condition, would
>>>> combine to create a higher than normal temperature in the combustion
>>>> chamber resulting in the abnormally high NO readings.
>>> Not to nit pick but lean air fuel mixtures don't burn hotter. They burn
>>> slower which causes associated component temperatures to rise. Lean
>>> doesn't cause higher NOx, the extra O2 in the exhaust from a lean mixture
>>> makes the reduction bed of a 3 way catalyst less efficient resulting in
>>> higher NOx at the tail pipe.
>>
>> Nit picking appreciated! It's always valuable to learn more precisely what
>> is going on. It makes perfect sense to me the way that you explained it.
>> Perhaps you could enlighten me on the effect of ignition timing on
>> emissions.
>>
>> I just got back from passing the smog test after initially failing it. The
>> failure was caused by improper vacuum to the EGR valve deom the carburetor
>> and a minor manifold vacuum leak. The vacuum being supplied to the EGR valve
>> by the Holley 2280 carburetor was insufficient to open the EGR valve, even
>> at over 2000rpm. This non-functioning EGR valve and the minor manifold
>> vacuum leak caused a measurement of over 3600 PPM of NOx. Anything over 1195
>> PPM is a failure. Below is the failed test results.
>>
>> --------------------
>> Percent of CO2
>>
>> 15mph 1333rpm - 10.8
>>
>> 25mph 1309rpm - 10.7
>> --------------------
>> Percent of O2
>>
>> 15mph 1333rpm - 5.9
>>
>> 25mph 1309rpm - 6.0
>> -------------------
>> HC PPM
>>
>> 15mph 1333rpm - 34 - PASS (maximum allowable is 134)
>>
>> 25mph 1309rpm - 26 - PASS (maximum allowable is 106)
>> -------------------------
>> Percent of CO
>>
>> 15mph 1333rpm - 0.01 - PASS (maximum allowable is 0.90%)
>>
>> 25mph 1309rpm - 0.01 - PASS (maximum allowable is 1.14%)
>> --------------------------------------------------------
>> NOx PPM
>>
>> 15mph 1333rpm - 3641 - GROSS POLLUTER (maximum allowable is 1095)
>>
>> 25mph 1309rpm - 3225 - GROSS POLLUTER (maximum allowable is 1140)
>> -----------------------------------------------------------------
>>
>> After failing the test, I checked the action of the EGR valve and found the
>> condition I described above. After correcting the condition I took the truck
>> back and passed the test with the following results.
>>
>> --------------------
>> Percent of CO2
>>
>> 15mph 1333rpm - 10.4
>>
>> 25mph 1309rpm - 10.7
>> --------------------
>> Percent of O2
>>
>> 15mph 1333rpm - 6.7
>>
>> 25mph 1309rpm - 6.1
>> -------------------
>> HC PPM
>>
>> *15mph 1333rpm - 132 - PASS (maximum allowable is 134)*
>>
>> 25mph 1309rpm - 65 - PASS (maximum allowable is 106)
>> -------------------------
>> Percent of CO
>>
>> 15mph 1333rpm - 0.01 - PASS (maximum allowable is 0.90%)
>>
>> 25mph 1309rpm - 0.04 - PASS (maximum allowable is 1.14%)
>> --------------------------------------------------------
>> NOx PPM
>>
>> 15mph 1333rpm - 626 - PASS (maximum allowable is 1095)
>>
>> 25mph 1309rpm - 606 - PASS (maximum allowable is 1140)
>> ------------------------------------------------------
>>
>> I'm a bit concerned that the truck passed the 15mph test for HC by only 2
>> PPM. I would like to try to lower the HCs from the tailpipe. The catalytic
>> converter is a brand new replacement. Would changing the ignition timing
>> lower the HC reading? Or changing the air fuel ratio through either resizing
>> the main jets or adjusting the float level? The ignition timing is currently
>> set at 7 degrees BTDC with the vacuum line disconnected from the transducer
>> at the computer, as per the shop manual.
>>
>> The carb is a new (not rebuilt) Holley 2280. It is the non-feedback version
>> of the Holley 6280 that came stock with this 87 Dakota V6. The mixture
>> control solenoid for the Holley 6280 is no longer available as a replacement
>> part so I tracked down a new 2280 on eBay and bolted it on. This
>> configuration no longer benefits from the input from the O2 sensor, but it
>> was the best I could do on short notice.
>>
>> The way I see it I have three main ways to lower HCs:
>>
>> 1. Advance or retard the ignition timing
>>
>> 2. Enrich or lean out the fuel mixture by increasing or decreasing the size
>> of the main jets in the carb.
>>
>> 3. Enrich or lean out the fuel mixture by raising or lowering the level of
>> fuel in the bowl via the float setting, which is currently stock.
>>
>> From the test results, I have more room to increase NOx than HCs so if
>> lowering HCs cause NOX to rise, there is some headroom there.
>>
>> Please feel free to nitpick away. It would go unappreciated.
>>
>> Jack
>
> I don't know why your HC's went up just from fixing an inop EGR valve ? High
> HC is from a rich mixture. Did you also mess with the charcoal canister vent
> line ? I would first try to lower the HC reading by leaning out the idle
> mixture screws, if that doesn't work then try the float level before you try
> switching main jets.
>
> One other question, was the engine and converter fully warmed up before the
> test ?
>

Hi Mike,

As to the last question, yes, the engine was fully warmed up as
indicated by the temp gauge. I don't know what constitutes 'fully warmed
up' for the cat. The engine was fully warmed up when the smog tech drove
it onto the dynamometer. He did a 15mph test and a 25mph test, during
which the engine was turning at about 1340 rpm, as indicated on the test
result sheet. I am somewhat mystified by that. I would expect the rpms
to be higher during the 25mph test, but it was actually lower by 22
rpms. Another one of life's unexplained mysteries. The test was long
enough to warm up the cat, I would guess.

There may also have been a minor manifold vacuum leak. The bolts fixing
the EGR valve to the intake manifold were slightly loose. If I had a
manifold vacuum leak during the first test, that would have introduced a
lean mix. Fixing the leak would richen up the mix. That would explain
the higher HC on the second test.

One bolt on the EGR was so loose that when I tried to loosen it with
what appeared to my eye to be a 1/2" hex head socket, I thought I must
have got it wrong as there was no resistance, so I put on a 7/16" socket
and that didn't fit. I tried metric and nothing that fit on the bolt
head gripped. That's when I tried my fingers and the bolt unscrewed easy
as pie. Being as I am my own mechanic I felt somewhat stupid, but soon
got over it, remembering that **** happens. So I probably had a vacuum leak.

I didn't mess with the charcoal vent line except to put a hose clamp
(damn, you're good) on where it connected to the carb. But to be honest,
I don't think it was all that loose, but I had a couple of extra small
hose clamps knocking around and figured what the hey.

As far as leaning out the idle mixture screws goes, how much does the
idle circuit figure in at 1350 rpm on the engine and 15 and 25 mph on
the speedometer?

Roger on adjusting the float level before changing the jets. I like that
because it doesn't require getting new jets, just bending metal.

The Holley 2280 carb that I put on this 3.9L (239 cu) engine was used on
Chrysler's 318 engine. It's the same body as the Holley 6280 that came
stock on the 87 Dakota, but it doesn't have the mixture control
solenoid, which is no longer available as a replacement part. As this
part went south on the original carb, and any rebuilt 6280 will have an
old mixture control solenoid, those of us who have this carb are left to
our own devices. My solution, as was that of others with the know-how,
was to install a non-feedback 2280. Just so ya know...

Do you have any light to shed on adjusting the timing either way,
advanced or retarded, to bring down HCs? I have found arguments both
ways while googling the web and the groups.

Thanks for your sharing your knowledge.

Jack

aarcuda69062 wrote:
> In article >,
> Simpson > wrote:
>
>> aarcuda69062 wrote:
>>> In article > ,
>>> Simpson > wrote:
>>>
>>>> This would explain the low vacuum at the EGR port. The
>>>> lack of EGR function, plus the vacuum leak, which would cause a lean
>>>> condition, would combine to create a higher than normal temperature in
>>>> the combustion chamber resulting in the abnormally high NO readings.
>>> Not to nit pick but lean air fuel mixtures don't burn hotter.
>>> They burn slower which causes associated component temperatures
>>> to rise. Lean doesn't cause higher NOx, the extra O2 in the
>>> exhaust from a lean mixture makes the reduction bed of a 3 way
>>> catalyst less efficient resulting in higher NOx at the tail pipe.
>>
>> Nit picking appreciated! It's always valuable to learn more precisely
>> what is going on. It makes perfect sense to me the way that you
>> explained it. Perhaps you could enlighten me on the effect of ignition
>> timing on emissions.
>>
>> I just got back from passing the smog test after initially failing it.
>> The failure was caused by improper vacuum to the EGR valve deom the
>> carburetor and a minor manifold vacuum leak. The vacuum being supplied
>> to the EGR valve by the Holley 2280 carburetor was insufficient to open
>> the EGR valve, even at over 2000rpm. This non-functioning EGR valve and
>> the minor manifold vacuum leak caused a measurement of over 3600 PPM of
>> NOx. Anything over 1195 PPM is a failure. Below is the failed test results.
>>
>> --------------------
>> Percent of CO2
>>
>> 15mph 1333rpm - 10.8
>>
>> 25mph 1309rpm - 10.7
>> --------------------
>> Percent of O2
>>
>> 15mph 1333rpm - 5.9
>>
>> 25mph 1309rpm - 6.0
>> -------------------
>> HC PPM
>>
>> 15mph 1333rpm - 34 - PASS (maximum allowable is 134)
>>
>> 25mph 1309rpm - 26 - PASS (maximum allowable is 106)
>> -------------------------
>> Percent of CO
>>
>> 15mph 1333rpm - 0.01 - PASS (maximum allowable is 0.90%)
>>
>> 25mph 1309rpm - 0.01 - PASS (maximum allowable is 1.14%)
>> --------------------------------------------------------
>> NOx PPM
>>
>> 15mph 1333rpm - 3641 - GROSS POLLUTER (maximum allowable is 1095)
>>
>> 25mph 1309rpm - 3225 - GROSS POLLUTER (maximum allowable is 1140)
>> -----------------------------------------------------------------
>>
>> After failing the test, I checked the action of the EGR valve and found
>> the condition I described above. After correcting the condition I took
>> the truck back and passed the test with the following results.
>>
>> --------------------
>> Percent of CO2
>>
>> 15mph 1333rpm - 10.4
>>
>> 25mph 1309rpm - 10.7
>> --------------------
>> Percent of O2
>>
>> 15mph 1333rpm - 6.7
>>
>> 25mph 1309rpm - 6.1
>> -------------------
>> HC PPM
>>
>> *15mph 1333rpm - 132 - PASS (maximum allowable is 134)*
>>
>> 25mph 1309rpm - 65 - PASS (maximum allowable is 106)
>> -------------------------
>> Percent of CO
>>
>> 15mph 1333rpm - 0.01 - PASS (maximum allowable is 0.90%)
>>
>> 25mph 1309rpm - 0.04 - PASS (maximum allowable is 1.14%)
>> --------------------------------------------------------
>> NOx PPM
>>
>> 15mph 1333rpm - 626 - PASS (maximum allowable is 1095)
>>
>> 25mph 1309rpm - 606 - PASS (maximum allowable is 1140)
>> ------------------------------------------------------
>>
>> I'm a bit concerned that the truck passed the 15mph test for HC by only
>> 2 PPM. I would like to try to lower the HCs from the tailpipe. The
>> catalytic converter is a brand new replacement. Would changing the
>> ignition timing lower the HC reading? Or changing the air fuel ratio
>> through either resizing the main jets or adjusting the float level? The
>> ignition timing is currently set at 7 degrees BTDC with the vacuum line
>> disconnected from the transducer at the computer, as per the shop manual.
>>
>> The carb is a new (not rebuilt) Holley 2280. It is the non-feedback
>> version of the Holley 6280 that came stock with this 87 Dakota V6. The
>> mixture control solenoid for the Holley 6280 is no longer available as a
>> replacement part so I tracked down a new 2280 on eBay and bolted it on.
>> This configuration no longer benefits from the input from the O2 sensor,
>> but it was the best I could do on short notice.
>>
>> The way I see it I have three main ways to lower HCs:
>>
>> 1. Advance or retard the ignition timing
>>
>> 2. Enrich or lean out the fuel mixture by increasing or decreasing the
>> size of the main jets in the carb.
>>
>> 3. Enrich or lean out the fuel mixture by raising or lowering the level
>> of fuel in the bowl via the float setting, which is currently stock.
>>
>> From the test results, I have more room to increase NOx than HCs so if
>> lowering HCs cause NOX to rise, there is some headroom there.
>>
>> Please feel free to nitpick away. It would go unappreciated.
>>
>> Jack
>
> Hi Jack,
> Does this truck have an air pump?

Yes it does. It injects air directly into the cat.

> The reason I ask is because
> there is an awful lot of O2 in the exhaust sample.

That's what I thought.

> If the truck has an air pump, that would account for it, but
> unfortunately the dilution from the air pump makes analyzing the
> gas samples difficult.
> If there is NO air pump, here;s what I think; too much O2, too
> much HC, not enough CO2 and not enough CO. That is a lean
> mixture. Richening it up a bit will drop the HCs, lower the O2
> and increase the CO2.

I'm confused. If there is excess O2 and excess HC, what prevents them
from combining in a burn to produce more CO2?

> A richer mixture doesn't contribute to more
> NOx typically as long as the other NOx treatments are functioning
> as they should.

That's seems to be in line what what I have read concerning the
formation of NOx, which is mainly a factor of a too hot combustion chamber.

>
> Advancing ignition timing hurts NOx, it tends to hurt HC and CO
> also, the exception being if the ignition system is marginal and
> HCs are caused by an ignition misfire, this is because it takes
> less voltage to ionize the plug gap the farther the piston is
> away from TDC (advanced).

I can't quite follow you on this. Did you mean that "it takes *more*
voltage to ionize the plug gap the farther the piston is away from TDC
(advanced)."?

> Late ignition timing tends to help HC
> and CO for the exact reasons given in my first post, hotter
> combustion chamber wall, hotter exhaust valve and port, but late
> timing can reveal marginal ignition components.

Plugs, cap and rotor are all very healthy looking, but the resistance of
the wires all measure below the lower spec for resistance in the shop
manual. 250 ohms per inch is the minimum spec. Mine are at about 200.

At this point, I should probably mention that I have an MSD Blaster
Ignition, model PN 5900, installed:

http://www.msdignition.com/ignition_1_5900.htm

More precisely, it was installed when the truck failed the test and not
installed when it passed, but I don't think it had anything to do with
the failure. The non-functioning EGR system definitely played the major
role, IMO. I unconnected the MSD unit for the retest because I thought
that perhaps the more powerful and longer duration spark it supplied
might be contributing to the hot combustion chamber conditions that
caused the high NOx reading.



>
> CO2 is the best indicator of combustion efficiency, higher is
> better so anything you do that raises CO2 shows you're headed in
> the right direction. I mention this because your CO2 reading are
> low, by about 3 percent or more.
>
> An efficient catalytic converter lowers CO and HC and raises CO2
> by its very nature, OEM catalysts are usually 90%+ efficient,
> aftermarket catalysts are usually only 40%-50% efficient.
> The only way to tell if the tailpipe readings are due to an
> inefficient catalyst is to take pre-cat and post cat gas samples
> and calculate the efficiency.
>
> Without an air pump diluting the sample a stoichiometric air/fuel
> ratio would read .5% CO and .5% O2 give or take .1% either way.

So the truck has an air pump, and given what we have hashed through here
so far, I feel that, just to be sure, I should replace the plugs, wires,
cap and rotor before concerning myself with the jets or the float level,
even if I think they are in good shape.

BTW, do you happen to know if plug wires can *lose* resistance over time?

I hope all is well in your part of the world.

Jack

In article >,
"Mike" > wrote:

> "Simpson" > wrote in message
> ...
> > aarcuda69062 wrote:
> >> In article > ,
> >> Simpson > wrote:
> >>
> >>> This would explain the low vacuum at the EGR port. The lack of EGR
> >>> function, plus the vacuum leak, which would cause a lean condition, would
> >>> combine to create a higher than normal temperature in the combustion
> >>> chamber resulting in the abnormally high NO readings.
> >>
> >> Not to nit pick but lean air fuel mixtures don't burn hotter. They burn
> >> slower which causes associated component temperatures to rise. Lean
> >> doesn't cause higher NOx, the extra O2 in the exhaust from a lean mixture
> >> makes the reduction bed of a 3 way catalyst less efficient resulting in
> >> higher NOx at the tail pipe.
> >
> >
> > Nit picking appreciated! It's always valuable to learn more precisely what
> > is going on. It makes perfect sense to me the way that you explained it.
> > Perhaps you could enlighten me on the effect of ignition timing on
> > emissions.
> >
> > I just got back from passing the smog test after initially failing it. The
> > failure was caused by improper vacuum to the EGR valve deom the carburetor
> > and a minor manifold vacuum leak. The vacuum being supplied to the EGR
> > valve
> > by the Holley 2280 carburetor was insufficient to open the EGR valve, even
> > at over 2000rpm. This non-functioning EGR valve and the minor manifold
> > vacuum leak caused a measurement of over 3600 PPM of NOx. Anything over
> > 1195
> > PPM is a failure. Below is the failed test results.
> >
> > --------------------
> > Percent of CO2
> >
> > 15mph 1333rpm - 10.8
> >
> > 25mph 1309rpm - 10.7
> > --------------------
> > Percent of O2
> >
> > 15mph 1333rpm - 5.9
> >
> > 25mph 1309rpm - 6.0
> > -------------------
> > HC PPM
> >
> > 15mph 1333rpm - 34 - PASS (maximum allowable is 134)
> >
> > 25mph 1309rpm - 26 - PASS (maximum allowable is 106)
> > -------------------------
> > Percent of CO
> >
> > 15mph 1333rpm - 0.01 - PASS (maximum allowable is 0.90%)
> >
> > 25mph 1309rpm - 0.01 - PASS (maximum allowable is 1.14%)
> > --------------------------------------------------------
> > NOx PPM
> >
> > 15mph 1333rpm - 3641 - GROSS POLLUTER (maximum allowable is 1095)
> >
> > 25mph 1309rpm - 3225 - GROSS POLLUTER (maximum allowable is 1140)
> > -----------------------------------------------------------------
> >
> > After failing the test, I checked the action of the EGR valve and found the
> > condition I described above. After correcting the condition I took the
> > truck
> > back and passed the test with the following results.
> >
> > --------------------
> > Percent of CO2
> >
> > 15mph 1333rpm - 10.4
> >
> > 25mph 1309rpm - 10.7
> > --------------------
> > Percent of O2
> >
> > 15mph 1333rpm - 6.7
> >
> > 25mph 1309rpm - 6.1
> > -------------------
> > HC PPM
> >
> > *15mph 1333rpm - 132 - PASS (maximum allowable is 134)*
> >
> > 25mph 1309rpm - 65 - PASS (maximum allowable is 106)
> > -------------------------
> > Percent of CO
> >
> > 15mph 1333rpm - 0.01 - PASS (maximum allowable is 0.90%)
> >
> > 25mph 1309rpm - 0.04 - PASS (maximum allowable is 1.14%)
> > --------------------------------------------------------
> > NOx PPM
> >
> > 15mph 1333rpm - 626 - PASS (maximum allowable is 1095)
> >
> > 25mph 1309rpm - 606 - PASS (maximum allowable is 1140)
> > ------------------------------------------------------
> >
> > I'm a bit concerned that the truck passed the 15mph test for HC by only 2
> > PPM. I would like to try to lower the HCs from the tailpipe. The catalytic
> > converter is a brand new replacement. Would changing the ignition timing
> > lower the HC reading? Or changing the air fuel ratio through either
> > resizing
> > the main jets or adjusting the float level? The ignition timing is
> > currently
> > set at 7 degrees BTDC with the vacuum line disconnected from the transducer
> > at the computer, as per the shop manual.
> >
> > The carb is a new (not rebuilt) Holley 2280. It is the non-feedback version
> > of the Holley 6280 that came stock with this 87 Dakota V6. The mixture
> > control solenoid for the Holley 6280 is no longer available as a
> > replacement
> > part so I tracked down a new 2280 on eBay and bolted it on. This
> > configuration no longer benefits from the input from the O2 sensor, but it
> > was the best I could do on short notice.
> >
> > The way I see it I have three main ways to lower HCs:
> >
> > 1. Advance or retard the ignition timing
> >
> > 2. Enrich or lean out the fuel mixture by increasing or decreasing the size
> > of the main jets in the carb.
> >
> > 3. Enrich or lean out the fuel mixture by raising or lowering the level of
> > fuel in the bowl via the float setting, which is currently stock.
> >
> > From the test results, I have more room to increase NOx than HCs so if
> > lowering HCs cause NOX to rise, there is some headroom there.
> >
> > Please feel free to nitpick away. It would go unappreciated.
> >
> > Jack
>
> I don't know why your HC's went up just from fixing an inop EGR valve ?

Why wouldn't the HCs increase? When the exhaust gas
recirculation valve opens, what is it allowing into the intake
manifold/ combustion chamber?
CO2, CO, O2, NOx and HC.

When the exhaust gas recirculation valve opens what happens to
manifold pressure?
It goes up.
What does the power valve do when manifold pressure goes up?
It opens and allows more HC (gasoline) to flow into the venturi
in the carb.

aarcuda69062 wrote:
> In article >,
> "Mike" > wrote:
>
>> "Simpson" > wrote in message
>> ...
>>> aarcuda69062 wrote:
>>>> In article > ,
>>>> Simpson > wrote:
>>>>
>>>>> This would explain the low vacuum at the EGR port. The lack of EGR
>>>>> function, plus the vacuum leak, which would cause a lean condition, would
>>>>> combine to create a higher than normal temperature in the combustion
>>>>> chamber resulting in the abnormally high NO readings.
>>>> Not to nit pick but lean air fuel mixtures don't burn hotter. They burn
>>>> slower which causes associated component temperatures to rise. Lean
>>>> doesn't cause higher NOx, the extra O2 in the exhaust from a lean mixture
>>>> makes the reduction bed of a 3 way catalyst less efficient resulting in
>>>> higher NOx at the tail pipe.
>>>
>>> Nit picking appreciated! It's always valuable to learn more precisely what
>>> is going on. It makes perfect sense to me the way that you explained it.
>>> Perhaps you could enlighten me on the effect of ignition timing on
>>> emissions.
>>>
>>> I just got back from passing the smog test after initially failing it. The
>>> failure was caused by improper vacuum to the EGR valve deom the carburetor
>>> and a minor manifold vacuum leak. The vacuum being supplied to the EGR
>>> valve
>>> by the Holley 2280 carburetor was insufficient to open the EGR valve, even
>>> at over 2000rpm. This non-functioning EGR valve and the minor manifold
>>> vacuum leak caused a measurement of over 3600 PPM of NOx. Anything over
>>> 1195
>>> PPM is a failure. Below is the failed test results.
>>>
>>> --------------------
>>> Percent of CO2
>>>
>>> 15mph 1333rpm - 10.8
>>>
>>> 25mph 1309rpm - 10.7
>>> --------------------
>>> Percent of O2
>>>
>>> 15mph 1333rpm - 5.9
>>>
>>> 25mph 1309rpm - 6.0
>>> -------------------
>>> HC PPM
>>>
>>> 15mph 1333rpm - 34 - PASS (maximum allowable is 134)
>>>
>>> 25mph 1309rpm - 26 - PASS (maximum allowable is 106)
>>> -------------------------
>>> Percent of CO
>>>
>>> 15mph 1333rpm - 0.01 - PASS (maximum allowable is 0.90%)
>>>
>>> 25mph 1309rpm - 0.01 - PASS (maximum allowable is 1.14%)
>>> --------------------------------------------------------
>>> NOx PPM
>>>
>>> 15mph 1333rpm - 3641 - GROSS POLLUTER (maximum allowable is 1095)
>>>
>>> 25mph 1309rpm - 3225 - GROSS POLLUTER (maximum allowable is 1140)
>>> -----------------------------------------------------------------
>>>
>>> After failing the test, I checked the action of the EGR valve and found the
>>> condition I described above. After correcting the condition I took the
>>> truck
>>> back and passed the test with the following results.
>>>
>>> --------------------
>>> Percent of CO2
>>>
>>> 15mph 1333rpm - 10.4
>>>
>>> 25mph 1309rpm - 10.7
>>> --------------------
>>> Percent of O2
>>>
>>> 15mph 1333rpm - 6.7
>>>
>>> 25mph 1309rpm - 6.1
>>> -------------------
>>> HC PPM
>>>
>>> *15mph 1333rpm - 132 - PASS (maximum allowable is 134)*
>>>
>>> 25mph 1309rpm - 65 - PASS (maximum allowable is 106)
>>> -------------------------
>>> Percent of CO
>>>
>>> 15mph 1333rpm - 0.01 - PASS (maximum allowable is 0.90%)
>>>
>>> 25mph 1309rpm - 0.04 - PASS (maximum allowable is 1.14%)
>>> --------------------------------------------------------
>>> NOx PPM
>>>
>>> 15mph 1333rpm - 626 - PASS (maximum allowable is 1095)
>>>
>>> 25mph 1309rpm - 606 - PASS (maximum allowable is 1140)
>>> ------------------------------------------------------
>>>
>>> I'm a bit concerned that the truck passed the 15mph test for HC by only 2
>>> PPM. I would like to try to lower the HCs from the tailpipe. The catalytic
>>> converter is a brand new replacement. Would changing the ignition timing
>>> lower the HC reading? Or changing the air fuel ratio through either
>>> resizing
>>> the main jets or adjusting the float level? The ignition timing is
>>> currently
>>> set at 7 degrees BTDC with the vacuum line disconnected from the transducer
>>> at the computer, as per the shop manual.
>>>
>>> The carb is a new (not rebuilt) Holley 2280. It is the non-feedback version
>>> of the Holley 6280 that came stock with this 87 Dakota V6. The mixture
>>> control solenoid for the Holley 6280 is no longer available as a
>>> replacement
>>> part so I tracked down a new 2280 on eBay and bolted it on. This
>>> configuration no longer benefits from the input from the O2 sensor, but it
>>> was the best I could do on short notice.
>>>
>>> The way I see it I have three main ways to lower HCs:
>>>
>>> 1. Advance or retard the ignition timing
>>>
>>> 2. Enrich or lean out the fuel mixture by increasing or decreasing the size
>>> of the main jets in the carb.
>>>
>>> 3. Enrich or lean out the fuel mixture by raising or lowering the level of
>>> fuel in the bowl via the float setting, which is currently stock.
>>>
>>> From the test results, I have more room to increase NOx than HCs so if
>>> lowering HCs cause NOX to rise, there is some headroom there.
>>>
>>> Please feel free to nitpick away. It would go unappreciated.
>>>
>>> Jack
>> I don't know why your HC's went up just from fixing an inop EGR valve ?
>
> Why wouldn't the HCs increase? When the exhaust gas
> recirculation valve opens, what is it allowing into the intake
> manifold/ combustion chamber?
> CO2, CO, O2, NOx and HC.

Yes, it is reasonable to assume that these gases would increase in the
exhaust stream over their presence in a non-EGR exhaust stream, but only
in proportion to the total EGR gases that are reintroduced to a fresh
intake fuel mixture, 5 to 15 percent from what I have read. In my case,
the HC content increased 388% at 15 mph.

>
> When the exhaust gas recirculation valve opens what happens to
> manifold pressure?
> It goes up.
> What does the power valve do when manifold pressure goes up?
> It opens and allows more HC (gasoline) to flow into the venturi
> in the carb.

My source tell me you got that backwards. As manifold pressure goes up,
the power valve is closed. As manifold pressure goes down, the power
valve opens.

My source is "Holley Carburetors and Manifolds" by Mike Urich and Bill
Fisher. HPBooks, 1987:

When the engine is called upon to produce power in excess of normal
cruising requirements, the carburetor has to provide a richer mixture.
Added fuel for power operation is supplied by the power system
controlled by manifold vacuum.

Manifold vacuum accurately indicates engine load. Vacuum is usually
strongest at idle. As load increases, the throttle valve must be opened
wider to maintain a given speed. This offers less restriction to air
entering the intake manifold and reduces manifold vacuum.

A vacuum passage in the carburetor applies manifold vacuum to a
power-valve piston or diaphragm. At idle or normal cruising conditions,
manifold vacuum acting against a spring holds the valve closed. As high
power demands load the engine, manifold vacuum drops.

Below a preset point, usually about 6 inches of mercury (in.Hg), the
power valve spring overcomes manifold vacuum and opens the power valve.
Fuel flows through the power valve and through a power-valve restriction
to join fuel already flowing through the main metering system from the
main jet. The mixture is richened.

http://books.google.com/books?id=Cly...p41-vp4k&hl=en

Scroll down to the yellow hi-lited text

Jack

In article > ,
Simpson > wrote:

> > Why wouldn't the HCs increase? When the exhaust gas
> > recirculation valve opens, what is it allowing into the intake
> > manifold/ combustion chamber?
> > CO2, CO, O2, NOx and HC.
>
> Yes, it is reasonable to assume that these gases would increase in the
> exhaust stream over their presence in a non-EGR exhaust stream, but only
> in proportion to the total EGR gases that are reintroduced to a fresh
> intake fuel mixture, 5 to 15 percent from what I have read. In my case,
> the HC content increased 388% at 15 mph.

My reply to Mike wasn't based upon your readings, it was based
upon his statement.
In your case, if the engine is running lean to begin with, the
addition of EGR is going to displace any burnable mixture that
might have stood a chance of combusting.

> >
> > When the exhaust gas recirculation valve opens what happens to
> > manifold pressure?
> > It goes up.
> > What does the power valve do when manifold pressure goes up?
> > It opens and allows more HC (gasoline) to flow into the venturi
> > in the carb.
>
> My source tell me you got that backwards.

Nope.

> As manifold pressure goes up,
> the power valve is closed.

As manifold -vacuum- goes -up- the power valve closes.
Manifold vacuum is a great way to determine engine health, it's a
lousy way to refer to how an engine and fuel system operate.

Right now assuming your truck is parked and not running, the
manifold pressure is high and the power valve is open.
Assuming that you're somewhere below 1000 feet altitude, the
pressure inside your manifold parked and engine not running is
probably somewhere near 28 or 29 inches of mercury.
If you were to then start the engine, the manifold pressure might
drop to 10 inches of mercury. If we subtract the 10 inches of
mercury running from the 29 inches of mercury measured with the
engine not running, we get 19 inches which would be a fairly
healthy "manifold vacuum" reading.

> As manifold pressure goes down, the power
> valve opens.
>
> My source is "Holley Carburetors and Manifolds" by Mike Urich and Bill
> Fisher. HPBooks, 1987:

I know the book.

> When the engine is called upon to produce power in excess of normal
> cruising requirements, the carburetor has to provide a richer mixture.
> Added fuel for power operation is supplied by the power system
> controlled by manifold vacuum.
>
> Manifold vacuum accurately indicates engine load. Vacuum is usually
> strongest at idle. As load increases, the throttle valve must be opened
> wider to maintain a given speed. This offers less restriction to air
> entering the intake manifold and reduces manifold vacuum.
>
> A vacuum passage in the carburetor applies manifold vacuum to a
> power-valve piston or diaphragm. At idle or normal cruising conditions,
> manifold vacuum acting against a spring holds the valve closed. As high
> power demands load the engine, manifold vacuum drops.
>
> Below a preset point, usually about 6 inches of mercury (in.Hg), the
> power valve spring overcomes manifold vacuum and opens the power valve.
> Fuel flows through the power valve and through a power-valve restriction
> to join fuel already flowing through the main metering system from the
> main jet. The mixture is richened.
>
> http://books.google.com/books?id=Cly...%22When+ the+
> engine+is+called+upon+to+produce+power%22&source=w eb&ots=SdPf9kX1c4&sig=bDsxQZ
> oaKdSk_aQ98Kxp41-vp4k&hl=en

Please understand, I'm not referring to manifold vacuum.

aarcuda69062 wrote:
> In article > ,
> Simpson > wrote:
>
>>> Why wouldn't the HCs increase? When the exhaust gas
>>> recirculation valve opens, what is it allowing into the intake
>>> manifold/ combustion chamber?
>>> CO2, CO, O2, NOx and HC.
>> Yes, it is reasonable to assume that these gases would increase in the
>> exhaust stream over their presence in a non-EGR exhaust stream, but only
>> in proportion to the total EGR gases that are reintroduced to a fresh
>> intake fuel mixture, 5 to 15 percent from what I have read. In my case,
>> the HC content increased 388% at 15 mph.
>
> My reply to Mike wasn't based upon your readings, it was based
> upon his statement.
> In your case, if the engine is running lean to begin with, the
> addition of EGR is going to displace any burnable mixture that
> might have stood a chance of combusting.
>
>>> When the exhaust gas recirculation valve opens what happens to
>>> manifold pressure?
>>> It goes up.
>>> What does the power valve do when manifold pressure goes up?
>>> It opens and allows more HC (gasoline) to flow into the venturi
>>> in the carb.
>> My source tell me you got that backwards.
>
> Nope.
>
>> As manifold pressure goes up,
>> the power valve is closed.
>
> As manifold -vacuum- goes -up- the power valve closes.
> Manifold vacuum is a great way to determine engine health, it's a
> lousy way to refer to how an engine and fuel system operate.
>
> Right now assuming your truck is parked and not running, the
> manifold pressure is high and the power valve is open.
> Assuming that you're somewhere below 1000 feet altitude, the
> pressure inside your manifold parked and engine not running is
> probably somewhere near 28 or 29 inches of mercury.
> If you were to then start the engine, the manifold pressure might
> drop to 10 inches of mercury. If we subtract the 10 inches of
> mercury running from the 29 inches of mercury measured with the
> engine not running, we get 19 inches which would be a fairly
> healthy "manifold vacuum" reading.
>
>> As manifold pressure goes down, the power
>> valve opens.
>>
>> My source is "Holley Carburetors and Manifolds" by Mike Urich and Bill
>> Fisher. HPBooks, 1987:
>
> I know the book.
>
>> When the engine is called upon to produce power in excess of normal
>> cruising requirements, the carburetor has to provide a richer mixture.
>> Added fuel for power operation is supplied by the power system
>> controlled by manifold vacuum.
>>
>> Manifold vacuum accurately indicates engine load. Vacuum is usually
>> strongest at idle. As load increases, the throttle valve must be opened
>> wider to maintain a given speed. This offers less restriction to air
>> entering the intake manifold and reduces manifold vacuum.
>>
>> A vacuum passage in the carburetor applies manifold vacuum to a
>> power-valve piston or diaphragm. At idle or normal cruising conditions,
>> manifold vacuum acting against a spring holds the valve closed. As high
>> power demands load the engine, manifold vacuum drops.
>>
>> Below a preset point, usually about 6 inches of mercury (in.Hg), the
>> power valve spring overcomes manifold vacuum and opens the power valve.
>> Fuel flows through the power valve and through a power-valve restriction
>> to join fuel already flowing through the main metering system from the
>> main jet. The mixture is richened.
>>
>> http://books.google.com/books?id=Cly...%22When+ the+
>> engine+is+called+upon+to+produce+power%22&source=w eb&ots=SdPf9kX1c4&sig=bDsxQZ
>> oaKdSk_aQ98Kxp41-vp4k&hl=en
>
> Please understand, I'm not referring to manifold vacuum.

Ahn... you're right.

You referred to manifold pressure, not vacuum.


--

Chuck Norris can fool all of the people all of the time and Chuck Norris
supports McCain.

McCain -- it rhymes with Hussein, Bahrain, cocaine, insane, wolfbane,
chest pain and chow mein.

In article >,
Simpson > wrote:

> > Hi Jack,
> > Does this truck have an air pump?
>
> Yes it does. It injects air directly into the cat.

I suspected so. The only way to make meaningful determination of
the gas readings is to take the gas readings again with the air
pump disabled.

> > The reason I ask is because
> > there is an awful lot of O2 in the exhaust sample.
>
> That's what I thought.

At this point, it's nether good or bad until you attempt to use
the gas readings as a diagnostic aid.

There's a thing called the Brettschneider equation which allows
with the input of 4 or 5 gas reading to accurately calculate the
air fuel ratio. The equation works whether or not there is a
catalytic converter ahead of where the sample is taken.
What the Brettschneider equation can not compensate for however
is false air introduced either by leaks in the exhaust pipes or
joints or an auxiliary air system such as is on your truck.

> > If the truck has an air pump, that would account for it, but
> > unfortunately the dilution from the air pump makes analyzing the
> > gas samples difficult.
> > If there is NO air pump, here;s what I think; too much O2, too
> > much HC, not enough CO2 and not enough CO. That is a lean
> > mixture. Richening it up a bit will drop the HCs, lower the O2
> > and increase the CO2.
>
> I'm confused. If there is excess O2 and excess HC, what prevents them
> from combining in a burn to produce more CO2?

Low catalytic converter efficiency.
The excess gases are in the wrong ratio.

Ever use a cutting torch?
If you set the flame as oxidizing, you won't get the results you
need. If you set the flame as carburizing, you won't get the
results you need.
One size fits all replacement parts...
I've seen vehicles fail emissions tests because the air pump was
too small, I've seen vehicles fail emissions test because the air
pump was too big. (the OEMs rate/size them by CFM)

> > A richer mixture doesn't contribute to more
> > NOx typically as long as the other NOx treatments are functioning
> > as they should.
>
> That's seems to be in line what what I have read concerning the
> formation of NOx, which is mainly a factor of a too hot combustion chamber.
>
> >
> > Advancing ignition timing hurts NOx, it tends to hurt HC and CO
> > also, the exception being if the ignition system is marginal and
> > HCs are caused by an ignition misfire, this is because it takes
> > less voltage to ionize the plug gap the farther the piston is
> > away from TDC (advanced).
>
> I can't quite follow you on this. Did you mean that "it takes *more*
> voltage to ionize the plug gap the farther the piston is away from TDC
> (advanced)."?

No, opposite that. If you have a weak ignition system, chances
are it will present itself or be more problematic with less
ignition advance.
if I see low firing voltage on a secondary waveform on a scope
and no indication of plug fouling, I first grab my timing light
and check ignition timing.
You can search for my posts in either of the Chevy truck groups,
you want to focus on exchanges between myself and a poster named
"snoman" for my attempts to get him to understand why his truck
runs better with the ignition timing jacked sky high contrary to
the hundreds of identical trucks I've worked on in the last 20
years that would just ping, rattle and set ESC codes when set the
same or similar.

> > Late ignition timing tends to help HC
> > and CO for the exact reasons given in my first post, hotter
> > combustion chamber wall, hotter exhaust valve and port, but late
> > timing can reveal marginal ignition components.
>
> Plugs, cap and rotor are all very healthy looking, but the resistance of
> the wires all measure below the lower spec for resistance in the shop
> manual. 250 ohms per inch is the minimum spec. Mine are at about 200.

Which shop manual specifies ohms per inch?

> At this point, I should probably mention that I have an MSD Blaster
> Ignition, model PN 5900, installed:
>
> http://www.msdignition.com/ignition_1_5900.htm
>
> More precisely, it was installed when the truck failed the test and not
> installed when it passed, but I don't think it had anything to do with
> the failure. The non-functioning EGR system definitely played the major
> role, IMO. I unconnected the MSD unit for the retest because I thought
> that perhaps the more powerful and longer duration spark it supplied
> might be contributing to the hot combustion chamber conditions that
> caused the high NOx reading.

Again, hot combustion chambers do not contribute to NOx formation.
Hotter combustion does contribute to NOx formation.
Hottest combustion typically occurs if the mixture is 2 percent
richer than stoichiometric.

> > Without an air pump diluting the sample a stoichiometric air/fuel
> > ratio would read .5% CO and .5% O2 give or take .1% either way.
>
> So the truck has an air pump, and given what we have hashed through here
> so far, I feel that, just to be sure, I should replace the plugs, wires,
> cap and rotor before concerning myself with the jets or the float level,
> even if I think they are in good shape.

It's cheap enough to do on that engine.
My hunch however is that the carb is a little lean.

> BTW, do you happen to know if plug wires can *lose* resistance over time?

Outside of leaking insulation, I can't say I've ever seen it in
37 years in the trade. The typical failure that would account
for high HCs would be an open plug wire.

> I hope all is well in your part of the world.

Ask me again tomorrow 8-) they're forecasting 8-14 inches of snow
for our area. We've already had over 100 inches this season
though most of it had melted in the last two weeks.

Good wrenching and let us know the results.