fuel mileage drop problem solved!

just an update on the recent radical drop in fuel mileage i experienced
after doing a rebuild on my Carter BBD (87 YJ 4.2L) from about 17-18mpg
to something like 5-7mpg! there were no obvious signs of excessive fuel
consumption (black smoke clouds) or leaks in the system, and she seemed
to run and start ok.
turns out to have been rather simple and rather complicated at the same
time. when i took the carb apart i noticed that the stepper motor didnt
move very freely - the shaft appeared to be sticking in the bushing or
something. i pulled the shaft back out so that the pins were in the
forward position and put it back together. the next day i did the
nutter which recommends placing the pins in the full forward position.
now when i got this beast the CTO had been broken off as well as the O2
sensor being disconnected, so i had a pretty good hunch that the
stepper motor had been inactive for a long time (hence the corrosion
and the sticking). thus i hoped that taking the computer out of the
loop would bring the fuel mileage back up (or something like that). i
also bypassed the charcoal canister (bad purge valve), retightened the
carb mount nuts and the manifolds, and rechecked that there were no
vacuum leaks.
well none of it worked - and needless to say with gas going for $3+ at
the pump it is alarming to practically be able to WATCH your gas gauge
drop as you drive down the highway! then a few days ago i remembered
that having the stepper motor pins in the full forward position richens
the mixture. so i figured - what the hell i'll put em in the full lean
position and see what happens.
and VOILA! gas mileage is back to ~17-18 mpg and she runs like a top!
so CTO and O2 no good -> stepper motor doesnt move -> i 'rebuild' carb
and inadvertently put the pins in full forward position -> stepper
motor cant move because corrosion/old/POS -> disconnect computer ->
stepper cant move b/c no signal (obviously) -> restore pins to full
lean position - problem solved!
whew. anyway i just wanted to post about this because while i was
researching on this list it seemed to me that i ran across alot of
folks posting about their problems, then getting some responses and
possible solutions and then nothing else. it would be nice to hear what
exactly the problem turned out to be and how exactly it was solved (or
not) - at least more often.
so here ya go.
cheers all and thanks for all the help.
bennett

Thanks. We all like to find out what happened after a thread like that,
especially if it is a success story!

Earle

"bobvonbob" > wrote in message
oups.com...
> just an update on the recent radical drop in fuel mileage i experienced
> after doing a rebuild on my Carter BBD (87 YJ 4.2L) from about 17-18mpg
> to something like 5-7mpg! there were no obvious signs of excessive fuel
> consumption (black smoke clouds) or leaks in the system, and she seemed
> to run and start ok.
> turns out to have been rather simple and rather complicated at the same
> time. when i took the carb apart i noticed that the stepper motor didnt
> move very freely - the shaft appeared to be sticking in the bushing or
> something. i pulled the shaft back out so that the pins were in the
> forward position and put it back together. the next day i did the
> nutter which recommends placing the pins in the full forward position.
> now when i got this beast the CTO had been broken off as well as the O2
> sensor being disconnected, so i had a pretty good hunch that the
> stepper motor had been inactive for a long time (hence the corrosion
> and the sticking). thus i hoped that taking the computer out of the
> loop would bring the fuel mileage back up (or something like that). i
> also bypassed the charcoal canister (bad purge valve), retightened the
> carb mount nuts and the manifolds, and rechecked that there were no
> vacuum leaks.
> well none of it worked - and needless to say with gas going for $3+ at
> the pump it is alarming to practically be able to WATCH your gas gauge
> drop as you drive down the highway! then a few days ago i remembered
> that having the stepper motor pins in the full forward position richens
> the mixture. so i figured - what the hell i'll put em in the full lean
> position and see what happens.
> and VOILA! gas mileage is back to ~17-18 mpg and she runs like a top!
> so CTO and O2 no good -> stepper motor doesnt move -> i 'rebuild' carb
> and inadvertently put the pins in full forward position -> stepper
> motor cant move because corrosion/old/POS -> disconnect computer ->
> stepper cant move b/c no signal (obviously) -> restore pins to full
> lean position - problem solved!
> whew. anyway i just wanted to post about this because while i was
> researching on this list it seemed to me that i ran across alot of
> folks posting about their problems, then getting some responses and
> possible solutions and then nothing else. it would be nice to hear what
> exactly the problem turned out to be and how exactly it was solved (or
> not) - at least more often.
> so here ya go.
> cheers all and thanks for all the help.
> bennett
>

If you still have the 'mixture' screws factory blocked so you can't set
the carb up properly, running it too lean like that will burn holes in
your pistons.

You should put them in the center and hope that is the best for where
the factory set the carb's mix at.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
Canadian Off Road Trips Photos: Non members can still view!
Jan/06 http://www.imagestation.com/album/pi...?id=2115147590
(More Off Road album links at bottom of the view page)

bobvonbob wrote:
>
> just an update on the recent radical drop in fuel mileage i experienced
> after doing a rebuild on my Carter BBD (87 YJ 4.2L) from about 17-18mpg
> to something like 5-7mpg! there were no obvious signs of excessive fuel
> consumption (black smoke clouds) or leaks in the system, and she seemed
> to run and start ok.
> turns out to have been rather simple and rather complicated at the same
> time. when i took the carb apart i noticed that the stepper motor didnt
> move very freely - the shaft appeared to be sticking in the bushing or
> something. i pulled the shaft back out so that the pins were in the
> forward position and put it back together. the next day i did the
> nutter which recommends placing the pins in the full forward position.
> now when i got this beast the CTO had been broken off as well as the O2
> sensor being disconnected, so i had a pretty good hunch that the
> stepper motor had been inactive for a long time (hence the corrosion
> and the sticking). thus i hoped that taking the computer out of the
> loop would bring the fuel mileage back up (or something like that). i
> also bypassed the charcoal canister (bad purge valve), retightened the
> carb mount nuts and the manifolds, and rechecked that there were no
> vacuum leaks.
> well none of it worked - and needless to say with gas going for $3+ at
> the pump it is alarming to practically be able to WATCH your gas gauge
> drop as you drive down the highway! then a few days ago i remembered
> that having the stepper motor pins in the full forward position richens
> the mixture. so i figured - what the hell i'll put em in the full lean
> position and see what happens.
> and VOILA! gas mileage is back to ~17-18 mpg and she runs like a top!
> so CTO and O2 no good -> stepper motor doesnt move -> i 'rebuild' carb
> and inadvertently put the pins in full forward position -> stepper
> motor cant move because corrosion/old/POS -> disconnect computer ->
> stepper cant move b/c no signal (obviously) -> restore pins to full
> lean position - problem solved!
> whew. anyway i just wanted to post about this because while i was
> researching on this list it seemed to me that i ran across alot of
> folks posting about their problems, then getting some responses and
> possible solutions and then nothing else. it would be nice to hear what
> exactly the problem turned out to be and how exactly it was solved (or
> not) - at least more often.
> so here ya go.
> cheers all and thanks for all the help.
> bennett

Mike Romain wrote:
> If you still have the 'mixture' screws factory blocked so you can't set
> the carb up properly, running it too lean like that will burn holes in
> your pistons.
>
> You should put them in the center and hope that is the best for where
> the factory set the carb's mix at.

BRILLIANT! Instead of using an Air Fuel mixture guage to AT LEAST get in
the ballpark for good power and MPG's, you want him to put it "in the
middle and hope".

--
Simon
"I may be wrong, but I'm not uncertain." -- Robert A. Heinlein

Simon Juncal wrote:
>
> Mike Romain wrote:
> > If you still have the 'mixture' screws factory blocked so you can't set
> > the carb up properly, running it too lean like that will burn holes in
> > your pistons.
> >
> > You should put them in the center and hope that is the best for where
> > the factory set the carb's mix at.
>
> BRILLIANT! Instead of using an Air Fuel mixture guage to AT LEAST get in
> the ballpark for good power and MPG's, you want him to put it "in the
> middle and hope".
>
> --
> Simon

I thought so, thanks.

Seeing as he still has the metal plugs blocking the mix screws and no
working stepper motor 'and' the factory setup had the pins in the center
of their travel when the mix screws were originally set, would you have
a better suggestion?

And I have yet to see any back yard mechanic have an 'air fuel mix
gauge'. The CJ's are Carburated Jeeps that were manually tuned since
they came out in 1949. Only since the sad demise of mechanics and the
rise of 'technicians' that only know what a computer tells them is wrong
do you have tools like that.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
Canadian Off Road Trips Photos: Non members can still view!
Jan/06 http://www.imagestation.com/album/pi...?id=2115147590
(More Off Road album links at bottom of the view page)

> And I have yet to see any back yard mechanic have an 'air fuel mix
> gauge'. The CJ's are Carburated Jeeps that were manually tuned since
> they came out in 1949. Only since the sad demise of mechanics and the
> rise of 'technicians' that only know what a computer tells them is wrong
> do you have tools like that.

It's not a computer Mike, everything that lights up and is attached to
sensors isn't necessarilly a computer... It's a scientific measuring
device for combustion temps that can tell you if you're running too lean
or wasting gas (and POWER) by running too rich. It's a HELL of a lot
more accurate than your gut feeling, and it's WAY WAY freakin more
accurate than your "hope" technique.

If "computers" and scientific data providing guages caused the demise of
mechanical yokles and non-adapting-stuck-in-the-past technophobes then
the world is a better place...

--
Simon
"I may be wrong, but I'm not uncertain." -- Robert A. Heinlein

I'm guessing Mike's reply will be something along the lines of "Back in the
early days of computerized carburetors it was commonly possible to manually
tune the carbs for better power, mileage and lesser emissions than allow the
early computer controls to do the adjustments"

That statement would be true..

Generally the only time a properly adjusted carb needed attention was when
it was rebuild time which was every couple years.




"Simon Juncal" > wrote in
message ...
>> And I have yet to see any back yard mechanic have an 'air fuel mix
>> gauge'. The CJ's are Carburated Jeeps that were manually tuned since
>> they came out in 1949. Only since the sad demise of mechanics and the
>> rise of 'technicians' that only know what a computer tells them is wrong
>> do you have tools like that.
>
> It's not a computer Mike, everything that lights up and is attached to
> sensors isn't necessarilly a computer... It's a scientific measuring
> device for combustion temps that can tell you if you're running too lean
> or wasting gas (and POWER) by running too rich. It's a HELL of a lot more
> accurate than your gut feeling, and it's WAY WAY freakin more accurate
> than your "hope" technique.
>
> If "computers" and scientific data providing guages caused the demise of
> mechanical yokles and non-adapting-stuck-in-the-past technophobes then the
> world is a better place...
>
> --
> Simon
> "I may be wrong, but I'm not uncertain." -- Robert A. Heinlein

umm... appears i set off a little bit of a firestorm here...
anyway, yes mike the screws are still blanked off - until sunday at
least when i will be pulling the carb to change out the rounded
mounting nuts with new hardened ones. i am hoping that adjusting the
idle screws will even out my idle somewhat - it still lopes a bit from
say 500-600 rpms...
BUT wait mike, i thought that the idle mixture screws only affected the
mixture AT IDLE! so running lean on the highway or at > ~1200 rpm
wouldnt be affected by their position at all... correct? in any case
she's been running like this for about 5 of the last 6 months without
any noticable issues. additionally, i would suspect that if it was
running so lean that i might hole a piston i'd be noticing a little
backfire on the overrun etc etc.
oh yeah - i DONT have a carb mixture measurement device, whatever it
is... i have seen vacuum gauges for doing carb 'timing' adjustment on
motorcycles with multiple carbs but otherwise i dont know any other way
of adjusting the damn things besides seat-of-the-pants, old-school,
shade-tree, guestimation. not to say that that's my preferred method -
just the only option i currently have. if anyone with a carb
thing-a-majig wants to come and help me tune this sucker on sunday by
all means - come on by!
cheers,
bennett

Simon Juncal wrote:
>
> > And I have yet to see any back yard mechanic have an 'air fuel mix
> > gauge'. The CJ's are Carburated Jeeps that were manually tuned since
> > they came out in 1949. Only since the sad demise of mechanics and the
> > rise of 'technicians' that only know what a computer tells them is wrong
> > do you have tools like that.
>
> It's not a computer Mike, everything that lights up and is attached to
> sensors isn't necessarilly a computer... It's a scientific measuring
> device for combustion temps that can tell you if you're running too lean
> or wasting gas (and POWER) by running too rich. It's a HELL of a lot
> more accurate than your gut feeling, and it's WAY WAY freakin more
> accurate than your "hope" technique.
>
> If "computers" and scientific data providing guages caused the demise of
> mechanical yokles and non-adapting-stuck-in-the-past technophobes then
> the world is a better place...
>
> --

What billy ray said....

Plus, the OP is trying to make a carb behave by adjusting the pins on
the 'stepper' motor. This means he has something like 4 or 5 different
static places or 'steps' to put the pins on.

If he had the idle mix screws in the equation, my answer would have been
different than telling him to put it back 'like the factory' originally
set it with the pins in the center.....

I guess if all you know is a computer printout, this doesn't make any
sense to you, but putting it back 'factory' makes perfect sense to
me....

Too bad computers seem to make people forget the basics....

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
Canadian Off Road Trips Photos: Non members can still view!
Jan/06 http://www.imagestation.com/album/pi...?id=2115147590
(More Off Road album links at bottom of the view page)

When you get your screws open, (You can flip the carb upside down and
cut 1/8" off the block where the screws are or drill in from the side to
pop the blocks out or if they are tin, just use a sharp punch to get
them out) you can put the pins back in and set the mix like a 'normal'
carb.

Just FYI, the idle mix screws do affect the mix at idle, but that tends
to have an effect all the way up. The stepper pins affect the air mix
through the whole throttle range. They work (have effect) at speed
also, via input from the O2 sensor and computer.

I go for a best lean idle mix. I start with a warm engine and the
screws both out about 5 turns and set the rpm at 600- 700. I then turn
them 1/4 turn in each, then give the throttle a rev, then another 1/4
turn. I continue while revving it between turns until the engine get a
lean stumble. You will hear it. I them back them out 1/4 turn from
there and she just purrs.

As you are doing this the rpm will change up normally. You are supposed
to stop and drop it back down ending up at 600-650 final. 500 is pretty
low.

The emissions sniffer likes them to have that lean rumble so I leave it
like that, 1/4 turn lean on both screws, come emissions time.

Getting the mix right will do wonders for the vacuum.

Mike

bobvonbob wrote:
>
> umm... appears i set off a little bit of a firestorm here...
> anyway, yes mike the screws are still blanked off - until sunday at
> least when i will be pulling the carb to change out the rounded
> mounting nuts with new hardened ones. i am hoping that adjusting the
> idle screws will even out my idle somewhat - it still lopes a bit from
> say 500-600 rpms...
> BUT wait mike, i thought that the idle mixture screws only affected the
> mixture AT IDLE! so running lean on the highway or at > ~1200 rpm
> wouldnt be affected by their position at all... correct? in any case
> she's been running like this for about 5 of the last 6 months without
> any noticable issues. additionally, i would suspect that if it was
> running so lean that i might hole a piston i'd be noticing a little
> backfire on the overrun etc etc.
> oh yeah - i DONT have a carb mixture measurement device, whatever it
> is... i have seen vacuum gauges for doing carb 'timing' adjustment on
> motorcycles with multiple carbs but otherwise i dont know any other way
> of adjusting the damn things besides seat-of-the-pants, old-school,
> shade-tree, guestimation. not to say that that's my preferred method -
> just the only option i currently have. if anyone with a carb
> thing-a-majig wants to come and help me tune this sucker on sunday by
> all means - come on by!
> cheers,
> bennett