1977 coupe deville 425 manifold vacuum behavior

Having a little trouble understanding manifold vacuum and what affects
it. When I start my car the vacuum as measured from a port directly
into the intake manifold is low, around 14 in Hg. As the car warms up
and the choke opens the vacuum reading gradually improves to around 17
in Hg.

I suspected a leak around the carb or the manifold to cylinder head. In
fact found one loose bolt....wasn't it. Used map gas to try to find a
leak but couldn't find one. I noticed that the timing was retarded
about 2 degrees from spec of 18BTDC at 1400 RPM. Also, I believe the
air fuel mixture is a little rich.
All hoses where replaced and the vacuum measured with all vac ports on
the carb closed in turn.

My question for a good mechanic is will retarded timing and/or a rich
fuel mixture have an effect on manifold vacuum?

Thanks for your help.

Sincerely,
Tich.

Yes of couse, but why do you think 17 ( is it a steady reading) is low on an
old engine, what are the effects of this in respect to performance.
"tich" > wrote in message
oups.com...
> Having a little trouble understanding manifold vacuum and what affects
> it. When I start my car the vacuum as measured from a port directly
> into the intake manifold is low, around 14 in Hg. As the car warms up
> and the choke opens the vacuum reading gradually improves to around 17
> in Hg.
>
> I suspected a leak around the carb or the manifold to cylinder head. In
> fact found one loose bolt....wasn't it. Used map gas to try to find a
> leak but couldn't find one. I noticed that the timing was retarded
> about 2 degrees from spec of 18BTDC at 1400 RPM. Also, I believe the
> air fuel mixture is a little rich.
> All hoses where replaced and the vacuum measured with all vac ports on
> the carb closed in turn.
>
> My question for a good mechanic is will retarded timing and/or a rich
> fuel mixture have an effect on manifold vacuum?
>
> Thanks for your help.
>
> Sincerely,
> Tich.
>
>



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Thanks for your response Shep.

>From your reply I've learned that as a car ages one would expect the
manifold vacuum not to be as robust as it once was. Must be valves get
worn and don't seal as well etc.

I just wonder now if a much lower cold engine manifold vacuum is
normal.
I'm envisoning that as the engine warms up the metal parts expand
slightly improving the vacuum seal to the outside. Is this normal on
an old engine.

With a cold manifold vacuum of 14 in Hg seem low for a 425 cid with 100
thousand miles on it.

Thanks again for your helpful response....I ain't no mechanic.
Any hints are very welcome.

Later,
Tich

Shep,

Forgot to answer the performance question. I've had the car about a
year and put on a thousand miles or so. It ran ****ty from the get go
but really crappy before I parked it about 3 months ago. The plugs were
undergapped about 10 thousands and very carbon fouled. Found the air
fuel mixture screws all "screwed up" if you'll pardon the expression.
The right screw was backed out from lightly seated about 1 1/2 turns
and bent. The right screw was about 4 1/2 turns. The curb idle was 125
RPM higher than it should have been also. I think after putting 126
bucks worth of parts on it it will run fine. Just waiting for a
combination valve to repair the brakes. The only reason I bought this
old Cadillac was for the 100 inch front seat wide enough for grandma's
butt to take her to appointments :-) Previously, I was having to rent a
car twice a week!

Thanks for your post.

Later,
Tich

tich wrote:

>
> Thanks for your response Shep.
>
>>From your reply I've learned that as a car ages one would expect the
> manifold vacuum not to be as robust as it once was. Must be valves get
> worn and don't seal as well etc.
>
> I just wonder now if a much lower cold engine manifold vacuum is
> normal.
> I'm envisoning that as the engine warms up the metal parts expand
> slightly improving the vacuum seal to the outside. Is this normal on
> an old engine.

That's probably not what's going on. A cold engine is much less
efficient than a warm one. Its basically "loaded" by thicker oil and
less efficient combustion (plus an over-rich fuel mix thanks to the
choke being applied in a carbureted engine such as yours.) Take that
added load off, and the vacuum increases. Perfectly normal.

Agreed.
"Steve" > wrote in message
...
> tich wrote:
>
>>
>> Thanks for your response Shep.
>>
>>>From your reply I've learned that as a car ages one would expect the
>> manifold vacuum not to be as robust as it once was. Must be valves get
>> worn and don't seal as well etc.
>>
>> I just wonder now if a much lower cold engine manifold vacuum is
>> normal.
>> I'm envisoning that as the engine warms up the metal parts expand
>> slightly improving the vacuum seal to the outside. Is this normal on
>> an old engine.
>
> That's probably not what's going on. A cold engine is much less efficient
> than a warm one. Its basically "loaded" by thicker oil and less efficient
> combustion (plus an over-rich fuel mix thanks to the choke being applied
> in a carbureted engine such as yours.) Take that added load off, and the
> vacuum increases. Perfectly normal.



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