engine vacuum

[thomas g. warner]

I have recently completed troubleshooting and repairing the engine on a 1976
palmbeach(455 CI), replaced the right rocker pivots that were worn, replaced
all of the vacuum lines, distributor cap, rotor, new silicon wires, spark
plugs, new exhaust manifolds, new carburetor, heat riser, and compression
check (165# in 2-8 cylinders and 155 in 1). When vacuum checking the engine
the vacuum needle pulses very fast irregardless of the engine RPM. A
mechanic friend of mine says it is the RV cam in the engine (overlap between
the exhaust and intake stroke). Engine has excellent power, idles nice,
starts well and is very quiet. If I had not done the compression check I
would swear it was a valve. Anyone else ever experience this, and is it a
potential problem or not?????

 

[patrick flowers]

>
> I have recently completed troubleshooting and repairing the engine on
> a 1976 palmbeach(455 CI), replaced the right rocker pivots that were
> worn, replaced all of the vacuum lines, distributor cap, rotor, new
> silicon wires, spark plugs, new exhaust manifolds, new carburetor,
> heat riser, and compression check (165# in 2-8 cylinders and 155 in
> 1). When vacuum checking the engine the vacuum needle pulses very
> fast irregardless of the engine RPM. A mechanic friend of mine says
> it is the RV cam in the engine (overlap between the exhaust and
> intake stroke). Engine has excellent power, idles nice, starts well
> and is very quiet. If I had not done the compression check I would
> swear it was a valve. Anyone else ever experience this, and is it a
> potential problem or not?????

Thomas,

Vacuum reduction due to cam overlap should disappear with increased rpm
and is usually only a problem near idle; producing that "loping" sound
that we products of the 60's and 70's love so much(although I doubt the
factory motorhome cam is radical enough to have much effect). It sounds
like you've replaced most of the likely culprits, so I'll take a few
shots in the dark.

Disconnect and plug off the vacuum lines to the following one at a time
and check for a change:

1. brake booster

2. heater controls

3. cruise control transducer(not the servo)

4. PCV valve(replace if you haven't already)

Only other things that come to mind are leaking gaskets on intake or
carb. However, since you say it runs and idles well, if you can't find
it, then I wouldn't worry about it!

Good luck!
Patrick
- --
Patrick Flowers
Mailtoatri63

The GMC Motorhome Page
http://www.gmcmotorhome.com

 

[thomas g. warner]

Rick thanks for the input. The needle fluctuates very little just a fast
vibration. I used some of my old motors manuals also and read the same
parts as you did. The thing that is weird is that the engine runs excellent
and quiet. If the cylinder head gasket was even partially blown between two
cylindes than I don't believe the compression would be that high. Valves
and gaskets have to be good to pull 155-165 #. The vacuum gauge was
connected at several different places, where the air cleaner is usualy
connected behind the carb, and in front where the pick up to the cruise
control normally goes. Did not make a difference.

Just returned from a 450 mile trip tonight and the engine ran perfectly all
the way at all speeds up to 60-70 MPH. When stopping it idles very slowly
and very quiet. Very smooth acceleration. Filled the tanks before leaving
although do not believe they were completely full and still have about 1/4
tank left, so mileage is probably OK, although I have nothing to compare it
to since have never had a GMC motorhome before. In fact this trip was the
first ever made by wife and I in one.

I am at a loss as have never seen this problem before. It bothers me because
if it is a problem I would like to fix it now before it breaks down on the
road.

>> When vacuum checking the engine
>>the vacuum needle pulses very fast irregardless of the engine RPM. A
>>mechanic friend of mine says it is the RV cam in the engine ........
>Thomas,
> It would be helpful to know over what range the gauge pulses. Absent that,
>I can offer the following:
> According to my trusty old (1960) Automotive Encyclopedia, "If the needle
>floats regularly between a low and a high reading, the cylinder head gasket is
>liable to be blown out between two adjacent cylinders." They show a vacuum
>gauge flicking from about 5" HG to a normal 19".
> They go on to suggest: " Burned valve - regular drop", showing a drop from
>18" to 12", or possibly "Loose valve guides - Fast vibration", showing a drop
>from ~19" down to 14". All this of course is approximate, and in my
>experience should be taken with a grain of salt, although your one slightly
>low cylinder might presage an intake valve problem. If you were my customer,
>I would suggest rechecking the compression, and vacuum, in 500 - 1,000 miles.
>These things tend to resolve themselves over time, although that's scant
>comfort if you're planning a cross-country trip. 8-0
> One other possibility: Where is your gauge connected? It should be
>connected near the carburetor, possibly teed off the brake booster line or
>some such. If it's way out on one manifold runner near a single cylinder,
>it's gonna pulse a bit normally.
> BTW, low-rpm "torquer" engines such as our GMCs actually have very little
>cam timing overlap, which is only effective at very high rpm, in which case it
>allows the inertia of the flowing fuel-air mixture to pack a bit more into the
>cylinders. High-overlap cams have also fallen out of favor due to emissions,
>as they tend to allow some of the incoming fuel to pass right on out the
>exhaust during the overlap period (especially with headers), increasing HC
>emissions and cutting gas mileage. High -overlap (also called 'long duration')
>camshafts are for racing engines that live up around 5 - 6,000 rpm or more.
>(Not to be confused with 'high - lift' camshafts, which we might use.) (See,
>more than you ever wanted to know!)
>Good luck,
>
>Rick Staples
>Louisville, Colorado
>
>