Help with motor vacuum
Thread Starter
Heel & Toe
Joined: Oct 2006
Posts: 17
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Help with motor vacuum
I am trying to trouble shoot a rapid needle fluctuate between 13" and 16"Hg. while my 350 engine is at idle. On acceleration the needle tends to smooth some but not all bounce is gone. I am taking the reading directly from the manifold and no gasket leaks are found. I am leaning toward valve guide wear but will check any other suggestions. The motor is a basic 350 in my 69 coupe. Thanks, Fred
Last edited by BLAZE69; Dec 27, 2006 at 12:09 PM.
Race Director
Joined: Sep 2002
Posts: 10,362
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From: San Diego - Deep Within The State of CONFUSION!
My vote would be valves - but not the guides. Valve guides are out of the combustion chamber. This type of behaviour is most likely due to carbon buildup on intake valves and/or leak in valve seats.
Run a wet and dry compression test.
Run a wet and dry compression test.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
I think this can also be a result of having a rather radical cam with some overlap. Other folks who have better engine knowledge can do better with the specifics. If you have a "mild" cam, I would agree with valve leakage.
Racer
Joined: Apr 2005
Posts: 464
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From: Charleston South Carolina
I would think that if it is a steady fluctuation it would valve related. My 383 has 10" of vacuum at 850 to 900 rpms and 12 at 1000 to 1050 rpms (b/c of the overlap). But mine is steady as a rock at either rpm level. Mine dosen't like to idle very well below 900 so I keep it at 1000. Do a leakdown test and you'll at least narrow down the problem.
Racer
Joined: Apr 2005
Posts: 464
Likes: 0
From: Charleston South Carolina
Well if I let the idle go down to where it will almost stall then yes the needle will fluctuate, but his idle is around normal and it still bounces a little while accelerating, so I'm still leaning towards a valve related issue. It could also be an atmospheric leak too. Spray some ether around where the intake gaskets are and see if the idle goes up. Heck even try around where the carb mounts to the intake. If there's a leak, the ether entering the system will make the engine run lean and idle will increase.
