I am having a problem trouble shooting a rapid fluctuating needle between 13" and 16"Hg. The vacuum tends to smooth out on acceleration but is very erratic at idle.I'm taking the reading directly from the manifold port and find no gasket leaks. Do valve guides sound like the problem ? Any ideas will be appreciated. Thanks, Fred
Fred,
When you say that you find no gasket leaks, did you mean that you checked the intake manifold gaskets for leaks? If not, then use WD-40 (while using the little red straw to minimize the mess). Spray at the joint between the intake edge where it contacts the cylinder heads. If the idle speeds-up or smooths out, then you have a leak. Carburetor cleaner works well also.
Now, it is important to check the spark plugs, for if they're fouled, then you'll misread symptoms, and it will send you on a goose chase. You don't need to put new ones in, but if they're fouled, then you must clean them out with a wire brush. While you're at it check the gaps.
A bad Idle fuel/air mixture adjustment can cause this. Q-Jet carburetors usually require between 2 2/3 turns out from bottoming to 3 1/3 turns out INITIALLY. Holleys usually require an initial of 1- 1/2 turns.
Sometimes old and cracked-up ignition wires will cause this as well. The number of misses increases dramatically. As you loose and gain cylinders in every cycle, it gives the appearance of a rough idle or surging.
On stock advance timing cannisters, the advance is set during manufacture. I've heard of defective cannisters causing this. If it is a after market cannister, the neutral setting is about 7 turns in after having back-out the setting screw. You will need a 3/32" Allen wrench for this.
A leak-down test would be good at this point if none of the above fixed the problem.
GerryLP:cool
