[66chevy2ss]

Supposing my problem isn't something else, consider the following (if you will):

My 350 has a "big" cam in it. It idles roughly. Now its difficult to get it to idle at around 700 RPM, which I'm told is approximately where the initial timing should be set.

So I've been setting the initial timing at 1000 RPM (where it idles fine). But the problem is (obviously) that the mechanical advance will upset this tuning.

Any ideas?

[bullheimer]

no, but i'm dumb. <img src="graemlins/pain.gif" border="0" alt="[pain]" />

[66chevy2ss]

Not as dumb as me, though. Trust me...

[willys36@aol.com]

Usual first culprit to look for is a vacuum leak. Also of the cam is too radical, idle transfer port is uncovered by throttle plate. Trick to fix the latter is to drill small (1/8") holes in throttle plates to allow the trottles to be closed down to let these transfer ports do their job. That is last resort though.

[66chevy2ss]

Could you tell me how to look for a vacuum leak?

I do have (I'm borrowing) a friend's hand-pump vacuum with guage, and a second guage with just the hose and pressure guage.

My car also has hooked up (and don't ask me why) a Summitt Racing vacuum guage by the tachometer.

[willys36@aol.com]

It's not that kind of vacuum. What I am referring to is a leaking gasket somewhere on the intake manifold that is exposed to manifold vacuum while the egine is running. Only real way to do it is with the engine running. Take a can of WD40 with the red extension tube (if you are like me, you will need to go buy another can. I always lose the tube). Start the engine and methodically spray the WD40 on all the intake manifold joints (carb, head joints), a little at a time, everywhere you can reach. If you hit a leak, the engine will momentarily speed up. On a Chevy, you can also have a leak on the under side of the manifold that you can't reach - only alternative is reinstall the manifold w/ new gaskets. Bummer!

[KULTULZ]

Originally posted by 66chevy2ss:

[quote]My 350 has a "big" cam in it. It idles roughly. Now its difficult to get it to idle at around 700 RPM, which I'm told is approximately where the initial timing should be set.

So I've been setting the initial timing at 1000 RPM (where it idles fine). But the problem is (obviously) that the mechanical advance will upset this tuning.<hr></blockquote>

The way you are setting basic timing is correct for your application (hi-lift/hi-overlap cam).

You are also correct in your assumption that basic timing will be somewhat different with the 300RPM increase, but if the overlap in the camshaft doesn't allow a 700RPM idle, so be it.

The only way out of it is to know the advance specs on the distributor and adjust from there. Too much trouble. You are in essence power-timing the engine. If you experience detonation (through engine rattle-plug deposit check-replacing burned pistons/cracked rings ), back intial timing off a degree or two until the pre-ignition subsides.

If you are coming across a vacuum gauge, it is the best diagnostic tool available. Go to this <a href="http://www.users.bigpond.com/ergoff/OTHER.HTM" target="_blank">-site-</a> and look under Vacuum Gauge Interpretation.