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Old 07-19-2012, 04:58 PM
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Originally Posted by starin View Post
It idles like it has a vac leak but no leaks so well see what happens
One at a time, remove all the vacuum hoses and block the port off. If the idle quality improves when a particular vacuum hose is removed and the port is plugged, you will know you just found the faulty accessory or hose. This included the power brake booster and any emissions equipment that may still be on it.

I don't remember you saying what you're using for a carb or intake, so this is a shot in the dark: If you have an EGR valve (mostly '73-up but who knows what year the engine you have is or what changes might have been made, etc.), it could be sticking open or leaking. Just disconnecting the vacuum line to the EGR doesn't mean it can't leak or be stuck open, either.

If the intake manifold was recently installed, it is not at all unusual for the intake gaskets leak vacuum. If this happens on the underside, it can be hard to tell by using propane or spraying WD-40, etc. because of how the ports are near the valley cover.

The carb can be internally losing vacuum because of wrong or damaged gaskets or the power circuit (power valve on a Holley, power piston on a Q-jet or step up springs on a Carter/Edelbrock) can be stuck in the open position which can cause a rich idle condition that imitates low vacuum.

What is the vacuum reading at idle? Have you pulled the plugs to see if they're sooty from a too-rich air/fuel ratio?
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