I recently swapped my old 350 heads for a set of vortecs. I also replaced the intake manifold with a vortec model but kept the old holly 650 carb (previous owner did the carb conversion). Upon starting the engine for the first time after the swap, the idle instantly shot up to around 4000-5000 RPM and held there.

I made sure that all of the intake vacuum ports were connected to accessories or plugged. There also doesn't appear to be any major vacuum leaks from the gasket or RTV.

What would cause such an insanely high idle? Could this be a carb issue? I have never really seen or heard of this issue before (at least not this extreme). To my knowledge vacuum leaks would cause a slightly high idle, not a 4000-5000 RPM idle!

Any ideas would be much appreciated.

 

Vacuum hose hooked up on the brake booster end? Also make sure the throttle linkage isn't stuck and holding the gas pedal down.

 

Yes, the brake booster hose is connected. I disconnected the throttle cable from the carb and the problem still persists.

 

The cable may have nothing to do with it

What have you done with the idle screw and its the throttle return spring holding it all the way closed?

 

Did you look at the position of the throttle blades? Could be hung up on a bad gasket or something in the carb linkage could be holding the primary or secondary side open.

 

i had the same issue first time i did this.. i found i had put the carb base gasket on wrong and it was sucking air in.

Pop carb off and really inspect it.. super high idle is most def air leak, if not something mechanical..

check the vac hose from any kind of choke pull off device.. small hoses tend to crack easy.. disconnect throttle cable altogether.

could be the butterflies are catching on a piece of gasket in throttle bores? had that happen before..

 

Just realized I never closed this thread out. Turns out I placed washers on the carb studs UNDER the gasket. This created an air gap between the carb and intake manifold, hence the high idle.

 

Oh wow... didn't see that coming. A left-off or split hose seemed pretty likely.