I've had a Holley 600 on my 66 with a 289 for the past few years, running it without any issues. Recently I've started to have troubles with it.

The car drives fine, accelerates fine, ect but the idles stays high around 1200 rpms. If I rev the accelerator the RPM will then go back down to aprox 900.

Once I start driving the car again and come to a stop the engine will idle at 1200 plus again until I rev the engine, it will then come back down does this every time I stop and go.


I have put a different carb on the engine to trouble shoot and the engine idled and drove fine at all times. Thus I have come to the conclusion that the problem is with the Holley and nothing else.

I checked for vacuum leaks around the carb and have found nothing. I took the ends of the carb off to look for clogged jets ect and have found nothing, the gaskets appear to be in good shape too.

Any ideas? At a loss here.

 

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remove the return spring and try to wiggle the front shaft . . if it has much play in the base, that is your problem . . more preload on the spring might fix it.

 

If it has a choke, there's a good chance that the fast-idle cam is not adjusted properly or sticking. You should be able to see this easily by looking under the choke housing to see the linkage arrangement.

 

Electric Choke? Clean the spring
Exhaust riser choke? Broken heat tube allowing exhaust into choke body needs cleaning and repair.
Per Mach428 clean the fast idle cam bushing check for wear.
As Barnett468 said, plastic throttle shaft bearings gummed up, worn or swelled.
Dash-pot (keeps throttle from returning too fast) worn so it sticks and holds throttle open.
Depending on setup, throttle cable support on engine bent or twisted, or shaft pivot gunk or bushing bad.
Pedal pivot worn.