I was driving my 68 (holley carb) accelerating quickly through the gears at about 3000k per shift. when I slowed down for a light the engine stayed running at 2500RPM. i tried to kick it down by blipping the throttle but no change. made a quick u turn and headed for home. I assumed it was stuck linkage or broken return spring but checking it after I got home (motor still running/racing) all was okay. I backed off the idle speed screw but little change. I checked the throttle lever on the carb body and it was at the end of it run in idle position. To get the car RPM down enough so I could shut it down I pressed hard on the primary pump lever which dropped it somewhat to the mid 1000 RPM then I shut the engine off (pump lever also seemed in it normal position). None of the primary or secondary throttle plates appeared to be stuck open. Came back later to work on the problem and now the car is ideling in normal range (havent tried to drive it yet). Any ideas on what caused this problem?

 

Distributor lose and move by chance?

 

just checked . all is tight no movement.

 

Advance weights stuck?

DO MA NEU!

 

vacuum leak ?

 

Secondary throttle plates open?

 

Checked secondaries not open. Checked advanced weights moving freely. How would vacuum leak result in increased RPM especially since it has returned to normal after shut down and restart?

 

Something momentarily stuck. Spray and clean all moving parts, linkages, butterflies ect. in and out with carb cleaner and drive it. Might never happen again.

 

look for small missing screws, maybe a butterfly screw or something wedged in the throttle plate or something.

 

a leak that opens with increased temps maybe.

Just a thought.

 

thanks for the ideas. Strange event. felt like I was driving the car with a cruise control but no way to shut it off (no, my car does not have cruise). I'll continue to check around. Cant figure what would keep the RPMs that high and not be a binding linkage problem or primarys/secondaries sticking open but all looked good when I check it. I was getting on it a bit more than usual and thats when this all started, dont know if that was the catalyst or just a coincidence.

 

most likley if it happened once it will happen again.

 

Usually if the engine increased RPM than it was able to get more air. A vacuum leak some-were. or the throttle plates open. . Check the carb mounting nuts for tightness. Carb flange for cracks, gaskets for rips. Check the Vac lines. for a cracked fitting / hose. Check the PVC Valve.

If the choke pull off stuck in the fast idle position what you said will happen. The butterfly can sometimes open and the high idle cam will not drop - usually sticky from dirt/oil. This will render the throttle stop screw useless.

Good luck and hope you find it.

 

This may sound a bit left field but your symptoms sound very familiar...

Last year I had an oil pump fail.
The first noticable symptom was high revs; up about 2,500 at idle. This was also accompanied by valve train chatter (because the lifters weren't pumped up).
The first time it happened, I didn't look at or notice the oil pressure (or lack of), but shut things down. On restarting the engine the next day everything was fine, but after a few miles and immediately after heavy acceleration the same thing happened; high rev's and noisy valve train. This time I noticed the oil pressure and sh*t a brick. Immediate shut down and got a tow home.

Stripped the pump out, no visible difference against a known good pump, but a bench test with the two side by side revealed very poor pressure against resistive flow and a very noticible change in power required to drive the two pumps (using a power drill with attachment).
It must have been this loss in load that allowed the engine rev's to rise.

I'm all back together now and everythings fine. I checked mains and big end shells and all were good. I think I was very lucky and dodged a bullet. Probably because the failed pump was still moving volume, just not sufficient to create normal pressure.

Just something to check in case you have the same issue. I wouldn't want to run any time with such low oil pressure!

 

i strongly recommend to anyone changing a pan gasket to change the oil pump at the same time, and use a good pump like a Melling, don't cheap out and use some pile of garbage pump, they are very cheap compared to a new engine. i used a Melling select and it came with an upgraded drive shaft.

 

I agree completely, but you know there's a hole group of forum members that believe in reusing the original Gm pump.

DO MA NEU!

 

there are groups of people that think all kinds of things , it doesn't make them right.

 

My pump that failed = Melling
Pump now pumping = GM
Type of pan = GM
Trouble caused by stock pan on this 383 stroker in 8 years of use and 35,000 miles (including track days) = none

It doesn't make them wrong either. I suspect that there are whole groups of people that understand their own cars and engines and to them part of the enjoyment of car ownership (Corvette or otherwise) is not pouring money at everything but engineering solutions to problems in a calculated manner. There is certainly a group of 1 like that sitting here

Stock does not automatically equal junk, and all that glitters is not gold...

 

I've never had a Melling pump fail nor has anyone I've known unless the pickup screen was bad and allowed trash in the pump or the pick up tube wasn't installed right.. that could happen to any of them.

 

I was asked how a Vacuum leak could allow an engine to rev.

A vacuum leak allows more air into the engine. Take the PVC hose off the carb and watch the idle increase. That would be a major Vacuum leak. The carb will compensate somewhat.