Conv389drv

My dad has a 61 with an pretty unusual carb condition. The throttle has always wanted to stick slightly open when you let off the gas, and not return to idle position unless you blip the throttle. It did this for a long time while he had a Holley carb on there.

He has since changed to an Edelbrock, but the sticking throttle problem continued. At first we thought the throttle linkage was binding somewhere, but my dad disconnected everything so that the carb was alone and not hooked to anything.

Here's the funky part: when the engine is running, the throttle shaft sticks. When the engine is off, it moves freely! It's as if engine vacuum were sucking the plates closed and you had to overcome this to move the shaft. That sounds pretty far out there to me, but it's about the only explanation I could think of. I have this same carb on my Pontiac's 389, and it doesn't have this problem.

The 283 is stock except for a Comp Cams high energy piece with a duration of 218 @ .050"
I can't imagine that this cam, having a relatively large duration for a 283, would generate enough vacuum at idle to affect the opening and closing of the throttle plates! Anybody have any theories or experiences with this problem?


PS: I haven't had the chance to put a vacuum gauge on it yet, but that's what I want to try next.

ghostrider20

Are you using any kind of adapter plates or carb spacers? Any type of speacial gaskets etc??

Mark

Plasticman

Conv389drv,

Does sound kind of strange (especially since it has occured with 2 different carbs., and one of those that worked fine on another engine).

Look for any possible contact between the throttle blades and the intake manifold or gasket when hot. Have to assume you already checked for that, but might as well ask just to be sure (but normally that only causes problems at full thottle).

Other item to look at is the throttle return spring(s). They may be weak or the wrong ones, etc. Might even be attached to the wrong hole position (less leverage to close the throttle).

Doubt that vacuum could cause a problem like this, but "anything is possible".

Certainly blades can stick if the throttle shaft is worn, and this normally occurs at idle, but sounds to me like your problem is "not returning to idle properly", and is not "sticking at idle". Was the Edelbrock a new carb.? That would eliminate the worn throttle shaft theory.

Plasticman

jerrybramlett

I would check the position and tension of the throttle return spring.

GM sometimes installed the spring to pull towards the rear of the car. This is not a good design. With both the throttle linkage and the spring pulling back on one end of the butterfly shaft, it ***** the shaft in the carburetor baseplate causing wear.

Regardless of how GM designed the spring to work, I would install it to pull toward the front of the car. That way it counters the force applied by the throttle linkage.

Try a stronger spring if that doesn't work.

Brian VH McHale

Check the motor mounts. I have seen these cause this problem.

Matt Gruber

jerry hit the nail on the head. i had the same problem and reversing direction of the spring was the cure.

LB66383

Is this all with the same manifold? Is the carb mounting surface on the manifold perfectly straight? If it's warped, it can distort the base of the carb when you tighten it down and cause binding.

Conv389drv

Thanks for the replies.

My dad has tested the carb by itself, disconnected from any linkages. Therefore, it's narrowed down to the carburetor alone. The Edelbrock is still a pretty new unit, so there shouldn't be any wear issues. And again, it did this with both the previous Holley and the new Edelbrock. I'll have to check for any contact or binding between the blades and manifold. The only thing that has been changed is the carb, so no spacers or adapters. I suppose it's possible that it could be a problem with the intake manifold, since that has been a constant factor. I might try getting a straight edge to lay across the mounting surface to see if there's any unevenness.

Thanks everyone for the ideas, it's given me something new to chew on. :)


On a side note, this problem causes some funny incidents at times when out driving the car. We'll pull up and stop at a light, and the idle won't come all the way down. So my dad will blip the throttle, and then people beside us will think that we want to race!! :lol:

clem zahrobsky

make sure the weights in the distributor are not sticking in the advance position as this will cause the idle to stay high :chevy

jerrybramlett

Clem, you win the cigar.

I'll bet the distributor has some of those light Mr. Gasket advance weight springs. They're not strong enough to pull the weights back every time at idle speed.

clem zahrobsky

jerry i do not smoke but you can send a bottle of good scotch!! :party:

Plasticman

Clem,

Outstanding deduction (since he has already had 2 different carbs on the engine)!

Plasticman

Kid_Again

...yeah, this one WAS pretty impressive...nice job...of course, the poster hasn't tried the fix BUT this is a very clever deduction

drspencer

Did anyone ever find a solution to this mystery?

I have a '71 350/270, 4-speed, no AC, completely stock, that has the same problem.

I recently had the carb completely rebuilt.

When installed, the throttle linkage won't 'kick down' all the way, and make contact with the speed/idle screw. There is a small space between the throttle linkage and the screw.

It idles at ~800 RPM until you blip the throttle with the pedal, or your finger, then it rests at the desired 600 RPM (the small space between the throttle linkage and speed screw now being gone).

It will do this every time, regardless if the throttle cable is connected/disconnected, choke connected/disconnect. Using a larger/stronger throttle return spring does nothing.

With the carb uninstalled, in your hand, the linkage is smooth and fluid, with no binding.

It should be noted that this problem only happens when the car is running. With the car off, the linkage completely returns. With the car running, it does not.

The car has perfect vacuum, with no leaks anywhere.

Any suggestions?

Thanks

W Guy

Another illusive cause could be the accelerator pump shaft which can wear against the edge of the hole in the carb lid and stick. You can unhook the linkage rod from the arm and gently work the linkage to see if the problem goes away.

If it happens with two different carbs, check that little plastic anti-squeak tab behind the gas pedal. It can get a wear spot in that catches on the throttle lever that rubs on it.

Verne

drspencer

How do I unhook the linkage rod from the arm, without mucking up this beautifully rebuilt Q-Jet?

Where is the anti-squeak tab? My problem happens regardless of whether the throttle cable is connected or not.

Thanks

W Guy

Spencer, In your case, it sounds very much like the throttle blades are catching on the manifold gasket. OR you're torqueing the carb bolts too much and deforming the base of the carb.

Unhooking the rod that operates the accelerator pump from the linkage arm should be self evident if you look at it. But you said it operates fine when on the bench, so that's not your problem.

Verne