The Stovebolt Site Forums The Shop Area Engine Shop rochester won't stay off fast idle

gotta 49 chevy 3100 with 235 4sp from a 54 3/4 ton. Recently, engine dies at idle. what figure is all along the carb hasn't dropped to low idle, so previous owner simply adjusted down to 450 rpm. This went unnoticed until recently. So after kicking down and resetting rpm, stepping on acel pedal and it won't come back down to 450 rpm. can goose at the throttle linkage and it drops down. some times by tapping acel pedal it will drop but can take 2-3 times. so you can imagine what this sounds like when on the road shifting gears. It seems like the throttle rod almost binds the throttle linkage which prevents it from fully returning to low idle position.
Anybody have some insight?

There are lots of moving parts and pivot points in the throttle linkage. There is also a return spring. If the spring isn't strong enough, the throttle won't return to idle. If it's too strong, you'll have trouble pushing the throttle down and it will cause excessive wear on throttle parts.

Check the spring first, then all of the pivot points. Pivot points should not be too sloppy, but they also can't bind. Fortunately, it's all mechanical, so you can troubleshoot it with the engine off.

All of Paul's advice/suggested are excellent.

Here is a way to test/observe:
- Disconnect the bell-crank to pedal linkage.
- The throttle return spring should still be connected (pulling the throttle downwards/"closed").
- Adjust the carburetor.
- Flick/turn the throttle quickly to a fast idle and release - does it return to idle?
- Turn the throttle slowly to a fast ide and release - does it return to idle?

When engines are replaced, the wrong accelerator rod is often used. They can be bent/welded/modified to work.
Have someone depress the gas pedal while you look at the hole in the floor where the accelerator rod comes through. As the pedal is depressed there should not be excessive up and down movement of the rod at the hole. If there is, you will need to bend the rod until there isn't. A rod which does not travel freely through the hole will cause your issue. Since there is a bell crank involved, this effect is multiplied.
Carl

Often on that type carb the throttle lever gets loose on the shaft. If that's the case you can remove the throttle body & braze the lever to the shaft.

Bad throttle shaft bushings and/or misalignment of the throttle plate in the housing can cause an erratic return to idle. Try shaking the throttle shaft up and down with the engine stopped. A little clearance is OK, lots of slop indicates the throttle shaft and/or the housing is worn.
Jerry

Does anyone have a diagram naming all of the throttle parts on the carb. If I start making part names up, everybody will think I'm crazy.
there seems to be quite a bit of play in the linkage. the lever this linkage attaches to has some sideways wiggle to it as well. so I'm probably dealing with wore parts on the carb and wallowed out holes on the linkage lever.
If I want to rebuild carb, where do I find the external parts? All the rebuild kits I see contain only the internal parts.

Are you dealing with a Rochester or a Carter carburetor? A worn out Rochester is scrap metal- - - -toss it and buy another carb from Ebay, etc. Carters are worth saving, within reason. There are literally dozens of those carbs on Ebay, from trashed-out junk to fresh rebuilds, recent and old stock off the shelves of closed-down parts stores. If you're dealing with major wear of hard parts like throttle shafts, housings, etc. it's better to find a more suitable example to rebuild.
Jerry

Simple enough, after removing linkage rod, I was able to get carb to stick on fast idle by operating carb manually and was able to drop it by simply wiggling throttle.
Based on Hotrod's post, I would agree, junk it buy a rebuilt. Would now be the time to convert to Carter? I don't want to start a huge debate. I personally know some Rochester guys, they are very loyal to the product. The Rochester tend to weep, are there drawbacks to the Carter, which I'm not familiar with.

Unless money is no object, you may want to remove that carb and inspect it closely. I'd bet there's either an obstruction or burrs in the throttle shaft or the butterfly valve. A little lite filing or sanding may clear it right up.

And you'd learn a little about carbs in the process.

the end game on this is the throttle shaft has been repaired in the past by brazing. This has come loose. Valve appears to seal well. I'll probably take it to local fab shop and have them re-braze it.
thanks to everybody for their input.