In my quest to sort through a few things on my car I found a date code correct carburetor from Coralsnake that I sent to a Holly carb guy to replate and rebuild. It came back about a month ago and looked good.
I put it on this weekend and the car fired right up and idles beautifully. The strange thing is that when the car is running the throttle shaft sticks so that if you put it to 2000 rpm it might hold it there. Its not a high idle cam issue because if I touch the throttle arm it will return to the stop and the normal idle. In driving the car the throttle feels stiffer. If the engine is off the throttle moves freely.
I have changed a lot of carbs in my day but have never seen this sort of issue. Anyone have an idea?
I don't know if this is your problem but it is a problem I have had several times in the past.
The forward float bowl has a mechanical vent that opens when the carburetor approaches idle. The throttle arm at the top pushes against the vent arm to raise the rubber stopper. When these carburetors are rebuilt the throttle shaft assembly is usually replaced due to wear. The new arms from Holley have a little too much of a step where the vent arm contacts the throttle arm. This step causes the throttle arm to hang on the vent arm, resulting in a higher than normal idle.
A little smoothing out of this area allows the throttle arm to close all the way instead of hanging up. Aa flat jewelers file fixes them in a few strokes.
Here's a picture with an arrow where the problem happens. This one did not have that problem as the throttle arm is an original.
Quote from: Royce Peterson on November 29, 2020, 06:54:18 PM
I don't know if this is your problem but it is a problem I have had several times in the past.
Thanks Royce but mine works fine engine off -- smooth as silk. Its only when it is running that it hangs.
Most common causes for this:
-lever vent as Royce mentioned
-twisted throttle shaft (common with early brass shafts)
-fast idle cam rubbing on the choke housing. Even at best there is almost no clearance.
- fast idle screw hanging up
-pressure on the accelerator pump lever/cam (easy way to test, remove cam)
-throttle plates rubbing bore unevenly.
-improper throttle body gasket (between main body and baseplate)
-improper gasket between carb and intake.
-VS closing link too tight. (Sometimes you need to bend it and get more clearance)
Be methodical and You'll find it.
I KNOW how frustrating this can be, I've done a ton of carbs and once in awhile one is troublesome in this regard.
I'm not certain I can think of why it would just do it running.
Heck, recently I built a 4609 for Mr Bill here on the forums, ran fine for me, no issues, yet he has a comparable problem which is frustrating us both.
If you need more detail on any of the above just ask!
You may have just used the wrong base gasket - You should check that first, if you are running the gasket with smaller primary holes the throttle will stick.
Thanks guys. These sorts of bizarre problems are always the worst. I have just never seen one that works perfectly off but hangs when running. I put the non date code correct carb back on while I sort through this one.
I've had trouble with the newly made Holley dash pots too. They seem to have a spring that is just too strong. Also the secondary throttle stop / shaft relationship can be off, allowing the secondaries to crack open even though the primaries are closed / on the idle stop. Sometimes replacing the secondary shaft is necessary, other times a little bending of the rod connecting the throttle arm to the secondary actuator can fix it.
As Drew said there is no way to diagnose it over the internet. A hundred other things could be causing the problem. There are undoubtedly some that none of us has seen before.
Quote from: eric lipper on November 30, 2020, 08:18:44 AM
Thanks guys. These sorts of bizarre problems are always the worst. I have just never seen one that works perfectly off but hangs when running. I put the non date code correct carb back on while I sort through this one.
As a side note I know you are concerned about date correct so appearance correct would also be a concern too. Your carb re-builder did the typical thing of plating everything Zinc Dichrmate gold . Not all of those linkage parts were done the same finish originally . You may want to research the period look of some of those parts so that they can be detailed properly while the carb is off. I would suggest a different new thread if wanting more in depth so that others can search easier in the future.
Thanks Bob. I will post a pic of what I have and if I go to further detailing will move it over to a new thread.
If you end up sending it this way, I'd be happy to replate minor pieces as you wish.
Typically the pump spring and screw need to be silver as does the throttle return spring etc. little stuff, but a pain from your end.