My 58 with automatic is making me nuts. Had 4bbl rebuilt by "some guy" with good internet reviews. When hot idle is set around spec, car stumbles and stalls on part throttle apply ….hot idle screw is not on stop below 1200 RPM or so, but idle goes lower almost "on its own" which I cannot figure out. When I turn idle up to 1300 , though, hot idle screw just makes contact and car runs fine……. but seems insane to run idle that high!

I'm thinking it could be a clog in circuits that allow transition from idle to part throttle.

Anybody have ideas before I spend many hours trying to rebuild carb myself?

thanks!

 

Try this at your own risk...with the engine fully warmed up increase the RPM's as high as you are comfortable with and then force the choke plate shut by hand until the engine almost stalls then release the choke plate to allow the excess fuel to clear. It will increase the suction and fuel flow inside the carburetor hopefully freeing restricted passages. Do this outside and be cautious of exhaust backfires and intake/carburetor flash of flames.

 

Have you checked for broken
or cracked vacuum lines?
This can happen when R/R a carb.

 

I have gone over external vacuuums a number of times and can't find anything....though my gut says somehow vacuum is related. No hissing at all anywhere, but I'm wondering if internally a line is blocked.

Sugar Bear: I gave that a bit of a shot earlier...no avail but I also didn't make much of an explosion. (had a bunch of carb popoffs when msessing with timing, though previously so there has been some shocking going on alreadly)
Thanks much for the thought.

 

Have you tried spraying WD40 or some other solvent around all the vacuum lines and at the base of the carb while engine is at a idle? Often vacuum leaks will show themselves with an increase in RPM's....Tedd

 

4GC are sensitive to the correct gaskets. There are at least three different float bowl to throttle plate gaskets and they cover or expose different internal vacuum passages for different engines. My guess is your guy put the wrong gasket in and you have either a vacuum leak or an unintentionally plugged vacuum circuit.

I had one on a 64 Starfire that drove me crazy too. I rebuilt that carb several times matching the new gasket to the one being replaced. The FSC section of the shop book didn't show a very good view of that gasket, but when I looked in the Cutlass section, it was plain as day what was happening. Put the correct gasket in and that car never ran better- until the fuel inlet stripped out and ended up junking the carb.

 

eventually traced it to leaks around secondaries -- gaskets appeared ok but found that castings need to be addressed. Picked up a rebuilt complete from Fusicks (Daytona Parts) that got me dramatic improvement. Left with just a small off-idle stumble now versus a "non drivable" situation I had before. Original rebuild was definitely problematic the more I got into it. (Done by guys in Pittsburgh)

Great thanks to those that offered help here!