I recently purchased a C3 from someone who put only 3,000 miles on it in 15 years. The car ran well, but idled very rough and the generator light would come on during idle. I also smelled gas when the car was sitting still. I figured that the car had sat so long that a tune-up was in order. I replaced all the filters, plugs, ignition wires, rotor, and cap. It helped a little, but the car still ran rough and I had to turn the idle up a little to keep the generator light from coming on. I was also still smelling gas when the car sat still.

Of course, I looked at the carburetor initially, but hoped that since it needed a tune up anyway, that would take care of the problem. After the tune-up failed to get the results I'd hoped for, I bought a rebuild kit for the carburetor.

The carburetor was very dirty and after much cleaning, I put it back together according to the instructions. I've rebuilt Rochesters before and not had any problems. However, after installig it back on the car, it has an irregular idle speed after it warms up. I know that's a classic sign of a vacuum leak, but the irregular idle wasn't there before the rebuild.

I suppose it's possible that the rebuilt carburetor is now functioning as originally intended and as a result, a vacuum leak has become apparent that was always there. However, before I start analyzing the vacuum system, I wanted any suggestions for mistakes or errors I may have made during the assembly and adjustment. Specifically, can an improper float adjustment cause an erratic idle speed? I didn't mess with the mixture screws on the base plate. Is it possible they need to be adjusted after a rebuild? I don't smell the gas as strongly as I did, but it's still there.

I looked into the carburetor while the car idled. The choke valve was fully open, as it should be for a fully warmed car. I noticed gas dripping from the venturi onto the valves in the base plate, and that's when I suspected the float adjustment was not correct. Is this normal? I've never had to look before, so I don't really know.

On a positive note, other then the idle, the car runs better than ever. It has smooth acceleration, runs nicely down the road, and takes less pedal to cruise at highway speeds. Based on this, I believe the rebuild helped, but I either have an adjustment wrong or need to make further adjustments while the car is warm. I have a 1979 with a Rochester carb model M4MC. Any info is useful. Thanks.

 

You said that you didn't mess with the mixture screws on the base plate. You should have removed these screws, soaked the parts in carb cleaner, then blown air through all the passages to make sure they are clean. Perhaps you can remove the screws and squirt some spray carb cleaner in there.
I believe that you are right about the float level possiblely being too high.

 

If you have fuel dripping from the main discharge nozzles at idle, you have a problem - there should be no fuel dripping from the nozzles. Fuel drip from the nozzles is indicative of one of the following conditions:

• Incorrect float
• Fuel-logged float
• High float
• Leaking needle/seat
• Excessive throttle opening due to incorrect timing or vacuum advance hookup
• Warped airhorn
• Plugged idle fuel restrictor tubes

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