Proper Steps to Diagnosing High Idle "Sticking"

[QuintupleTurbo]

Hey everyone, I've had my '71 Spider (1.9L motor with ADFA Carb) for a little over a year now and have been loving ownership.

I'm having an issue with my idle speeds. When starting the car up cold, the idle will creep up to about 1,500 rpm and will stay there until the vehicle is warm.

The torrential downpours of the Bay Area have halted (for today) and I was getting ready to go out for a spin when I couldn't get the idle to drop back down. After blipping the throttle, the car stayed at 1,500 and wouldn't go back down. My guess is something is either letting excess air in, or keeping the throttle plate open. Also, for what it's worth, when turning the car off, the engine almost acts as if I stalled it after I turn the key to "OFF". There's some sputtering and shaking that occurs after I turn the key.

What would your course of action be here to diagnose the issue? Once the engine cools off I'm going to start going through all of the checks.


Thanks!

[18Fiatsandcounting]

When starting the car up cold, the idle will creep up to about 1,500 rpm and will stay there until the vehicle is warm.

This sounds normal. With these older cars, the automatic choke will keep it at a high idle until the engine is warmed up. That high idle is usually around 1200 to 1400 rpm, so yours might be a bit high but not terribly so. However, once the engine is warmed up, the choke should "kick off" and the idle should return to its normal 850 rpm. If it doesn't, the choke might be sticking.

As for the "run on" that you experience when turning off the ignition, there are several possible issues. Could be that your idle cutoff solenoid on the ADFA carb is stuck in the open position. Could be an issue with the idle being too high (related to the choke issue?), and in extreme cases, it could be caused by something in the cylinders that is way too hot and is acting as a spark plug to keep the engine firing even with the ignition off. That last issue is pretty extreme with Fiats and I don't see it very often. Premium gas can often address this issue.

Once the rain clears, you're always welcome to come out to Livermore and have me take a look. There's also a variety of parts, usually free.

-Bryan