Technical What causes erratic idling when warm?

[smart51]

The last couple of times I've been out in the car, after about half an hour, the engine has stalled when coming to a stop. It doesn't always happen, and never when cold. The engine can be kept running with a bit of throttle.

I've tried adding a bit of choke and it kills the engine, so it isn't running lean. I've tried leaning the idle mixture but it made no difference. So I turned in the idle screw to keep the engine going. Then I had to stop to let a horse past. The engine sounded like it was idling at 2000 RPM. Later it went back to 'normal' and stalled as I approached a junction. For reference, I was doing between 25 and 45 MPH along country lanes. Not pushing it at all really.

What could be changing so that the idle speed is sometimes low and sometimes not, but only with a warm engine?

 

[bugsymike]

The last couple of times I've been out in the car, after about half an hour, the engine has stalled when coming to a stop. It doesn't always happen, and never when cold. The engine can be kept running with a bit of throttle.

I've tried adding a bit of choke and it kills the engine, so it isn't running lean. I've tried leaning the idle mixture but it made no difference. So I turned in the idle screw to keep the engine going. Then I had to stop to let a horse past. The engine sounded like it was idling at 2000 RPM. Later it went back to 'normal' and stalled as I approached a junction. For reference, I was doing between 25 and 45 MPH along country lanes. Not pushing it at all really.

What could be changing so that the idle speed is sometimes low and sometimes not, but only with a warm engine?

Does sound like a carb issue.
Any clues if you inspect the spark plugs after a run. They should be nice and clean, maybe a light brown, if sooty check for float sticking or dirty needle jet causing slight flooding.
Others on Forum are more 500 specific than me

 

[bugsymike]

Any smell of petrol at that point?

 

[smart51]

Any smell of petrol at that point?

None, even when tweaking the carb. The car is otherwise running beautifully.

 

[bugsymike]

None, even when tweaking the carb. The car is otherwise running beautifully.

Going in the opposite direction then , air leak around carb base or throttle spindle?
Sometimes a length of clean fuel hose to your ear aimed around the carb may locate that sort of thing.
Re the spindle , when engine idling off choke try rocking/pushing the spindle without opening the throttle to see if it affects the idle speed.

 

[Goldnrust]

Inconsistent idle speed in my past experience has often been wear in the carb, most often the throttle spindle.

I'd check for other air leaks between the carb and the engine also. Warping on the base of the carb is common on 500s, and that can cause air leaks. Might be worth checking.

 

[smart51]

The throttle spindle is new, though that's not to say that the casting isn't worn. With the engine struggling to idle, I might try a drop of oil on the spindle to see if that seals it.

Good call on air leaks. I'll look out for that. I'd also wondered if the throttle linkage might be a bit sticky just shy of closed, so that sometimes it closes further than others.

 

[bugsymike]

The throttle spindle is new, though that's not to say that the casting isn't worn. With the engine struggling to idle, I might try a drop of oil on the spindle to see if that seals it.

Good call on air leaks. I'll look out for that. I'd also wondered if the throttle linkage might be a bit sticky just shy of closed, so that sometimes it closes further than others.

You mentioned sticky spindle this can happen after replacement.

 

[bugsymike]

Replacement spindles or butterfly's can often cause problems, pattern butterfly poor fit in throttle body due to mismatch on bevel where it touches/closes, distortion in spindle where not fully supported during "peining" of the brass retaining screws, screws actually dropping out causing an another air source through venturi , apart from the usual worn bushes/hole, poor fit spindle etc.

 

[smart51]

A few drops of oil on the throttle spindle and accelerator linkage and the problem seems to have gone away. I can't say which bit fixed it, but one bit of it did.