YZ250

I have not touched the idle setting on my race car in years, then suddenly today, the car was dying due to it trying to idle at just a few hundred RPM. 4 or 5 turns on the idle screw brought the idle back up to normal, and the car ran perfect on a test drive.

Does the huge change in idle screw setting indicate something might be in the process of breaking?

Here is a little more background. I went to start the car this morning after sitting for about 6 weeks, but no start with a crazily swinging tach, which typically means low battery voltage. I charged up the battery, and still no start with the tach still swinging. I disconnected the battery to reset the DME, and after re-connecting, the car started up and ran normally except for the above idle issue.

Tom M'Guinn

Well, something must be up to require so much more air. You might try setting checking the idle with the ISV and O2 disabled via the diagnostic port, then see what happens when you re-enable them. O2 sensor might be causing it to go rich, or ISV might not be working. Or the TPS or AFM might be malfunctioning. Or a squirrel build a nest in your air box. Or... Do you have a wideband? That might help add clues...

Porsche Ober

idle stabilizer switch under valve cover? I hear they go bad around 15 years in...idk

YZ250

No wideband. Looks like I need to do some homework.

YZ250

Air filter. Turns out it had a crazy amount of track rubber and track grit. I have no idea why it was fine starting several times over several months, and then not fine about 6 weeks later. Anyway, changed the air filter and all is fine. Checked out the ISV, and it is working properly. Idle screw is back to normal.