Posted by Gordon Maney on Tuesday, October 11, 2005 at 9:21PM :
In Reply to: Not Run-On posted by dave horvath on Tuesday, October 11, 2005 at 2:37PM :
If you have never timed it with a light, I would urge you to do so. If the timing was retarded sufficiently it could contribute to your symptom.
You can adjust idle speed and idle mixture. The idle speed is a stop screw that determines how far the throttle valve can close. The mixture screw is a brass screw that screws into a carburetor casting. It has a fine, pointed end and is very fragile. It determines how much liquid fuel can flow in an internal idle passage. It is not an air screw, as some believe.
Start the engine, let it warm fully. Adjust the idle speed screw to slow the engine as slow as seems practical. Don’t slow it so much that it is struggling to stay running.
Shut off the engine. Turn the idle mixture screw, with your fingers, or a very light touch with a screw driver, until it bottoms. Count as you go by half turns, to see how far out it was when you go to it. Write down what you determine. Do not tighten it beyond that, as you can easily damage the tip of the screw.
Back the screw out 1 1/2 turns. Start the engine.
Make your initial adjustments by half turns. Constantly keep in mind where you are. Talk out loud to yourself, saying where you are. Turn in a half turn...... Listen..... Did it become smoother or less smooth? Back out to the starting place..... Then back out a half turn from that. Listen...... Is it smoother, or less smooth. Move both in and out from your origin, by half turns, to find the place where it runs the smoothest. You will likely end up somewhere between 1 1/2 and 2 1/2 turns out. It can vary.
When you get to the best place you can find, then make 1/4 turn changes, and finally, as you listen ever more carefully, smaller than 1/4 turn changes. But.... Always talk to yourself so you know where you are. DO NOT just start screwing around, so to speak, just turning the screw continuously. You have to pause in a location and let it function there. Always stop at your various positions.
As it runs smoother, you can go back to the idle speed screw to slow it more. The slower it runs, the more evident the effects of the screw changes will be. Once you have the mixture adjusted, you can set your final idle speed.
You don’t need a tachometer to do this, you can do all of this by ear. You will know a reasonable idle speed when you hear it.
If changing the idle mixture screw causes no noticeable effect, then you have a dirty or otherwise non-functioning idle circuit. You might try removing the screw completely and pressing the end of an air hose nozzle against the hose and direct a good blast of air into the hole. That can sometimes clear the passageways.
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