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View Full Version : Air screws/fuel screws on Mikuni carbs


Rubi
01-03-2009, 03:57 PM
I read somewhere that if the screw is before the slide, it is an airscrew, and if it is after the slide it is a fuel screw. Is this correct?

On some 34mm roundslide Mikuni's I've got, I have the airscrews turned in all the way. They are tight. The sled runs quite well this way. Should I install the next size larger pilot jet if the screws are turned in all the way? The pilot says 30 on it. What is the next largest size?

On my TM38 rack carbs, I think the screws are after the slides. Does this make them fuel screws? Which way would I turn these to lean them out some?

PolarisRich
01-03-2009, 04:18 PM
If the engine is running poorly just off of idle, the pilot jet screw can be turned in or out to change the air-fuel mixture. If the screw is in the back of the carburetor, screwing it out will lean the mixture while screwing it in will richen it. If the adjustment screw is in the front of the carburetor, it will be the opposite. If turning the screw between one and two and a half doesn't have any affect, the pilot jet will have to be replaced with either a larger or smaller one. While adjusting the pilot screw, turn it 1/4 turn at a time and test run the snowmobile between adjustments. Adjust the pilot circuit until the snowmobile runs cleanly off of idle with no hesitations or bogs.

Rubi
01-03-2009, 04:20 PM
Is it ok then to have the airscrew turned in all the way? I thought I heard somewhere that you should go up one size of pilot if you need to turn it in closer than half a turn.

PolarisRich
01-03-2009, 04:30 PM
That would depend on what side of the carb is the screw on. If it's in the back of the carb then you will have to go one size larger on the pilot. If it's in the front then you should go one size smaller. You should really have at least some play in the screw for adjustments.

Rubi
01-03-2009, 04:33 PM
It is an airscrew on my roundslides. It's turned in all the way to make it richer.

PolarisRich
01-03-2009, 04:38 PM
It's a "pilot jet screw"! What side of the slide is it on? air box side or engine side (before or after the slide)

What your telling me is, it's on the back side right?

Rubi
01-03-2009, 04:42 PM
Airbox side.

PolarisRich
01-03-2009, 04:46 PM
So to me it sounds like you are leaning out the mixture with the "Pilot Jet Screw" not richening it. So you could try to go one size smaller and back the screw out 1/4-1/2 turn but it dosen't hurt to run it as you are!

PolarisRich
01-03-2009, 04:51 PM
Carburetor troubleshooting is simple once the basic principles are known. The first step is to find where the engine is running poorly. It must be remembered that carburetor jetting is determined by the throttle position, not engine speed. If the engine is having troubles at low rpm (idle to 1/4 throttle), the pilot system or slide valve is the likely problem. If the engine has problems between 1/4 and 3/4 throttle, the jet needle and needle jet (most likely the jet needle) is likely the problem. If the engine is running poorly at 3/4 to full throttle, the main jet is the likely problem.

PolarisRich
01-03-2009, 05:41 PM
Ok Rubi I did a little research and I found the anwser you were looking for...




The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is located near the back, it regulates how much air enters the circuit. If the screw is turned in, it reduces the amount of air and richens the mixture. If it is turned out, it opens the passage more and allows more air into the circuit which results in a lean mixture. If the screw is located near the front, it regulated fuel. The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling, the next smaller size pilot jet will be needed.


So it sounds like you need to come down one jet size.

So back is a "air screw" and front is a "fuel screw" Huh who knew, thanks Rubi! I learned something new today!!!LOL