Hot Weather Fuel Mixture Screw

I noticed that in the summer hot weather (90 - 95) degrees that my 426 is real hard to start when hot, even with the hot start open.

With the air thinner due to heat, are people leaning out the fuel mixture screw? I tried going from 2 1/4 ro 1 3/4 and it helped a little. Thinking about going even leaner.


uh-huh. And I also dropped the needle 1 clip position and went to one size smaller main. Cleaned everything up real nicely. A friend of mine with a 426 has repeatedly fouled his plugs since it's gotten hotter and rainier. But he refuses to think it could be that his jetting is too rich. Go figure.

I learned what many of you already know. The jetting (especialy the pilot circuit) has everything to do with how easy your bike starts. I played with clip positions, different needles, and mains in an attempt to dial my bike in perfectly. I found a "happy" point and my bike ran strong, no fouled plugs, and it started fairly easy. It wasn't until I dropped down a size on my pilot that I found what I had been missing. (especialy for the TX Summer)

My bike now starts first or second kick everytime with no choke. I often do not have to use the hot start when its warm.

These bikes are not and should not be hard to start. They are easy to start when the jetting is dialed in. It takes some trial and error, but it is worth the effort.

MX974, don't be affraid to drop in a smaller pilot for the warm weather. It might be what you need. Also keep in mind that perfect jetting for today may not be for tommorrow when the temp might drop or raise 10-15 degrees. It helps to have a T handle on your fuel screw. Once you learn what your bike likes, you will know just where to turn it.


Just out of curiousity...what jetting did you end up at? Main, pilot, needle/clip, fuel screw, etc? Since we ride the same tracks and both have aftermarket mufflers, it would be interesting to see what your at. Thanks,


I don't know how far you guys are from stock, but I find that my completely stock '01 runs best on hotter days (high 80's and up) and on cooler days it starts to get lean. Minor fiddling with the fuel screw has so far fixed any problems. If you're not running stock jetting normally then it may be worth giving the stock settings a try.

yes I know what you are saying, I have one of the Kouba T-handle fuel screws now and my bike is running alot better now no popping under decel like it was, starting seems to be fine too. But I haven't got the screw exactly set where I want it yet because it is a little loose and when I crank the throttle it turns the screw in. So It ends up resting at about 1.25 turns out. I know it needs to be about 1.5 or a little above but it doesn't seem to mind it too much. It has been in the 90's all week here. I need to fix the screw so it sits a little tighter in the hole anyone have any ideas??? Thanks, Frank (Loctite???) :)

Maybe a little tiny piece of teflon tape around the threads to make it fit a little tighter? I've never tried this before, try at your own risk!

Frank is the spring and the o ring on your screw?

There should be a spring, an o ring and a little tiny flatwasher also, all on the end of the screw if ya take it out. the stack from bottom up would be spring, washer, o ring.

good idea thumpalot, new their was something I could do, I bet u that is just what I need!!! I think loctite would be a little extreme!! LOL thanks as usual, Frank

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