Home Forums 2-Cycle Racing KT100 too lean? Reply To: KT100 too lean?


Jim White

I roadraced Yamahas for 20 years. The last few using the carb trigger to lean the hi needle down the straightaway. I ran 110 with 8oz of oil like you did. Not sure how it compares to sprint racing but many times I ran over 450 head temp. Looking back at some data I see one race where I ran 14 consecutive 3 mile laps between 470 and 500 and with egt in the high 1200’s with no damage. I also only used CHT as a reference and actually tuned using EGT. I could always tell when it was running it’s best. Not so much from the gauge but at top RPM the fins just gave off a distinctive ring that told me it was happy.
That being said, in my opinion your flex color is pretty close to ideal. In the top picture of the header on the engine you can see what’s known as the wet line. Some people tune off the length of it. Once again it’s another reference to look at and learn from. I would be happy when mine looked like yours does. You also said you were turning some of your best lap times. That also leads me to believe you were in a pretty good state of tune. Now when you are running that close to the edge you have to be careful. Depending on weather those settings may be too lean some days and even still too rich on others. This all also relates only to the fuel you ran that day. Go back to pump gas or whatever and it all changes.
As you sound to be new to this I would probably be safe and follow others advice and richen it just a tad as you learn, but now you do know the feel and sound of having a pretty good tune. As you learn more you will eventually get a little greedy and stick one. My engine builder told me along the way that I would (and did) eventually stick one. But he also said that’s not a bad thing. At some point you have to find the limit so you know how far you can go.
I would be highly surprised if you damaged anything although running near the edge also lessens piston and bearing life as you are making more power. I would probably at least pop the head and check out the bore and piston crown (the color of which is another tuning aid). I would always do that after every race regardless.
Now that just my opinion…you’re mileage may vary 🙂

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