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kx250 break in

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Hi im new to this forum. I just got done rebuilding the top end on my kx250 and this is my first time doing so. I would like to know what others do when breaking in there new top end. Just FYI the guy i bought the bike from had the carb jetted lean and i havent changed that yet. Im a little unsure if this is good thing to have while breaking it in.

Thanks in advance for any responses.

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What do you mean by "jetted lean"? And has the bike ever melted a piston? Does it run noticeably lean, or is it simply that the previous owner jetted it correctly for the weather and altitude there, and that happens to be a little leaner than stock?

As for break-in, it depends mainly on what type of piston you used. Cast pistons don't require much at all in the way of break-in, while forged pistons should be heat cycled.

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Well according to the factory manual the bike should have a 165 main and a 50 pilot jet with the needle in the second clip. The previous owner had a 140 main, 40 pilot and the needle in the first clip. the air screw is at 1.5 turns.

Its never melted a piston and the altitude around here is anywhere from 0 to 1000 ft depend on where you ride. It just put in a forged piston.

I just thought that the jetting seemed lean after learning what the jets where but i am new to the jetting game so it could be just me.

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I agree Chokey that is way lean. My 300 cylinder jets the leanest of mine and I live at 5200 ft and run a 45 pilot, NALF in the second pos, and a 150 main. Also do 3 heat cycles with the new forged piston, warm it up for say 5 minutes at verying throttle then cool completely then do it again and cool completely and recheck the head and cyclinder nuts then the next time ride it for say 10 easy minutes at verying throttle and let it cool again. After that ride it easy for the next half hour then go for it if all is well.

Ed

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Thanks for the help. ill wait until i can get into the local shop on tuesday to pick up some jets that arent so lean.

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