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KTM MX & Off-Road Dirt Bikes
'99 380 cuts out at high RPM after new top end
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[QUOTE="burtlamborn, post: 112160, member: 21044"] Ok, I'm looking for some brainstorming ideas. I have a '99 380 which I've had since it was new. This winter I decided to replace the piston and rings. It still ran really well, but after two years I figured I'd pull it apart and see what things looked like. Things looked pretty good: one ring end gap was 0.013", the other was 0.019". (Spec is 0.012 to 0.015".) I replaced the piston and rings with Wiseco. Ever since replacing the piston and rings, it has run poorly at high rpm and doesn't rev out as far as it used to. Can anybody shed some light on this? Hear are my thoughts: Jetting: I don't thing it's related to jetting. I keep very accurate records, and I'm running jetting that I've used before at this altitude and temperature with good results. Just in case, I've played with it a little bit, with no improvement. Reeds: It ran great before the top end, now it doesn't rev out. The reeds are still stock, and perhaps are wearing out a little, but that would be a gradual decline in performance, not an overnight change. I don't think it's the reeds. Power valve: I have removed the cover to make sure the power valve is actuating properly, which it is. I guess it's possible that I put it together one tooth off or something, but I thought I was careful about that. Piston: I noticed that the Wiseco piston's skirt was shaped quite differently from the stocker. Could this affect port timing or something and cause it to cut out at high RPM? X & Z: I *think* I got these exactly as they were before. I'm sure I could be off by a few thousandths, however. It's fully broken in, and compression is good. (190 psi at 4500 ft altitude.) I've replaced the plug with a new one just in case the problem was related to the plug starting to foul or improper plug gap. No improvement. One last detail: I'm doing an oil experiment. I noted the wear and deposits after two years using Golden Spectro and MC-1. After doing the top end, I switched to a really cheap oil and I'm monitoring compression to see if the top end wears faster with the cheap oil. Hmmn, I can't see how that should affect high RPM performance, but I though I'd mention it in case. Ok, any comments you might have are welcome. Burt [/QUOTE]
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KTM MX & Off-Road Dirt Bikes
'99 380 cuts out at high RPM after new top end
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