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No power at full throttle

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My 91 ( old style model ) XT 250 is driving me insane with carbie issues - I have had the carbie off 30 + times and it is a pain in rear as it requires seat and tank removal each time.   Originally it was running too lean so I  replaced all jets and needle (supposedly stock)  it went like crap so reinstalled originals.    I played with float height and needle clips and now I have the opposite problem.

Current issue is no power over 50% throttle - went to town purchased new plug  installed and then took it up a long steep hill at full throttle - turned of ignition and coasted to a stop.   What I note about  the plug is that one side of the insulator is still white the other side starting to blacken up.  I rode home approx 10 kms and the plug looked leaner but still noticeably black on one side.  I marked the plug and checked the soot is happening at about 7 o'clock on the inlet side.   I was going to reduce main jet but thought I would post this and get some input

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Set carb up to standard clip setting using new needle, main and needle jet. I removed the coil and rectifier cleaned and ensured a good earth and replaced plug and cap.     I checked inlet valve it was on the tight end .05mm so reset to 0.08mm.    Now running slightly lean and  slightly better but still with flat top end.  Have ordered new coil ... failing that I will have to try a new CDI.

One note - I do not have the pumper connected but can't see why that would have any impact.

Edited by keychange

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In a way the porcelain on that plug looks awfully white . Plug reading isn't as clean cut reliable on 4 strokes like on 2t's . I'd put a bigger main jet in and see how it changes , if better go another size bigger till it pulls good up top , or go the other way . Generally if the main is lean you'll have a lazy motor . It'll rev slowly and just sort of run out of steam at some rpm and not be real powerfull . If too rich it'll rev up but surge and make a lot of noise but seem a bit low on power . If on the extreme rich side it'll puff black smoke every time you crack the throttle and the tailpipe will be full of loose fluffy soot along with the fender .

You can test a little as well by taking the air cleaner off and removing the spark arrestor to gain flow temporarily to see if it gets worse . Taping up some of the airbox intake and/or putting the stock exhaust on(if aftermarket is on now) to test the other way.

Doubtful  cdi is bad , Yamaha is generally reliable in this area and often failure show up as no start/no spark or running good up to 3500-4000rpm and then hitting a wall , maybe spitting/backfiring , like it's hitting a rev limiter. Likely all jetting or carb issue .

.

Edited by jjktmrider

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