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03 CRF450 running lean

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A few weeks ago I noticed it was running hot... I tore it down and found that the plug was white and the left intake was at zero... I sent it to the shop for new valves and figured that is why it was running lean...I didn't check the plug when I got it back and it seemed to run well. After a crash destroyed my stock header, I put a Yoshimura full system on last week (with stock jetting)...when I got home from the track I pulled the plug and it is scary white again! :cry: Is this strictly because of the pipe with stock jetting or do I have another issue? Thanks

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Stock jetting can be lean at low throttle positions. Try turning the fuel screw out 1/4-1/2 turn and compare.

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I'm sure you've checked & made sure you have no fuel restrictions like clogged tank filter or dirty fuel bowl or jets, correct? If so Yoshimura tech says you should start with a 172 or 175 main then read the plug [the test for the main jet is a long WFO run without throttling down, kill it at WFO, then check the plug color]. If you've made any other changes like airbox mods, pistons, cams, porting, etc. you might need to increase the main again. We haven't seen one need more than a 178 yet, but some do well with a needle swap. If adjusting the screw helps it run better at idle & off idle great, but you don't want to make the HUGE $$ mistake of adjusting the screw to mask a lean plug without checking the mid/top end jetting. PM me if you need more info.

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