2000 yz426 oil leak

i have a 00 426f that i had a complete rebiuld and it leaks oil behind the front sprocket i replaced the oil seal around the counter shaft there and was going to replace the collar too but i was wondering if had any ideas why it would be still leaking. also what is the head pipe on 2000 made of cause mine glows red after a min or two of sitting there especially in slow riding is that normal or is the jetting wrong cause it backfires too and runs real hot real fast!!!!

There is also an o-ring that goes on the countershaft that seals the spacer. Yamaha p/n 93210-22298. The header is stainless and the glow is normal. Check the spark plug for mixture if it is too lean and also check to make sure the header is seated on the head. Be sure to check that the header gasket was put back in after the rebuild. Good Luck!

thank you very much for your help

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