95 XTZ660 not running smoothly

[Chef]

Hi everyone,

I have a 1995 Yamaha XTZ660. The bike previously ran well, started easily, and was reliable. However, it was consuming oil, which was visible on the dipstick, and riders behind me could smell burnt oil.

To address this, I replaced the valve stem seals with the cylinder head still installed. When comparing the old and new seals, there was a significant difference in the inner diameter.

After reassembling everything, the engine idles normally. However, when I increase engine speed to approximately 4,500 RPM, the revs suddenly drop to around 2,500–3,000 RPM. At that point, the engine produces black smoke and exhibits symptoms consistent with misfiring. With full throttle, the engine sputters up to 7,000 RPM while producing a large amount of smoke. Upon removing the spark plug, it is completely black and heavily fouled with soot.

I've already checked the following

-New spark plug installed
-Vacuum leaks checked
-Wiring loom inspected
-New fuel lines installed
-Cleaned the carb

I am considering replacing the ignition coil and HT lead next, but I am not convinced this will resolve the issue.

I’m reluctant to give up on this bike, but at this point I am running out of ideas.

[donnybach]

I’d start at the basics and check valve timing and compression, valve clearances.
Then start looking for air restrictions.
Although the carb was cleaned, have you checked all the settings again? Might be massively over fuelling in the mid range, maybe leaking in through an overfilled bowl.
Check settings against standard, and do a fuel height check too.