Motorcycle Repair: 1983 nighthawk carb problem, needle jet, excessive fuel

Question
1983 CB650SC Honda nighthawk. I've had the carbs cleaned twice and each time it runs great. Then after maybe 1000 mi, the #1 plug fouls. I clean it or replace it and its good for 10 minutes before its covered in black soot. To get it to run now, I've turned in the pilot screw all the way in. It's been suggested the float is leaking and over time (its plastic) and its flooding that carb, but the mechanic never saw that. There was a dirt problem, so I've installed a large gas filter, but that is after everything.  As its set now there is no fouling and its got lots of power.  I've also drained the carb bowel after every ride and had no problem. Any idea why all of a sudden I needed to screw the pilot screw all the way in to stop fouling that plug? Do I need another carb cleaning?  Any ideas would be helpful because my mechanic has given up...
Thanks

Answer
Hi Lar,

I suspect the needle jet may have bell-mouthed and is metering excessive fuel in that carburetor. Look for an "oval" or "oblong" wear shape (front to rear) on the jet's main bore where the needle jet makes contact with it.

Look for an air restriction on that cylinder.

Check fuel service level and float height.

Respectfully,
Mark Shively