right plug keeps going bad/fouling??

[69er]

racking my brain on this one;

bike runs perfect, then after 30 miles or so plug goes bad/fouls..put a new plug in and runs fine.

pamco ignition
compression 135lbs both cylinders
carbs rebuilt/jeted/floats adjusted/synced (not my first rodeo)
pods and open pipes
timed
valves adjusted
cam chain adjusted

it is just the right side...

I haven't tried hotter plugs. (shouldn't have too!)

 

[mouser]

I had a similar problem. It turned out that the throttle flap on my chronic left side did not quite close perfectly. I had always checked them without the carbs bolted together. Once I had them bolted to the little angle support and held them up to the light. I spotted the prob..I adjusted them and my problem disappeared...

 

[gggGary]

What does the plug look like? Oil fouled is popular. Valve seals are typical suspects.
If the deposits are light and fluffy then a leaky float valve can be the cause.