It is hard to figure as they use rather weak magneto ignition systems

Paul, and Perry I always ran 100LL in my yamaha FZR 1000 race
bike, at 10:1 compression and NEVER even hinted at fouling a plug.
This was running a total loss battery ign with crappy stock coils!
Point is Perry must have something in conflict with the lead or
perhaps your mixture isn't as lean as you think Perry. On my race
bikes the 100LL always left the exhaust a light tan unless the mixture
was so rich it would barely run. Bill Jepson.

something about the combusion chamber flow patterns with a 'port' type engine?

Ken Powell

Ken, Yamaha published jetting charts to use with aircraft fuel including 100LL.
In fact they suggested running leaded race gas as well. At least as
late as '93-'94. The PREFERRED fuel was the ELF super zoomie
unleaded. The stuff ws so exotic that casually smelling the fumes
could hurt you!  Bill Jepson

"7" colder plugs when I had

the fouling really bad.  But I also had fouling with the "9" plug

Perry Mick

Are these ND or NGK plugs? If so a LOWER number is HOTTER
Bill Jepson

Paul Lamar
 
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