I have just found out something new and i dont know if this helps anybody but, on all the hondas they come stock with bigger jets in the carb than what you should have, to prevent blowing up and that causes the hondas to foul the plugs once in a while or often. If you have this problem switch to a smaller pilot jet in the carburetor. I have yet to see that fail.

 

First time I ever heard that.
I thought the design of the intake( small diameter), and the air flow in the heads, and rev limiter, is what keeps the motor from overreving. To the best of my knowledge the mixtures are correct., from the factory. I have a 1970 honda with original carb and jets, and SAME spark plug...its 36 years old, and never fouled, or been replaced. In fact I cant remember ever fouling a plug on a 4 stroke honda.
The pilot jet only effects the idle circuit, it wont effect any throttle opening over 1/8 open. Did you mean main jet?

 

Maybe because that is an older style but I have heard of many cases where the jets were the problem and that was the case on mine. Changed the jets and havn't had to change the plug yet.

 

I never would have thought the pilot jets were the problem untill a guy that fixes 4 wheelers and helps me with mine sometimes told me about it and I changed them and sure enough.