Motorcycle Repair: Fouling plug, air screw, expansion chambers

Question
I have a 1987 CR250.  When I put in a new stock plug BR10ES it fouls within the hour.  Runs great while it lasts.  Doesn't matter if I idle or rev up hills.  It looks oily.  When I put in BR8ES it never fouls instead it pings under extended heavy load like riding in sand.  I guess this is to be expected as the plug is too hot.  It looks white.  Any suggestions?  Don't say get a BR9!  Then I'd have the worst of both?  Too rich?  Adjust needle clip?  Old 2-stroke oil?  Mix less than recommended?

Thanks in advance,
Steve

Answer
Steve,
 Before trying the 9 make sure you are running the stock expansion chambers and that the e-clip on the main needle is at the center groove.  Keep oil ratio as reccomended, use fresh fuel and oil.  Be sure the plug is gapped correctly.  If you are still fouling you can either be suffering from a weak spark or an overly rich mixture.  Check the setting on your fuel/air screw (usually 1 1/2 turns out from seated).  Be sure your fuel level is constant and correct and that you do not have plugged up overflow tubes from the carb.  The 8 was obviously to hot and causing pre-ignition, the premature firing from an overly hot cylinder head causing the pinging and white deposits.  A 9 may be best if not a 10.  I'm not sure if this model still had points, if so be sure they are timed and set correctly.  If they look burnt, replace them w/ new ones along with a new condesor.

Hope this helps,
mike