I have a completely stock 05' YZ 125 and run the stock BR9EG plug. My problem is when I start the bike cold it fouls the plug. I kick start the bike, choke it for 10-20 seconds and rev it and the plug fouls. Once the bike is running I'm fine, but going thru too many plugs.
try shutting the choke off faster, if it wants to die leave it on but never touch the gas til it will idle on its own. this helped me in cold weather b4 jetting. but you do need to jet it sounds like
had the same problem on my 05 cr125....dropped 2 sizes on the pilot, and 2 on the main and it runs much better and doesnt foul plugs any longer...-Hammy
hammy what size pilot are you using? I have the same bike and have tried a 55, 50, 45 and now I got a 40 which I hope will stop the plug fouling at lower speeds and throttle openings, but in my manual it says the pilot jet range is 45-65? I think my dealer must of ordered an 04 jet, will this fit and be ok?