Ok, for starters I have searched through a ton of threads and this is what I have questions on, I am aware of hot start lever / exhaust leak checking..... I have a 04' YZ450F and whenever I get on it hard it and let off the throttle completely it backfires / afterlites w.e. you want to call it (flames out muffler, 2-3 "pops"). Starts up one kick, idles fine, and has good throttle response. It seems to only happen when im on it hard - throttle 3/4 to all the way open, then let all the way off. Now I've read that its normal if its not excessivley doing so and thats the big question. Is this condition considered excessive if it does it every time im on it hard, but it doesnt if I dont open it up all the way before I let off ? It has stock jetting, the previous owner put a FMF pipe on it (didnt change jetting) and I have only rode the bike in cooler weather (had some popping, definately less... but I wasnt comfortable on the bike so I was never on it full throttle like I am sometimes now). The only conditions that have changed are me running 87 (all I had) instead of 93, and the fact that its about 15 degrees warmer than what I have been riding in. I would like to also get some clarification on the 4S jetting... some say that running lean can start melting things, others say its almost impossible b/c 4S will run so bad with lean conditions. The bike isnt puking any coolant, wouldnt it be doing this if it was running hot enough to do damage b/c of lean condition? Thanks!