I have searched on here and did some switching around of parts with my friends hybrid with no answer to my question. Here it is my quad will not run with out my timing all the way retarded, if it is even the slightest bit advanced it will backfire so bad it will break your leg. My friend and i are both scratching our heads on this one. I have traded multiple cdi's, carb, stator, rotor, and coil. My friend ran my cool head on his bike and he can run his in the stock position. It used to have 87 cases but i changed to the 88+ cases and it still does this, runs the same with my 87 cylinder as well. By this time we thought it was because of porting, but when my friends head was off i mic'ed the exhaust port from the top to the opening and width of the port and it is the same as his jug. The bike runs great when its retarded but i just want to figure this out. Any info would be great, thanks. Oh its a 1987 LT250R frame with 88+ cases 87 tranny LT500R motor coolhead LRD pipe boysen rad valve bored flat slide mikuni carb.