back fires when starting problem

[FLASHLIGHTBOY]

I have a ZSTAR 110 CC MODEL... when you start it up SOMETIMES it will back fire and blow white smoke out of the carb,,,, pre-igntion... It trys to fire to soon... but then it is ok after that one time.. you wait for a long time and maybe it will do it again... I change the carb... I raise the rod in the carb (like it says in the carb setting section here) to try to lower the amount of fuel setting.. could it be the CDI .. or ingition coil??? any help would be good.. P.S. sometimes there is a big bang from the motor trying to fire up.. but it runs great when it starts and runs.???

[forchetto]

Slipped ignition timing could cause this, but the only way would be for the key that fixes the position of the rotor to shear in the crankshaft. Unlikely.

Also unlikely is the valve timing slipping by the cam chain jumping a tooth.

A lean mixture can cause this. Try richening the low end by tweaking the mixture screw. If that doesn't do it I would check the valve clearances. A tight inlet valve could cause it.

[FLASHLIGHTBOY]

OK. I figured it out.... when the custoemr was trying to start the motor. they would push on the throttle lever.. this caused a too rich mixture and cause the engine to back fire.. I just turned the throttle screw up .. and bingo it great.. hours of work to figure this out.. as it blew smoke out the carb and I thought a intake valve was not sealling