Motorcycle Repair: 1980 Honda cb750c Backfires all the time, 1980 honda cb750c, air fuel mixture

Question
QUESTION: I recently bought very customized bike. It has been chopped, lowered, front end raked out, big fat 300 tire put on the rear! Anyways the motor is a 1980 Honda cb750c. The pipes have been cut about halfway so I know the gas/air mixture isnt right. Ive tried setting the adjustment screws all the way in and all the way out and nothing seems to help. It also leaks a little oil from one of the Cylinder Head side covers (part #2 on OEM diagram). Have any advice so I can ride this around town and not worry about it?  Thanks.

ANSWER: Dave, the air mixture screws only control the mixture from 0 to 1/8th throttle.  The needle jets and main jet will need to be changed.  You probably need to raise the needles about .03 inches with some small washers and go to the next size larger main jet depending on just how lean it'r running.  If it's not too lean, you may be able to just raise the float level 1.5 mm and be alright.  Rejetting is a trial and error process unless you have an air/fuel mixture gage.  

THe oil leak probably won't do anything but make a mess.

Regards
Rich

---------- FOLLOW-UP ----------

QUESTION: Thanks for the reply. I was also wondering about timing. Would that have much to do with backfiring?  

Answer
Dave, yes the ignition timing could cause back firing but you should always adjust the valves and check the ignition timing first thing when troubleshooting any performance problem. Just to rule those out as possible problems.  However, a lean fuel mixture due to an unrestricted exhaust which is what you described will also cause backfiring.