What causes backfire?

 

well there are 2 types of back fire. Threw the Intake valves, or threw the exhaust valves.

If it backfires threw the intake, it means your timing is too advanced, the spark fires before the valve is closed, causing a backfire. If the timing is too retarded the spark will fire while the exhaust valve is open, causing it to backfire threw the exhaust.


Its not a big problem on our cars, however on my 1968 Mustang, the distributor was worn out and it had a points type of ignition and I could not get the points gap (dwell) set correctly because of the condition of the distributor. This caused my engine to be either extremely advanced or retarded. Not only would have keep the rev's high (above 1000 or it was dead) but it was very hard to start. Its very dangerous, and since I didnt have my Air cleaner on (also doing carb adjustments) I had several 5+ foot flames shooting out of he carb (the first flame was that big because there was lots of gas in the carb from me trying to get it to go. the less gas would only be about one or 2 feet high, and would be more of a blueish color because there wasnt as much gas to burn so it could burn cleaner and quicker.

MrGone

btw, because it is a 1968 and I do not have to be emissions compliant to have it be street legal, I can hook up a spark plug and ignition coil to my exhaust pipe and have flame throwers

 

ignition timing off, clogged fuel injectors/lines, mis-timed fuel injector pulse firing, bad gas, cold start... several things as well as nothing can cause it.