1977 xs400 backfiring

I have a 77 400 backfiring on the left cylinder replaced plug, set timing,adjusted valves, coil is reading 20kohms on the secondary 4ohms on the primary, checked compression, rebuilt carb put in jets replaced carb boots checked for air leaks. does anyone have any ideas?

Plug either new and wet or older black and possibly wet. Too much fuel or weak ignition. Too cold a plug. Choke left on too much. Checked compression means nothing if it is too low. We have to have the numbers or worthless saying it.

if it were a car engine and higher mileage to do it then possibly dead worn cam lobe. Bikes don't commonly do that but not impossible.

Depends as well on where backfiring, back through carb or in the muffler. On decel and through the pipe says air leak in exhaust pipe.

Thanks for the reply, It is backfiring through the exhaust just idling only on the left side It does pop a little on the intake when you raise the throttle mainly when its warmed up, the plugs are brand new ngk plugs when I pull them out they aren't really wet they have some black carbon on them think the coil might be weak
The spark is not real good on the left side but the resistance measurements are very close to the coil on the right side the bike shows 20k miles on it Someone put pipes on the bike with cherry bomb mufflers probably in the 80s sometime seams like that would effect both cylinders

If they use ethanol in your local fuel that is likely contributing to it, it messes the carbs up pretty quick. Even faster if bike not ridden daily.

They aren't really wet is questionable enough to me to say they are. And getting black quick adds to it.

I only use 95octane no ethanol fuel in all my smaller engines just for that reason,

Lucky you..........You might check what the octane booster used there is. Ethanol is commonly used for that too and if the law doesn't mention ethanol as a must the oil company uses it to simply boost octane and has no requirement to report it. Because it is the safest and by far the cheapest way to boost octane now. They can't use MTBE, aniline, and others any longer, too toxic.

They use ethanol to boost octane because the fed subsidy makes it whoppingly cheap as compared to other methods.