I have a honda 250sx threeweler and i have correct timing and gas and fire but it won't start it backfires?

I have a honda 250sx threeweler and i have correct timing and gas and fire but it won't start it backfires?

Best guess is that it isn't getting enough fuel, why it back-fires.
Clean the carb, especially the pilot jet and if it has aftermarket exhaust think about a larger main jet.

Install a brand new spark plug.
Dirty plugs will spark outside of the engine, but not under compression.
Check the air filter.
Clean or replace as necessary.

Bent valve, sticking valve, or valve gap. If the other is for sure correct. Easy one to check is valve gap. Which reminds me I gotta check my yz250f valves.