Go-kart engine trouble?

I cannot get my go-kart engine to start, it will fire every once in a while, but other than that, nothing. the carb is spotless, gas is fine, im pretty sure its getting spark, could the on/off switch be malfunctioning? what esle could be wrong? cause i'd rather not take it to a repair shop. i just wanna get it sold!

i put some gas in the carb, and nothing happened, then i disconnected the switch, put some more gas in the carb, (after letting it set for a few) and all i got was a nice flame to shoot out of the carb. what caused that?
it was a brand new spark plug

is it a 4 stroke or 2 stroke? it sounds like not firing at right time pull the fly wheel off check the points or coil clearance..spark firing when piston going down it not going to stay running just back firing…..