Clic bike backfireing?

ive got a bsa c25 engine from the 60's and most of the time i try to start it it backfires and hurts my ankle, lol. is there a way to stop it or to reduce it happening? i put the engine in a bsa c15 scrambler, with a huge undersweped exhaust with a huge funnel on the end. is it cos im allowing to much fuel into the carburettor (im currently pumping till it appears out of the hole in the mouth).