Help!!!

Ive been trying to fire up the GPX 250 Im (STILL!) doing up. Its been sat in our damp garage for about 4/6 weeks. Altho it turns over fine, all that happens is there is a slight puff of smoke/steam from the exhausts and occasionally a HUGE back-fire blasts out of one of the pipes AFTER I stop turning the engine over. Im not sure which tho.

Ive tried with the choke on /off, throttle open/shut and all the various combinations of the above.

The engine is trying to fire but it just wont catch.

Any POLITE surgestions????