So my buddys got a d15b and we just installed a 75 shot (he's had a fake 50 (nos recommedations for quite some time now and never had probs) and during our first pull it backfired in the intake, blew the airfilter apart, and popped a vacuum cap off. We started spraying around 4000 rpms and it backfired at around 5000. So we did it again but didnt start spraying till around 6k and seemed to work fine untill the clutch started slippin. So we got the clutch fixed and now the same thing has happened again.

I really dont know how this happens, i assume the nitrous/fuel mix auto ignites when it hits the back of the intake valve or something. Can someone explain this and tell me how to keep it from happening again.

its a fogger kit w/ 10 pd bottle, 48 nitrous jet and 30 fuel jet. Havnt really searched it so make it easy on me.