[78 trans am]

I have over 100 runs on nitrous (150 shot) on my street 403 olds, & only 10 of those runs have been on F&L SP1 (110 octane), but I've backfired twice & its been on the race fuel. I was wondering what you guy think it could be? It's a stock bottom end 403 w/ a cam, 350 heads, 9.5-1 compression. its a mid 11 sec car on nitrous. timing is set @ 17-18 idle 36 @ 2,000 rpm. I am running a NX Hitman

here's a cool little video my bro put together, backfire included http://www.youtube.com/watch?v=-8wNSR5yzgw

 

[Bellman]

Is it back firing out the exhaust, or thru the carb ??

 

[78 trans am]

the carb

 

[Drag Chevette]

its lean....