carb backfire issue - Ford Forums

miketami · 03-16-2008, 05:30

need help. i have a 351 c .30 over, mild cam 566 lift hydrolic roller, 2500 stall convert. 750cfm 4v carb (started with a holley 650 and put a pro form 750 center in), 2.5 exhaust with 2 stage flowmaster. edelb intake.

I have mechanical secondaries, 75 jets in the front and 84 in the secondaries. timing is 16 deg now.

has taken me forever to get this thing running right after the build. My problem now is backfire through carb ONLY when driving down the road and quickly kicking in the secondaries.

sitting snapping the throttle, no problem, climb into the secondaries, no problem. But snap the throttle after you are driving and it will backfire every time.

also input on what I should expect from a dyno run would be helpfull.

thanks from a newbie.

cjperformance · 03-18-2008, 18:09

It sounds like the secondarys are running too lean, if no backfire when free revving but does backfire under load then start with checking for instant pump shot as soon as secondarys open, then an increase in secondary pump shot, followed by increase in secondary jet size, in that order. Cheers,