88 roller 350 with vortec heads, hei, holley 650 vac secondaries, #70 main jets, timing set at 15 deg base, fuel pressure 5psi. Newly built engine.Carb backfires when you go from idle to WOT. Most of the time its not flame, just a cough from the carb. idle screws currently turned out 1 1/2 turns, choke is set proper, and accel pump lever set to .015". Float level good. I've upped my jet size, played with idle screws, isolated the secondaries, played with squirter size and adjustment on lever but NOTHING is helping to get rid of this backfire. Doesn't fall on its face, just stumbles and backfires thru carb and then keeps climbing in rpm. HELLLLP!

 

If it is a backfire, sounds like a timing or spark issue to me (HEI module going south, not enough initial advance, etc.). If it is just a stumble and not really a backfire, then it is most likely the accelerator pump. Make sure the arm is touching the pump and that even the slightest movement of the throttle provides a stream of fuel from the squirter.

 

I'm having the same exact problem with an Edelbrock Performer on a 305, was hoping to find some advice here. I went through several carb adjustment (metering rods, springs) with no appreciable change. If I roll into the throttle, the RPM will climb to the redline w/o issue, but it sputters, backfires, or just straight dies if I get on the throttle hard.

I've check the timing, but haven't changed it. This motor ran fine with the same timing using a GM Q-Jet. Plugs and wires are newish and have been used in this motor with a different carb. The plugs appear to be indicating a rich condition, but I'm not an expert at reading plugs.

 

It turned out that my plugs were the culprit. Put them in last September and have only about 1000 km on them and cars been parked since november. They were run rich for too long and ended up fouling them. Changed the plugs and took care of almost all my problems. May not be the same for yours, but something to think about. If they're heavy with soot or gas, may want to change them out.

 

My problem was in the carb. I changed the plugs and distributor cap with no effect, so I swapped carbs with my other car and the problem didn't occur with the second carb. I still don't know what it was. Both carbs are the same CFM, but the second one is a Holley, go figure. Both cars seem to run fine now.