Running a demon 750cfm vacuum secondary non marine on a 502 HP500 with merlin single plane intake instead of dart intake manifold and standard HEI distributor with advance plugged. I believe I have read that the HP500 ran a 800ish carb with standard jets.. Question is would jetting still need to be staggard with the 750 that I'm running? The other night I started getting a backfire when coming off the throttle after WOT top speed. Found the accelerator pump leaking. Replaced that and today it seemed like it was getting to much fuel and it started backfiring if I got over 3k and backed off throttle. Decided to remove carb and go though it. Would be good time to change the jetting.

Is it back firing thru the carb or popping out the exhaust? If it's back firing thru the carb either the timing is very retarded or your hanging an intake valve. If it's popping out the exhaust it's lean. Very lean. Or your hanging an exhaust valve open.

It was popping through carb. I have adjusted all valves and set timing when I changed the carb to the 750. It will run good at WOT but once I back off the throttle and it's above 3k rpms it will backfire through carb until it shuts down. To start the engine again it takes a minute. Once it's started it runs fine. I'll check ignition timing again once I get the carb back on.

You really should have an 800-850cfm carb on a 502/hp500. A 750cfm is marginal on a mildly modified 454.