Finally got my 502s running, Dart pro one heads, dart single plane intake, holley 800 (jetted the same as a HP500) and comp cam intake lift .640, exhaust .623 duration @.05 intake 230 exhaust 242, lobe separation 112.0.

Timed same as a HP500.

Problem, both engines will backfire when throttles are pushed off idle unless pushed very, very, slow.

Backed the timing down from 14 to 12, but did not seem to help very much.

Will likely have to replace power valves in primaries.
Any help with resolving backfire is greatly appreciated.

sounds like the accelerator pumps need to give a bigger shot of fuel.

Correct length spark plugs??? Short plugs in a long plug head will cause backfires through the carb.

Sounds like you need bigger accelerator pumps to me. You need the 50cc pumps if you don't have them.

Timing Timing!!! whether its in the advance, plug wires ,cam timing or vacume advance. Start from square one and make sure its right!!!!!!!!! Something has to be Popping it off early to be coming back through carb.. I would lean twards cam timing if everything else checks out.

If the exhaust valve is closed and timing is right how does this cause a backfire???
Fuel build up?

1 agree.try 36degree at 3000rpm then set air mix . do you have a spacers under the carb?????

Short plug in a long hole acts like big time retarded timing, takes a long time to burn the fuel through the plug hole and out into the combustion chamber that causes backfire. My situation would backfire out the intake. Don't ask me how I know all this.

BTW pushing the plug tip into the combustion chamber is how splitfires plugs get better fuel economy/power, the split fires are extended tip plugs that essentially advances the timing.

Push the arm for the primary accelerator pump down. It should squirt fuel immediately.

Where is the backfire occuring out the intake or exhaust. Does the motor idle OK? What happens after it gets on plane? Does it run OK? Reach rpm that you would expect for your setup?

check the adjustment on the acc pumps before doing anything else.

Backfire out the carbs, then engines run fine. Going to check out all the suggestions this Friday.
I hope the cams were installed correctly, I did not install. The Comp cam card says the cam specs are for cam installed @ 112.0 Intake center line. The valve timing @.050 Intake Open 3 BTDC Close 47 ABDC. Exhaust Open 53 BBDC Close 9 ATDC

Thanks to all for the suggestions.

Fred