Slight POP out the carb?

I have a 77 B here that will give a slight pop out the carb when accelerator is gunned real fast? Right as pedal is hit it will give a quick little pop right off the throttle and not everytime either. I've leaned and enrichened carb plus moved timing back and forth with little dial on distributor(25D) but no change? Any ideas would be appreciated. Thanks again....

Have you set the valves? If so, worst case would be a burned valve.
Edit: set the valves then run some Sea Foam through the gas and go for a drive. If that doesn't clean it up, look at other things.

Is the dashpot piston lifting cleanly. If it sticks down it will give a lean mixture which can cause popping back.

Yes I recently adjusted rocker arm assembly to specifications and also a valve job has been done approx. 2 years ago. Dash pot seems to lift out with no trouble except the suction wanting to pull it back as I withdraw it plus there is oil in it. I can't figure it to save my life.

I agree with Chris, sounds like valve adjustment or timing.

I ahd the same thing on the '79 I used to own and it turned out to be the cam, #3 was almost round replacing the cam fixes it.

Just out of curiosity, if it was the timing would that mean it needs to be advanced more or less?

The most likely causes of popping in the carb are

Lean mixture either through adjustment or leaks

Ignition retarded

Dashpot piston not lifting when throttle opened

Incorrect valve clearances, but if they were far enough out to cause popping back I would also expect the engine to be running a bit rough.

Does it run fairly smooth? If there was a burnt valve or worn cam it should run rough. Talkin to a couple of people and you may simply be running rich or loosing air flow somewhere. When you adjusted the mixture did you have a gas analyzer or adjusting the the needle withthe piston lift test John Twist teachs. Lift the piston slightly and if the RPM's increase it's running rich. If the enigine starts to stumble it's running lean. Too much fuel can casue a popping through the carb. You may simply have a worn needle, float to high or float valve sticking a little.

Ok ,,the oil in top of piston acts as a damper ,,resisting sudden movement,,when throttle slammed open
that resistance increases largely the vacuum caused by increased velocity of air across the bridge
ENRICHING mixture ,,,like an acceleration pump in conv carb .
Backfire can be caused by ,,lean mixture,,valves ,,,timing..
A momentary backfire ,,once or twice ? oil in damper or mixture +++
Always start with easiest things first before screwing around and wrapped around the axle .

The car runs good. It's just that slight little pop when I first hit the throttle. I know it isn't valve setting. If I remember right I have timing set at 15 degrees btdc. I have a 25D distributor in it(which doesn't come with this year) with the vacuum advance hooked to carb base. It does it intermittingly. I can gun it one time and it will do it and then it may not do it again for 5,6 or 7 times. It seem like it does it more often when car is good and hot after driving it a while. I've tried testing lifting dash pot but I can't hear a difference even turning needle valve in and out? But like I said, other than that motor runs great..