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so I have been having an issue with a backfire through the carb at low rpm. It Seems to only happen on hot days as soon as i give it gas from a stop. I have rebuilt and jetted the carb aslo tryed fattening her up thinking it was a lean condition. I have also tried a various ranges of timing. The distributor has a mechanical advance im thinking of switching to vacum. All and all ive had it with this carb im ready to start piecing together an efi set up. Any ideas?

My guess is it is a lean condition. And being this only happens on accel after a stop, I'd say this is related to your accelerator pump. While I'm confident EFI would serve you well, you should be aware of what it'll take and what you are considering. EFI is tried-n-true and can be dealt with using tuning tools if you don't have a stock engine. And as a DIY tuner, I'd be glad to talk you into doing EFI tuning. But I do have to be honest, EFI isn't likely to be the cheapest option compared to either finding what's causing the leanout (vacuum leak) or just replacing the carb with a new one that is known to be calibrated correctly and will work.

What mods does the engine have? If it has an aftermarket cam, what are the cam specs? If the engine is bone stock, the problem may be the carb is just too big. A Holley 650 cfm carb is good for a moderately built engine but is a bit big for a stock engine. A 500 cfm carb is plenty for a stock engine, will get better fuel economy, and better responsiveness. A bigger carb doesn't force more air into an engine. If the engine can't pull down what the carb can allow, the engine's only going to pull what it can.

I bought the motor from craigslist so im not sure of the measurments of the cam i was told it was mild it does heave comp cams rockers and aftermarket springs so I took the guy seriously when he told me it was cammed. IT is GT40p motor. I have always had efi 8 fox body mustangs and 2 sn95's . its what I am most comfortable with. while I admit carbs are great and some of the fastest strip cars ive seen had a big ol holley on them. My truck is an every day day driver. also i live in Ohio so every day could and usually is a different weather condition we go from dry to moisture levels in the 90"s in a matter of hours. my truck runs beautifully on cool dry nights but most of the time im driving it during a work day its hot and humid so it backfires horribly. I have tracked down so far a stock explorer upper and lower intake with fuel rails still on it for 100. a cobra wiring harnes for a 93 mustang with the five speed computer for another 100. I still have my old cold air intake and mas air from a 93 cobra. I am still on the hunt for a good T-5 I am kicking myself for selling my last borg warner when I was in a tight spot. But the swap from auto to 5 speed is something ive done dozens of times. I know im gonna have to go back to the in tank kind of fuel pump i think i have a walbro 190 sitting around somewhere. I know im forgetting a few other things

I am with cgrey and say your accelerator pump is not working properly

I had similar problem, carb spacer was not sealing on intake manifold side.
Spray carb cleaner around carb base when running and see if idle changes.

I have checked for leaks with carb cleaner and tuned the accelerator pump it has a size 35 nozzle on it and the yellow cam. I am still leaning tward the old distributor on it with the mechanical advance. Also I agree with the too many cfm suggestion 670 is a little high i know it should be twice the displacement.

I know with my Holly, the back two adj screws are only turned about a half turn out.. I think the front two are only out about 1 1/2 to 2 turns out.. There is a very fine line in the adjustment as to if it will backfire or hesitate.. Carbs can be very finiky, I think hollys are more tempermental than edlebrocks..