help please!!!!!!!

Check your fuel filter and your timing. Do a visual inspection on the motor and make sure everything that is supposed to be attached is.

Is it back firing through the exhaust or the carb? It might be the timing being off. If that doesn't work the condensor on the distributor could be fried. That could cause a backfire through the carb even with good timing.

Check those things mentioned above, first. Could also be that the timing chain slipped a tooth. How long since last rebuild and timing chain replacement?

My Hypothesis:

Crap in the gas tank and fuel line, like residual old gas, will cause those symptoms. You said you drained the tank, but unfortunately the stuff that temporarily clogs up your fuel filter will settle to the bottom of the tank and fuel line. The stuff looks and acts like clear mercury. The car will run fine in the driveway, but start driving it around and/or stirring things up by adding more gas and the little blobs get sucked out of the tank into your fuel filter. Let the car sit a few minutes and the blobs dislodge from the filter and roll to the lowest point in your fuel line.

Thoroughly flush out the fuel line, replace fuel filter, flush out or replace gas tank. Flush out and inspect (or replace) sending unit.