Sounds like a fuel problem. If it was ignition, stomping on the gas would put more demand on the ignition system and the misfire would still be there or it would be worse. No that I've said that it will end-up being the ignition system.
Hold the rpm at 1500 where it begins to misfire. Take a can of carb spray or a small propane bottle and spray a little into the carb and see if the misfire goes away. If does you have a fuel problem, if not its electrical or mechanical.
Next, run it again at 1500 where it is misfiring and pull one plug wire at a time with the best insulated pliers money can buy. Each wire pulled off should drop the rpm and make it misfire even more. If one wire makes little or no difference that is the cylinder with the problem.
Stick a vacuum gauge on it and see if it reads steady. That will tell you if the valves are opening and closing like they should be.
I borrowed this from another site about the causes of back firing in the exhuast. They make several good suggestions. There has to be fuel in the exhaust pipe for it to backfire. That can only happen if its too rich or a cylinder did not fire causing the gas flow on out into the exhaust.
Why does my engine spit or backfire?
First off, you need to determine whether it is backfiring out the exhaust or back up through the carb. We usually refer to an exhaust backfire as a "backfire", and backfiring through the carb as "spitting" or "coughing".
"Backfiring" is usually caused by a spark plug "sparking" when it isn't its turn and the exhaust valve is open. If your air/fuel mixture is too rich and you have unburned fuel in the exhaust system, cross firing from one spark plug wire to another can occur if they are touching each other and when this happens while the exhaust valve is open, it will ignite the rich / unburned mixture in the exhaust manifold and tail pipes and result in a big bang.
Having timing that is too late (retarded) can cause this same thing sometimes if your engine is running too rich. When the exhaust valve opens, the mixture isn't done burning so it ignites the unburned fuel in the exhaust system and causes a loud bang.
Back firing can also be cause by a cracked distributor cap, or one that has carbon tracking inside which causes cross firing between the terminals inside, which in turn, sends spark to a spark plug that isn't ready for it yet.
Backfiring through the carb (spitting or coughing) usually occurs in the morning when a carb's air/fuel mixture is a bit too lean. This usually goes away once the engine warms-up. It is also commonly caused by the accelerator pump inthe carb not squirting enough fuel before the main jets start working. If you spit the instant you blip the throttle it is probably the accelerator pump in the carb not working, plugged up or out of adjustment.
As with backfiring, coughing or spitting can also be caused by a bad ignition system, such as cross firing, which sends a spark to a cylinder that has the intake valve open. When that plug sparks out of turn, it lights the fuel in the cylinder and the pressure has to go somewhere... so if the intake valve is open, it goes right back up through the intake manifold and out the carb with a "spit" and sometimes even a flame.
When and how it backfires or spits will give you an indication for where to look.
