Once the float bowl in the carb fills, it would run then starve out if the fuel pump was just getting weak. You would expect trouble under higher speeds and loads at first if you were losing the pump. It could certainly be something else. Is it backfiring? If it is, is it through the exhaust or the carb? Try a little more choke. It should pull a lot of fuel through the main circuit with the choke on. If it doesn't seem like it is getting too much fuel when you are approaching 3/4 choke, the main circuit is suspect.

If the motor is idling, can you rev it? Will it run up in the RPM's in neutral under no load without misfiring? We are looking to see if it is engine load/speed related. Are you able to check the timing? If you can check it several times, and see if it is drifting radically indicating distributor trouble.

If it is idling well, I am wondering about that high speed circuit. I am thinking the carb needs another look. Drop me a PM. Maybe we can take a look together. I have an old Sprint Car buddy out that way I need to stop out and see.