I think a way to test if it's the carb, or the ignition would be to:

Start the engine.

Bring it right up to the speed where it dies.
Now back out the "L" needle 1/2 a turn.
Now try to raise the rpms above the max you did before.

If that works you can continue that all he way to full power if you want, but that will prove it's a plugged "H" circuit. If you still can't increase above what you had before then it's a ignition problem.

I know the engine will run to full power on the "L" circuit alone because the first engine I tuned I really screwed up the carb. I had the motor putting out good top end power, but it idled terribly. I decided to start over and found the "L" needle was out 4 1/2 turns and the "H" was out 1/2 turn..