I've been chasing this issue for about a year.
My engine idles very well, runs wide open very well, but getting from idle to wide open is nearly impossible.
No matter how fast or slow I move the throttle from idle to wide open, it begins to stumble just off idle, may or may not "catch". If it doesn't, it dies. If it does catch, it's Katie bar the door: it will run full power until the cows come home. Yo, and if I leave it idling, it will idle until the other cows come home.
What have I done: disassembled the carburetor about five times, blown it out with compressed air about five times, tried every setting of the idle screw about five times. And just for fun, replaced the fuel pump. One time. Fuel delivery to the carburetor seems to be ok. The fuel lines are clear, and when after moving the throttle forward, and the engine dying, the carb bowl is full of fuel.
Electrical: on the understanding that the ignition system is a magneto-powered CDI system, I've not confirmed that either the battery or the charging system is up to snuff.
So: what am I missing?