throttle surges

[Waif]

Be sure she's breathing throughout.
Fuel cap vent. Through to carb. As well as a happy gravity path. A hot soft fuel line (not saying you have one) can collapse when hot if venting is poor or under much draw. Or kink.(I've a story of a Chevy with a funny gas line.)
Air intake stack.The screen underneath can get loaded up. Oil bath air filter might want peeked at also..
How or when insects had a bad party in my oil bath one year remains a mystery. But my cup runnethed over!

How's the heat on your carb and fuel line? A too hot line might benefit from a heat sink.

Ethanol fuel seemed weak to me.Like not being able to get consistently dialed in under load. Been running rec fuel a long time.

[Eugene]

I was dragging for about an hour with it Monday when it started to miss. Checked the spark plugs after it cooled down. They were coal black.

Following is stolen from the internet. Better written than I could say it.

Soft, black, sooty dry deposits on plug indicate carbon fouling. Carbon fouling is an indication of a rich air-fuel mixture, weak ignition, or improper heat range (too cold). Carbon deposits are conductive and can create a path for spark plug misfire.

Double check, then triple check the ignition system.

I'd also compression test the engine and valve tappet adjustment.

Easy stuff first. Timing light, check ignition timing - and check the advance. Swap coil out with a known good one. Spark plug heat range?

[52DAVE]

I looked in the service manual and it looked like the idle mixture screw adjusts fuel flow. In that case in would decrease flow. Am I wrong? Does it instead control air flow?

[staninlowerAL]

Idle mixture screw controls air flow. High speed jet is fixed.