Question on numbers stamped on piston

[Sportster-2001-951C-Stock]

Sounds like your carb(s) aren't flowing off-idle, low speed fuel circuit and pilot holes are likely restricted and need cleaning (THOROUGH, not cursory cleaning). Don't forget to confirm metering needle valve pop is correct and these carbs aren't forgiving of off-brand parts or the wrong parts.

Give the choke a quick blip as the engine is falling on it's face during acceleration, it will perk up and rev out if it's lean hesitating (running lean due to crankcase air leaks ie: crank seals worn or malfunctioning carbs will roast pistons). Fuel lines must be air tight so low air, pressure leak check them to confirm.