Have you tried a partial choke?
If that improves it, it's likely a lean fuel condition.
Start with checking the fuel availability to the carb. With the engine off, fuel valve open, remove the drain plug in the bottom of the carb. Catch the flow in a clean glass. It should start with a full flow, then slow to a trickle. If the flow stops, or slows to a trickle, there is a fuel delivery problem. Could be the sediment screen, possibly a restriction in the tank valve, or the screen in the carb inlet fitting.
Look at what was caught. If cloudy, it has water contamination. If rusty or dirty, the tank is contaminated and will need to be cleaned. Whatever was caught will also be in the carb. It may need to be eased apart and cleaned out. Easy job, just don't loose anything!
Has the air cleaner been serviced? There is a wire mesh above the oil bowl that is often overlooked. Also check the entire pipe and canister for mud dobber nests and rodent nests.
Once everything has been checked and remedied, and you are happy with the rest of the components, as in properly tuned, valves set, etc, then the carb jets can be adjusted.
Adjust the idle jet first. The engine needs to be at slow idle, down around 400RPM. Turning the screw in richens the mix, out leans it. You are adjusting air bleed, not gas. Back it out to rough idle, then back in to best idle, then slightly more in.
Assuming it has an adjustable main jet, adjust it next. The main screw adjusts fuel flow, so backing it out richens the mix.
To adjust, idle the engine down, then suddenly open the throttle. If it dies, back the main jet out 1/2 turn, try again. Keep repeating until it will take sudden throttle without hesitation. A single puff of black smoke is the goal.
If the main is not adjustable, or it won't adjust rich enough to take throttle, something is clogged in the carb. It will need the main jet and distribution tube removed and thoroughly cleaned. Dried up ethanol gas tends to form a hard, sticky residue that must be scraped out of jets and fuel passages.
