Good performance depends on several things; fuel, air, ignition, and compression. It sounds like you've already verified the fuel, compression, and ignition. The only thing left is air. Getting too much or too little will affect performance. Since you've already leaned out the carb as much as you can, it sounds like you're not getting enough air. The fuel/air mixture has to be right for the engine to perform well. What you're describing is poor performance under load.
Are you using the original oil bath air cleaner? If so, try running (briefly) without it. Those things can get gunked up pretty good and really restrict air flow. If running without it solves the problem, then you can try soaking the oil bath cleaner in solvent overnight to clean it out. Or, replace it with a paper filter, which would require buying a filter assembly (or scrounging for one that fits.)
Do not run without an air filter for very long, because it can ruin your engine. The air filter protects your engine by keeping particulates out of the engine, where they can scratch the cylinder walls and the piston rings, reducing compression. They can also get into the oil where they could damage bearings.
Note that if this works, you may have to richen the carb some to get it to work well without the air filter. If so, that would be a clear sign that the air filter is the problem.
