As Mark said, if it runs better when you choke it, it is running lean. There are essentially three things that could cause it to run lean at speed but not at idle:
1. Restricted fuel supply. I suggest you remove the float bowl, put a cup or tin under the carburetor, turn on the fuel, and see how fast it runs out of the carburetor into the cup. If it doesn't run full-flow, you have crud in the bottom of the tank, the filter, or the fuel tap (since you've already cleaned the carburetor).
2. Restricted main jet or carburetor body drillings. You said you've cleaned the jet. You need to be able to squirt carburetor cleaner through all the carburetor body drillings. I suggest you don't remove the Welch plug until you've checked everything else, though (both items 1 and 3).
3. Intake pipe leak. A leak in the intake pipe or the gasket where it mounts to the carburetor or cylinder can cause lean mixture. In most cases that kind of leak causes hard starting and poor idle though, to an even greater extent than lean running at high speed.
All this is based on your statement that it runs properly when you choke it. If it seems to run a bit better but still not properly, that would be a different situation and you would need to check your ignition timing, which is quite a complex procedure with that engine.
