There are a couple possibilities. First, if you're running it as recommended with a castor oil fuel, you may have some varnish around the carb. Does the throttle barrel turn freely over the entire range or is it binding?

Can you turn the engine freely when not in compression? You could have a rusty or gummed up bearing.

You say the full throttle is only a few hundred rpm above idle, and the idle is too high. How fast is it actually turning?

Did you adjust the needle valve settings after you took it out of storage? You say they are at the factory settings. Does this mean you never adjusted them before?

The gasket on the intake could be causing an air leak. I would definitely replace it if it looks worn.

Brad