Got Exactly the same problem with my T500. I found the problem was that the guy who ported the barrels also took some metal from the piston skirts and widened the intake ports too much so that the gas seal was non-existent. Somehow the engine ran but would only idle with some artificially large throttle openings set by the idle screws. Blip the throttle and the engine would race and not return to idle speed.
Threw away the barrels and got some new pistons - problem gone.
Good luck with your Yam ....Yam , oh sod it, you know what I mean