I ran one of these back in my pattern days, a rear exhaust version. Strong, strong engine. Still have it, in fact. To think along a slightly different line, I wonder if the idle adjustment may be messed up on your carb. I don't remember the details of that carb, but I imagine there is an O ring involved, and it may be dried up. Worth playing with, perhaps. I think one of my pattern buddies had a problem with his, and we filled the tank with denatured alcohol, took the plug out of the engine, and spun the engine with a starter, which caused it to run the alcohol through the pump and engine, blowing it out of the plug hole. It seemed to clean out whatever had gotten into the pump, and ran fine thereafter. Maybe worth a try.

Good luck. Don't give up on it yet, it is a fine engine. Clair