The engine not decelerating when you throttle down quickly is an indicator that the mixture is too lean. Based on the engine's age and the fact that is had been out of service for such a long time and started right up is in itself a testimony to the Zenoah quality, however I think you should consider giving the carb a complete overhaul.

Though it's not rocket science, if you think you are not qualified to do the job any garden power tool or karting repair shop will do. Better still if you have a flying buddy that is so inclined and will help, go for it. Just wondered, what size prop are you using and what rpm is it turning.

Karol