I would recommend getting a fuel filter inline with the carburator, flush the carb and remote needle housing with the needle removed.

Check to see if there is any play in the crankshaft bearings. I have a friend that got three sets of bearings that were bad within three tanks of fuel and the engines would not run right it would surge. Horizon was very good about it all and my friend finally seems to have an engine with good bearings and the piston/sleeve fit is very nice on the latest replacement.