It's your carb, it either still has crap in it or an air leak at the base gasket.
Does it have a fuel pump, if so it could be restricting fuel.
How about checking the float level also, all this could be the problem, it doesn't matter what carb you have a carb is a carb.
Don't buy a new Chinese carb they don't work.
Take the carb to a reputable mower shop and have them clean and install a kit. This will save you a lot of headaches.
Don't go drilling jets it ran fine for many years with what is there, if you drill jets you are just covering up the real problem.