If you can't adjust the air fuel mixture by turning the idle mixture screws 'in" and it won't kill the kille the engine that means you're gettinga leak from somewhere else. Start by taking a can of carb spray and spray around the intake and carb and see if see if the idle changes, Your looking for gasket leaks. If you don't find any than you probably have something plugged up in the carb which is really common. Take it apart and gentlly take the carb cleaner and blow out each passage. Reassemble the carb being careful not to strip out the fittings. Everything is fragile, you need to be careful with it. The idle mixture screw you want to lightly bottom out ( don't mix them up) and then open them n both 1 1/2 turns. Once the engine warms up you want to turn them in each a little at a time until the engine starts to slow and then back them out until it smooths out, both screws need to be exactly the same number of turns. If that's the original carb you want to keep it, it was calibrated to go with that engine. If you start changing carbs you're just going to create even more problems. Also before you rebuild it check the butterfly shaft to make sure does.nt have to much slop in it, if it does you might want to look for another carb.