Ok, if it is running badly, there are a few things i would check first. Are the bolts that hold the carb on tight, and is the gasket ok? if there is a problem here it will draw air in and run lean.
valve clearances can make it a problem too, also if the flywheel key is sheared or half sheared it will run badly.
So by the sounds of it you are using a new carburettor, so throttle shaft wear shouldn't be an issue. What is the engine doing exactly that it required tuning?
The screw under the float bowl is the high speed mixture screw. the one up on the very top is the low speed mixture screw, then you're idle adjustment is on the throttle shaft itself.
I always start with both screws at 1.5 turns out, get it started, then adjust high until it runs properly. (clockwise on the screw on the bottom will lean it out and make it pop and or start to die, counterclockwise will make it rich, blow black smoke and die, you want it about halfway between rich and lean.
once it is running nicely start low, it is usually easier to have the idle screw in a fair way so it doesn't just die. same as before, lean will make it hunt up and down, rich will choke it. once you find the mid way start lowering you're idle speed until its idling nicely at i think approx 800rpm
you may have to adjust high again after low, as they do affect each other.
Now all of this will have to be done likely with the air filter off. once its running nicely sit the filter on, and you may need to lean both down a little bit as the filter will restrict the intake flow a little bit.
If it will not shut off it is not a tuning problem, it will be something to do with the stop switch. is it on a key or is it just on the throttle control at the back? it should stop regardless of rpm.
