It has to run with choke half-way on. I've checked so well for any leaks? In any other case of porting intake (that attaches to jug) , would you need to change jets? Could there just be a leak? Sealed intake gasket with o-ring on carb like it was before.
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Dude - you should have said this at first. You have an air leak or somehow a fuel restriction problem. Fix it and get back with us. Make sure you are using a real fuel filter, a good fuel valve and that your carb parts are spotless. You said it was running fine before, or so I thought. Something so gross, yeah don't go messing with jet sizes until the obvious problems are fixed.