My carb behaves the same...I believe it is improper functioning choke opener, but haven't been able to prove it. If I do a carb tune-up, as I reset the fast-idle at the last step, it will hold steady at 1,800 rpm (with the choke fully set)...but then go to start it the next morning and that strange symptom returns...starts fine, but right after begins to decel/stumble a little so I have to push the choke in to 1/2 way.I get a weird thing though when the choke is fully pulled that it starts to idle really rough until I push in the choke slightly then the idle smooths out.