l would reset H/L & Idle screws to factory spec (in manual), then do exactly as treesmith says. Getting the low ticking as fast as it can while idling will get you in the 'area' where your stumble problem will go away. Then l would richen it ever so slightly as much as l can without loosing throttle response (we're fine tuning within say 1/8 of a turn at this point). By the way 1fiddy your favourite 'tard' with cs vid had me & many others rolling on the ground. Very funny indeed