What jets are you running, i have found my 650dp on my mild 360 magnum likes a 70/76 split from the 67/73 factory, you may want to jet it up & see, if its an abrumpt stumble then its most likely a "lean" transition, you have already mentioned that you installed a "larger" squirter, PV with a weaker spring & they helped, both of these changes "fattened" things up a bit, so your going in the right direction, thats why i say jet it at least 69/75 or 70/76 & try it, your stumble may magically disapear. As for the 750 PF MB, i'd say "yes" on your combo, but you'll be mighty surprized how well those 650dpers run once dialed in.
