850 or 950 Carb
A QF 750 flows more than 750... I think with the setup you have, the 950 would be an improvment. I dont see the 850 doing much for you going from the QF. You cannot overcarb a 4" engine with a 3800 stall... I run 770 Avengers (wet flowed) as the low end/small carbs on anything with a 4" crank.