I just went through a similiar situation.
The BG carbs are a great piece. BUT BUT
They have tuneable air bleeds that control the amount of vacuum that basically controls the fuel curve. As the air bleed is enlarged, it will not only work to lean out the mixture, but delay the discharge of fuel at the booster. ie lean spot
Conversely, reducing the size of the air bleed size richens the mixture and activates the main metering system ealier.
SO your 850 is a 2 circuit carb High speed air bleed and idle air bleed. The high speed air bleed which is where you are having the problem is where you most likely want to activate the main metering circuit earlier. You should have .033 air bleeds, Try .028's
Look up the model carb on line and it will have tech specs.
Look at the HS air bleed.
Hope this helps.