Well that just reeks of valve lash or very rich
Why are you running the 850? Big power? Hi rpm? Or just cuz?
Wait-a-minute!
The gas sloshes into the carb? That is not supposed to be able to happen; the vent stack goes into the airhorn, some nearly 2 inches higher than the fuel level. I think this is a big clue! You're talking exactly which 850?
Notice the carb shop said it was all good, I bet they ran it on a stationary engine,while you are in a moving vehicle, with the fuel constantly sloshing.
I wonder if that Percy block is not routing the vent properly, or the 850 itself has a defective vent system.
If you take the carb off,plug both vent stacks and the inlet, then you should be able to turn that carb upside down with no loss of fuel. The only way out for the fuel, when upside down is, or should be, thru the stacks.
Well; the main lines and the accelerator circuits might dribble a bit until they are empty.
Each bowl holds about 50cc, and if all or most of it drains, that is a big issue! Find out why!