Justin,
I have the vm Carb. I am at 17.5 on pilot and needle set at highest position. I was having a stumble at about 1/4 throttle, just a hesitation and I moved needle down to lean a notch and It got worse. Finally ended with it all the way up, at the richest setting before stumble went away. Methinks 20 on the pilot is too rich and 190 may have been too rich. For me 170 was too lean on the main. I'm in the mountains at about 2500 ft though.
At least the vm carb is easy to tinker with.
I would play with different needle settings.I can't see how the petcock could change your carb though, the vm doesn't have a vacume port like the stock carb right?
All that could change would be the speed of the fuel flowing to the bowl right?
It took me a few days tinkering to get it right but it's worth the time.
My mileage basically stayed the same although I tend to twist the throttle now more than I did with the stock carb.
Seviersavage