Mikuni VM carb on my 87 Trail Boss keeps pouring fuel out of the overflow. Within 10 seconds of opening the petcock, gas pours out of the overflow, not just drips. Even with the engine idling, it's not enough to keep up, only around 1/2 throttle. I rebuilt the carb with a kit. Included the needle and seat. I thought it was the floats, but they float fine in the bowl when it is off. I hooked up the gas line with the bowl off still, and I have to really squeeze the valve shut in order to stop the gas flow. Definitely more force than any float could provide. Rubber tip on the needle seems fine, and I don't notice any issues with the seat surface. What am I missing here?

 

I have had bad ones in a new kit before, not common but it happens. Also check for a small piece of crud stuck there to keep seal from closing. Sometimes cleaning them loosens crud and allows it to get trapped. Most seats have a gasket under them, are you sure your has one?

 

There is a gasket that seals the seat to the body of the carb. I noticed that the new seat was about 1/16" longer than the old seat. The wire connecting the needle to the float arm would hit the face of the seat before it completely sealed against the seat. I took a 5/16" drill bit and drilled a hole in the face of the seat about 1/16" deep. The needle then had plenty of clearance. Put it all back together and there's no more leaking,

 

Did you replace the o-ring UNDER the needle valve seat? With todays alcohol fuels they harden and commonly fail.