1641 won\'t idle properly

I'm putting a 1641 in my 62 split bus and I can't get it to idle properly.

When I rev it and then let the throttle lever go, the throttle leaver returns to it's proper idle position but the revs slow to about 1500rpm and then it takes forever to slow below that, when it does, it stalls.

I can't get it to idle lower without stalling.

If i restrict air going into the carby with my hand, it'll idle almost properly. This leads me to think it's sucking air somewhere. But when I do this, it starts pumping brown smoke from the exhaust, therefore running rich.

Is it just running very rich and therefore sucking more air to keep up with the fuel? or is it that to get it to run I've accidently got it running rich?

The motor is new and so are all the gaskets. As far as I can remember, I positioned everything right and tightened it all down properly.

Any suggestions taken.

Thanks.

Ross.