Since it you said it really runs great other then the off idle surge tells me you are on the high side of the jetting. If it surged all the time and ran crappy but still got 4 MPG, I would think it was lean. You need to step down a lot more and open the carbs more if they are lean. The off idle might be the power valves opening to soon or not at all. Vacuum will still be the same as if it has one carb, but the airflow thru each one will be about half. The lack of airflow can really mess with them.
You will need to pull the tops off each carb and take a look. There were a ton of model B's made so you have to be sure you have a matched set. Jets, floats, ventures, power valves, etc.....
I have a Uni-syc gauge I could loan you. These let you set the throttle valves just the same so each carb has the same airflow. Then you set the throttle linkage.
Set the timing up some, get it to idle as best as it can using only the distributor. Turn it till it speeds up, using a vacuum gauge here set the timing for the most vacuum. If it starts hard, back it off a little. We have better gas now then way back when, so you stand way more timing then the books call for.
I also have a air/fuel ratio gauge that sticks in the tail pipe. Its real handy for setting the idle adjustments. I have rigged it so I could drive around and check the mixture. Mine was way rich, then I went way lean. It was easy to see on the gauge.
:et me know if you need to borrow them, Joe.
