For my bench tests I made a plate that attaches to the bottom of the carb bodies when the float bowl is removed. The plate is large enough to straddle a 4-cup piece of disposable Tupperware. An electric fuel pump sucks mineral spirits out of a 1 gallon jug and delivers it to the carb via a T fitting connected to a pressure gauge. I set the float height to 18mm as discussed earlier and I tweaked the float orientation so the float seams were parallel (more or less) to the flat bottom side of the carburetor body.
The front carb worked just as you would expect. As the Tupperware filled up the float closed the valve and everything came to a stop. The fuel level was about 5.5mm below the flat underside of the carb body.
The rear carb did not do as well. Even though this carb also has a new float valve and was set to the same 18mm height, the fuel level kept rising until mineral spirits started overflowing the Tupperware. I fiddled with the float valve itself for a while and had no success. In frustration I added another sealing washer under the float valve and reset the float height to about 19mm. This time the valve shut off as it should. Like the front carb the mineral spirits was about 5.5mm below the flat bottom surface of the carb body.
This is consistent with what I observed last week when the rear carb was running rich and revving the engine (w/o air cleaners) seemed to cause a few tiny drops of fuel to come out of the rear carb's intake. (This happens with SUs on A-series BMC engines but I did not know it could/would happen with Strombergs on Triumphs).
I now have confidence that the float valves will shut off. All I have to do now is wait on parts. I'll update this thread again once I have my new needles installed.
Doug L.