First I would like to say that you don't need this tool for balancing carburetors on the MGA (or MGB or MG Midget). The reason this tool is not particularly important for MGs is because the MG has a balance tube in the intake manifold between the carburetors. If you completely close one carburetor the engine will run nicely at idle speed with air intake from only one carb. Angles of the throttle plates at that point are not much different, only about one turn of the adjustment screw. With throttle shafts locked together, when at wide open throttle that small angle difference in the throttle plates is pretty much negligible. The easy solution is simply to open each idle screw 1/2 turn before you lock the throttle shafts together.

However, the tool is relatively inexpensive (if you find the right source), and does what it is supposed to do quite well. It is also relatively easy to use, after you remove the air cleaners). Therein lies the primary problem with using this tool. If you don't need to use it, there is not much benefit in removing and reinstalling the air cleaners. I have one of these tools, but the only thing I use it for is to prove that you don't need it. This tool may be much more useful for multi-carburetor engines that do not have any balance port in the intake manifold, where each carburetor would be feeding separate intake port(s) with no intake air cross over.

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What it Does and How it Works:

The Uni-Syn unit can measure "relative" air flow quite accurately. What it does is to place a very low restriction venturi at the entrance to the carburetor. The venturi produces a slight vacuum signal that generates a small amount of air flow in the signal pipe. The signal pipe is held vertical, and it is slightly tapered (larger at the top), and it contains a "pea" with a diameter slightly smaller than the signal pipe. The annular area around the pea changes with height of the pea in the pipe. This annular space is a flow restrictor. As flow increases the pea rises higher until the annular space increases and differential pressure across the pea is just enough to support the weight of the pea. The lighter the pea the more it moves, and the more sensitive is the instrument. By adjusting the very low restriction venturi aperture you can change the vacuum signal until the "indicator" physical position falls near the center of the indicator range. Height of the pea then provides an accurate and repeatable indication of air flow rate through the carburetor inlet, although it does not tell you the absolute flow rate.

But you don't actually care about the absolute air flow rate, only that you want to make it equal for all carburetors. After getting the tool set up to read dead center on the indicator for the first carburetor, you place the tool in front of the second carb. Adjust the idle screw to give the second carb the same indicator reading, and therefore the same air flow as the first carb. Repeat for any additional carburetors (if more than two carbs). When all carbs have equal air flow, then lock the throttle shafts together. For adjusting idle speed thereafter, simply adjust all idle screws the same amount.