What percentage of full deflection do you typically see on your gauges or have you tried them on the FJR's TB?
A differential gauge is probably the way to go unless you want to spend big bucks on an accurate (electronic or mechanical) absolute vacuum gauge. A home-made liquid manometer or a device like the Morgan CarbTune is probably the simplest; especially if absolute vacuum is not (as) important. I saw the electronic gauges when looking for the CarbTune and thought I would ask if anyone had tried them. While mercury sticks are favored by some, I wouldn't use them. Materials are hazardous and could cause real damage if the liquid gets sucked into the engine! Water (or transmission fluid) in a manometer wouldn't do any harm and either is actually far more sensitive because of the much lower density.
I don't remember for sure. Last time I did the FJR was a year or two ago. I have the Motion Pro unit and what ever the height was at it was plenty sufficient. I want to say it was 30-50% but don't quote me on that. The tubes are pretty tall for their diameter. I blip the throttle to make sure idle comes back to a consistent reading and closing the throttle at elevated RPM causes extra vacuum that can suck the fluid out of the gauge if you get too overzealous. The gauge is sensitive enough that the level will oscillate a very little bit even with the orifices are installed in the lines. For less than $100 it has paid for itself between the FJR and my old ZR-7 I originally bought it for.
Again, absolute accuracy is immaterial for this application. Adequate sensitivity in the particular measurement range is critical - as is reproducibility. Likewise, all four channels have to read the same as each other even if there is a calibration error. Some sort of signal damping is also needed for a digital gauge - either mechanical (small orifice or vacuum reservoir) or electronic via an averaging algorithm. You are absolutely correct in your comment about perceived accuracy of a digital readout. I have a nice digital tire gauge that consistently gives me the wrong reading! (Although it looks good!)
