Precision glass tubing with mercury columns are the gold standard for accuracy. As laboratory instruments that is what is used to calibrate precision vacuum measuring devices.
Given the hazardous nature of mercury they are no longer favored as mechanical workshop test equipment. I still have a set though.
Twin Max, Carbtune, and others are now favored. I have no idea which of these is the most precise. I have an old Twinmax that works fine and is sufficient.
Frankly, I don't know what I would buy if I didn't already have what I have.
Variations in vacuum and actual inlet airflow due to valve adjustment, deposits in the intake path, and other causes is likely to be a bigger factor than the accuracy of either a Twinmax, or Carbtune, or whatever.
Finally, Airheads and Oilheads will have a buzzy spot. Period. It is due to the fore-aft offset of the two pistons, rods, and counterweights. There will at some rpm always be a rocking couple imbalance. Frequently it is around 4,000 to 4,200 rpm. Perfect carb synchronization won't make it go away.
Last edited by PGlaves; 12-07-2012 at 02:12 PM.
Paul Glaves - "Big Bend", Texas U.S.A
"The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell