[SOLVED]manometer vs gauge dials

[keratos]

I've not used a dial type (always suspected they'd give the sort of results you have), but since the vacuum fluctuates all types exhibit jitter to some degree.
Both mercury and rod-and-spring (Morgan) wobble noticeably, in fact the latter depends on the jitter to overcome stiction. You could, in theory, increase the damping of mercury ones to the point where the movement isn't visible, but it would make them slow to react to adjustments of the carbs. So far as accuracy is concerned I didn't find any problems with either type of manometer (set-up carbs, swapped connections... readings stayed the same).
I did make a (differential) water manometer for the job once when I was too much of cheapskate to buy a Carbtune... and found the carbs were sufficiently out of balance that the 5ft of head I'd designed for was not adequate. Fortunately I'd included restrictions (to cut down on jitter) and that reduced the rate at which the engine sucked the water in so there were no ill effects. :lol

Appreciate the insightful post. Useful info. thank you