I just put a vaccum gauge on my motorcycle (1974 honda cb125) I drilled the intake manifold and put a hose nub in, pluged the hose going to the gauge in a few spots with filter foam to slow keep the gauge needle from going crazy.
it idles around 10 inches of mercury, and that is simply the highest I can get it, and normaly driving is around 3-4 inches, down hill throttle fully closed it jumps up to around 15-20" depending on the hill.
if I open it wide open, it drops down to 0-1".
so should I just aim for keeping at at 4" for best mileage?
I was also thinking that if I can find an affordable motor controler that hooking that up with a vaccum switch that kicks in around 1" to give it an extra boost of speed under acceleration might be fun.
I hardly look at it anymore since I kind of know where it is sitting at any given point. However, I keep it installed since it also acts as a diagnostic tool (and it looks kind of cool).
Vacuum at idle is largely dependent on ambient and engine temperature. When I first installed the gauge it was cooler outside and my first test was with the engine cold. On a hot day with the engine warm the gauge hovers around 19 in. Hg without a problem (17 to 22 is normal for cars - I have no clue what is normal for a bike).
So, overall I guess I'm trying to say that you can try to keep the needle at a certain point, but even slight throttle changes, inclines, declines, and rpm highly affect the reading.