[Excaliber]

I think you mean "Accel" distributor? There is very little vacuum available when the throttle is pretty wide open, it is virtually zero vacuum at WOT (wide open throttle). A 'lumpy' cam profile is the main reason for a low vacuum signal. At idle it may well be 10" of vacuum or even less. Thats why some guys cannot use a Power Brake vacuum booster and are forced to go with some types of mechanic hydrualic power brake booster (often called 'juice brakes'). Vacuum advance is extremly difficult to 'feel' or 'sense', it comes and goes based on throttle position. While on MOST cars it will improve mpg during steady CRUISE rpm, you will not 'feel' this, you have to 'measure' it with careful mpg calculations.

IF your cruise rpm 'inches of vacuum' is BELOW the threshold of the distributors vacuum advance unit requirements, you will get ZERO benefit from it's use. Vacuum advance starts at 15" and your cruise vacuum signal is 10", equals no benefit. Of course when you CLOSE the throttle vacuum goes WAY HIGH, perhaps 20" and you get full advance. Which again has virtually no impact on mpg or performance when the throttle is closed.

SOME vacuum advance units can be adjusted for a lower signal AND 'total advance' they introduce, most cannot be adjusted. You need a good vacuum gauge to determine your cruise, idle, full throttle and closed throttle vacuum signal. Use a LONG hose and place the gauge on the windshield (use tape perhaps) or in the cockpit and note the readings. Compare the cruise vacuum signal to the specs on your Accel distributor to determine if your 'in the ball park'. Check to see if the Accel distributor can be modified for the vacuum signal strength required.