Vacuum advance on 80 special

[justinc1081]

the vac advance unit will move if I slowly rev motor, but it doesn't move at all if I blip the throttle, or keep it at higher revs above 3k.
Does this thing even need to be hooked up? Been battling #2 carb rich problems and 10 mpg. Wondering if maybe this vac advance is not working correctly and might be part of the problem.
Also my harbor freight multimeter really doesn't like it when engine is running lol
Tried to check voltage on battey while running to see if charging was working now, with leads of MM off everything the volts were jumping all over the place

[MPittma100]

Vacuum Advance

the vac advance unit will move if I slowly rev motor, but it doesn't move at all if I blip the throttle, or keep it at higher revs above 3k.
Does this thing even need to be hooked up? Been battling #2 carb rich problems and 10 mpg. Wondering if maybe this vac advance is not working correctly and might be part of the problem.
Also my harbor freight multimeter really doesn't like it when engine is running lol
Tried to check voltage on battey while running to see if charging was working now, with leads of MM off everything the volts were jumping all over the place

The vacuum advance works off of "ported" vacuum - meaning when the throttle is open. No vacuum applied at idle. And YES it needs to be hooked up. When you check your timing with a light, the vacuum advance is part of the total timing curve. Holding the throttle just above idle you will see it move to full advance. You can also check it with a vacuum source.

Verify that the mechanical advance is working. Either of these can be a contributor to fuel mileage, but the rich condition in only one cylinder is likely a carburetor problem.

[justinc1081]

Alright, figured the carbs were coming off again anyway, advance works with slow throttle application but rapid throttle movement it doesn't move at all

[jetmechmarty]

Test the advance pot for a leaking diaphragm. It should hold vacuum.

[TopCatGr58]

Alright, figured the carbs were coming off again anyway, advance works with slow throttle application but rapid throttle movement it doesn't move at all

Justin,

As stated earlier, the Vac. Adv doesn't kick in much with almost ANY kind of throttle input especially with NO LOAD on the engine. The ADV unit is mostly to provide additional adv. during low load cruising speed/rpm with minimal throttle input...the butterflies barely open...then vacuum is supplied/sent to the adv. unit, and it kicks in to provide more adv. to take advantage of the lean fuel to allow MORE time to burn it during the cycle to provide more fuel economy. But once you open the throttle up...vac. levels DROP and the vac. adv. releases=retards back to the lesser level of timing advance of ~35 degrees which is the MAX adv. needed/desired to attain the best POWER timing setting!!

T.C.

[MPittma100]

Vacuum advance

Alright, figured the carbs were coming off again anyway, advance works with slow throttle application but rapid throttle movement it doesn't move at all

That is normal. Hold the throttle at 1500-2000 RPM. Vacuum advance should advance fully and stay there.

[justinc1081]

Ah ok, well that makes sense so it's working perfectly then, slow throttle and it moves with throttle, more juice and it goes back to doing nothing
Thanks guys!