vacuum advance

[newb70f250camper]

trying to figure out if my vaccuum advance on distributor is bad or if there is no vacuum for it. is it possible that the vacuum port from carb is plugged? how do i check

[fordman]

suck on the hose. do it both ways. with it hooked to the carb only and with it hooked to the advance only. it is hard to suck if that advance is stuck or bad i think. or maybe its the other way around. when hooked to the carb it should be ported vacuum. which means it will only be easy to suck on if that port is open to let air go through the hose.

[69rangerman]

You have to really be a sucker to get that dizzy plate to move.

If you have a timing light and a tachometer you can slowly increase the rpm to 1500. As you move past 1500 into the 2000 and higher range you should see one of two things:

1. The timing mark will either stop moving at around 20 btdc no matter how fast the engine is going (the vac advance is not working or not hooked up)

2. The timing mark will continue advancing up to the limit on the balancer (35? BTDC and then slow and stop), this usually happens at 2500-3000 rpm.
This indicates that the vac advance is working and has "taken over" from the mechanical advance which is usually 15-20btdc.

I know it sounds like alot of trouble, but it's pretty easy and very noticeable as you watch the timing mark while increasing the engine speed. Mine sort of takes a "jump" as it hits 20 btdc and then ends up north of the 35 mark.

Good luck

[fordman]

yea i couldn't get a good one to work by sucking on it so i took it off and i was able to get it to move the rod.

[fastEdsel]

I chased one of these for a long time. The engine hesitated every time I went to accelerate, backfired and was just plain annoying to the point I started to drive my Chevrolet again. I knew I had a vacuum leak but just didn't think it could be the distributor because it was supposed to be new/rebuilt. I thought it was good to go. Wrong! The diaphram was leaking vacuum and I found it by blowing on the line to the dizzy. It blew as easy to the dizzy as it did to the carb. I took the dizzy back to NAPA who gave me 100% warranty, I installed a Pertronix Igniter and parked my Chev. I did try the timing light trick but it was all over the place, of course. Now it runs great, the timing light is right on and vacuum is right on the money.

[DuckRyder]

69 rangerman is on the right track.

Aditonally, you can hold the engine at a steady speed around "cruise RPM" (IE: 2500 or so) and then disconnect the vacumme advance; obviously you should see the timing drop.

[willowbilly3]

I don't use it often but I find a hand held vacuum pump and gauge to be indisposable for this trouble shooting. Then you can get the dist. spec from a good manual (I use an old Motors) and dial in the vac advance to factory spec.