Testing factory vacuum advance

[TheLimeRanger]

So while not a very technical way to to test the function of the vacuum advance, I decided to use my mouth to see if it’s working. So with the distributor cap off I applied suction. The arm does not move at all. And I applied ALOT of suction. Should I assume this thing isn’t working?

 

[savage]

I've done it that way, it take a lot of vacuum to move it , but you should see a little wiggle. The best way is using a vacuum gun.

 

[TheLimeRanger]

I've done it that way, it take a lot of vacuum to move it , but you should see a little wiggle. The best way is using a vacuum gun.

I was just able to get it to move. So I guess that’s not the problem lol

 

[savage]

I was just able to get it to move. So I guess that’s not the problem lol

Does it run? If so ,if you have a timing light and your vacuum is hooked up to vacuum source. Unhook the vacuum line and plug it check timing with light, then hook back up and see if it advanced. Are you carbureted or fuel injected , only asking is each one uses different locations for vacuum.

 

[MarsChariot]

Here is a nice summary of how to test the vacuum advance semi-quantitatively. Otherwise, I use a vacuum hand pump to (1) see if it moves the link in the distrubutor, and (2) to see if it holds a vacuum. I recently had one go out because it was no longer holding vacuum.
http://grantorinosport.org/bubbaf250/perf/perf02c.html

 

[TheLimeRanger]

Does it run? If so ,if you have a timing light and your vacuum is hooked up to vacuum source. Unhook the vacuum line and plug it check timing with light, then hook back up and see if it advanced. Are you carbureted or fuel injected , only asking is each one uses different locations for vacuum.

She runs but has a hesitation when I give it gas. Almost a stumble. I stock 74 with duraspark. Can’t figure it out. Changed thr ignition module (old one puked it’s guts). Same problem persists. I hooked a timing light up to it and I’m at 8 degrees BTDC with the vacuum disconnected at idle.

 

[savage]

She runs but has a hesitation when I give it gas. Almost a stumble. I stock 74 with duraspark. Can’t figure it out. Changed thr ignition module (old one puked it’s guts). Same problem persists. I hooked a timing light up to it and I’m at 8 degrees BTDC with the vacuum disconnected at idle.

My 74 is set at 12 degrees BTDC. You said you disconcerted the vacuum hose did you plug it or is it above the throttle plate? I would move it to 10 and drive it and if it stops hesitating good, if its a little better try moving 1 degree at a time. One it starts to preignition back off till it stops. You didn't say if it was carbureted or fuel injected.

 

[Bronco-Brian]

You might be getting too much full advance timing as mechanical advance is in addition to the vacuum advance (under certain load conditions) - have you checked with a timing light? It takes 10-15 inch of vacuum to pull the vac advance - you cannot create that much with your mouth. There is also a spring inside the vac advance that controls movement vs vacuum. This can be adjusted with an 1/8" allen wrench thru the port. I've had a similar issue you are noting - mine was caused by damaged wires at the module - every time it moved with vacuum advance, the wire(s) would lose contact and engine would begin to miss until I let off the throttle and it returned to no advance position. These can be finicky to troubleshoot as more than one issue may be present. You will need to isolate the issue first. Try w/o vacuum advance hose first and check full mechanical timing at 3000rpm. Then connect the hose, and repeat. Anything past about 35 degrees total advance is going to cause hesitation or backfire.

The other high probability cause of hesitation (for carburetors) is accelerator pump on carb is not working. Be sure to look at that too.

 

[TheLimeRanger]

Thank you! I’ll be spending some quality diagnostic time tomorrow lol

 

[B RON CO]

Hi, X2 on Bronco Brian. These engines usually do better with more initial timing. @12* is usually good. As long as there is no pinging under load.
And make sure the accelerator pump starts to squirt as soon as the throttle starts to open. Engine warmed up and shut off, look for two nice pump shots down the center of the carb.
Good luck