Trying to understand vacuum advance

[Roger Bray]

Hey Guys,
I've trying to trouble shoot some issues with my engine and I wondering if I have a problem with my distributor.

Please take a look at this video and see what you think.
Any help would be appreciated.

https://www.youtube.com/watch?v=v5GD_eqsY40

Thanks,
Roger

[olddog]

The gauge should be reading inches of mercury for vacuum. If it is reading psi or psig your most likely reading pressure. Are you sure you are pulling vacuum? You can put a finger on the hose to tell.

An engine would pull more like 19 inches of mercury ("Hg or in-Hg) at idle, unless the cam has a lot of overlap in the valve timing. You may not be pulling enough vacuum for it to move.

[Barnsnake]

Assuming there is only one hose connection on the diaphragm assembly:

If you are sure you are applying vacuum and it holds the vacuum, it should have moved the distributor trigger plate.
The logical assumption is that the diaphragm pot is seized.
If it doesn't hold the vacuum, the diaphragm is ruptured.

In either case, it needs to be replaced.

[Roger Bray]

I'm sure that I am applying vacuum. I does hold vacuum.
Looks like I'm off to the auto parts store tomorrow.

Thanks for your help

[RoyTheBoy]

OldDog,

For a quick test just suck on the vacuum tube and you should see the advance plate moving as you suck and release ;-)

Cheers

Roy

[Roger Bray]

Hey Guys,
Once again thanks for all of your help.
Anyway I went to the auto parts store today. I brought in my vacuum advance pot and the guy that works there confirmed that it was shot.
A new one would run me $25. However a new distributor is only $45 dollars and I'll get a $15 dollar core credit ... so I went with a new distributor. I figure the one in my car is 10yrs old and if the vac adv is shot something else might also start giving me trouble.
I've found a good video by MSD that explains the process so I'm going to study up and change it out tomorrow.

Thanks,
Roger

[Wbulk]

Roger, many of those hexagon vacuum canisters are adjustable. I have found that many are also not adjusted correctly. Stick a 3/32 allen wrench in the vacuum port of the canister and see if there is an allen screw to adjust it. Sometimes it takes a delicate touch. You want about 10-15 deg. advance out the the unit. You can drop the idle way down so it just barely runs and hook the canister to manifold vacuum while watching your timing marks with a timing light to see what it's set at. Or you can just use your vacuum pump to. Don't let the idle get to high or the centrifical advance starts coming in.

Wayne

[PeteF]

A new distributor for $45 sounds too good to be true. I would beware of it, especially of it is made in China, which many car parts now are. A failure or malfunction could strand you far from home. Compare prices to known good brands like Mallory or MSD. Good luck whatever you decide.

[Roger Bray]

Hey Guys,
Points well taken.
I've been looking at my current setup and I'm wondering if the play I have in the rotor is normal.

https://www.youtube.com/watch?v=FoS9s7TY89c

Thanks again,
Roger

[Barnsnake]

That rotational motion is the centrifugal advance.
If the shaft moves sideways it's an indication of wear.

[Wbulk]

Read this. It will help understand how it works.
Dist. Advance Re-curve

[Roger Bray]

Wayne,
Fantastic information. Thank you!

I followed your advice and fixed my vacuum advance pot.
I've documented on youtube for my future reference or for anyone of the same skill level as me that would like to see it.

https://www.youtube.com/watch?v=xrEcXskEVps

Thanks again for all of your help,
Roger

[HighPlainsDrifter]

Hi,
Factory distributors are set to higher vacuum readings for a stock cam. IE;
It will probably start advancing at 10 to 12 inches of vacuum and have maximum advance at 20 inches of vacuum. It could pull 20 to 30 degrees more advance under high vacuum.
Performance cams have low vacuum and should have a performance mechanical advance only and not use the vacuum advance.
Good luck,
Perry.