Join Date: Apr 2002
Thanked 3 Times in 3 Posts
Re: 87 4.2 vacuum advance
No, it shouldn't pull it all the way.
The vacuum advance should be hooked up to ported vacuum on the carburetor.
Ported is above the throttle blades, manifold is below the blades.
The throttle blades are the little brass/gold colored round things you see when you look down the throat of the carburetor.
Anyway, you should have little to no vacuum at idle with the vacuum advance hooked up to ported vacuum on the carburetor. As vacuum increases, signifying increasing load on the engine, the advance mechanism on the distributor should start pulling as more vacuum is applied.
You want no vacuum advance at idle, and more as engine speed increases.
Manifold works the opposite way...lots of vacuum at idle and drops off the higher the RPM's go. The distributor on these 258's has very little mechanical advance, which is why the vacuum advance is needed.
On my 90 YJ, at idle the vacuum advance mechanism is not pulled at all. As engine speed increases it does indeed get pulled as it's supposed to, but it only pulls a max of about 1/4" to 5/16". So the 3/8" you had posted earlier sounds pretty close.