Vacuum advance problem

[yellow85cj]

Fresh rebuild AMC 258 i6 / 4.2l w/4.0 head.
Timed at 8*. Idle 650 RPM. Vacuum at 19 -20. Runs fine. Distributor is a reman'd.

First time the engine was started was fine till vac adv was attached. Then it idle'd way up missed and ping'd. Thought that it may be the vacuum on the dist. Couldn't seem to adjust it so swapped it for one from an old dist. That old one seems to slow down the idle and cause missing and rough running, then stalls.

Thought the vac advance would just cause the engine to idle up a 100 or so.

Don't have a clue as to where to start looking for a solution. Any suggestions?

 

[gert]

my memory is getting a bit fuzzy but bear with me..... 2 types of vacuum ports on a carb, manifold and ported. sounds like you are on the manifold which gives full time vacuum, you need to be on the ported vacuum which only sends vacuum to your vacuum advance when you are at part throttle. also my old mopars had a tinny Allen head screw in the vacuum advance so you could adjust your total advance up or down depending on what you need. Put your old vacuum advance back on and switch vacuum ports on your carb it should run much better.


Let us know how it goes

 

[gert]

on second thought, test both vacuum advances for leaks by pushing the arm all the way in and plug the port with the tip of your finger the arm should not move, if it does the diaphragm has a hole in it and its trash. I would use the one thats good.

 

[yellow85cj]

Ported or manifold, same thing.

And Gert, you lost me. The vacuum sucks, causing the arm to move and advance the timing. If the vacuum had a hole, would it not do anything to the timing? Be like an open vacuum port.

 

[elwood blues]

Read through this post;

https://www.jeep-cj.com/forums/f2/vacuum-advance-adjustment-distributor-23788/


There's some very good info and it may help you dial your problem in.

 

[mtnwhlr]

Ported or manifold, same thing.

Wrong, watch this video I hope it helps.
https://www.google.com/url?sa=t&rct...=O0OIVchIRT0&usg=AOvVaw02fgwFoc4l3SAfUSk_-08L

 

[yellow85cj]

Wrong, watch this video I hope it helps.
https://www.google.com/url?sa=t&rct...=O0OIVchIRT0&usg=AOvVaw02fgwFoc4l3SAfUSk_-08L

I understand the difference, the same thing happens with either ported or manifold vac.

 

[Lumpy Grits]

Not sure if this will help-
Ignition and Timing

LG

 

[slomo]

Ported comes off the carb base plate. Sounds like you are running full manifold vacuum. Holley recommends running ported only.

slomo

 

[thundley57]

I understand the difference, the same thing happens with either ported or manifold vac.

If the same thing is happening when you're plugged into manifold or what you think is ported vacuum then you aren't truly plugging into ported vac. You should have zero vacuum from ported vacuum while at idle. I run ported vacuum with the vac advance adjustment screw turned all the way out which in my particular case starts moving the vac advance at 8" and is full in at 13". I also have a vac gauge installed in the cab plugged into ported vac.

 

[yellow85cj]

Ok, so I'm thinking that the 2 ports at the base of the carb ( drivers side, one front, one rear) are ported vac, the port on the PB line is manifold.

Not so. Thanks to all that were trying to tell me this. Front port at the base of the carb is ported, rear is manifold.

Hooked up to the ported and works pretty well. After futzing with the mixture settings runs better.

So what I've learned about ported and manifold vac. At least I hope this is the way it goes. Manifold vac is high when at idle, increases in rpm, decreases vac.
Ported vac is 0 at idle increases with rpm.

So this begs the question, if people are running manifold vac (high at idle), how? Hook up to distributor and it immediately advances timing. What am I missing?