87 4.2 vacuum advance - Off-Road Forums & Discussion Groups
Jeep-Short Wheelbase All discussion of short wheelbase Jeeps: CJ, TJ, YJ and JK

Reply
 
LinkBack Thread Tools Display Modes
post #1 of (permalink) Old 10-17-2002, 06:08 PM
bchiker
Guest
 
Posts: n/a
87 4.2 vacuum advance

I need some help! I've been working on my jeep, trying to get the thing to idle smoothly and maybe get some more power out of it. It's stock all but the pulse air system capped off or removed. I've got the carb running fine. But, here it come's my question. After I time it and rehook up the vacuum advance line. I see the vacuum diaphram pull and linkage under the distributor move. Shouldn't this happen at a higher R.P.M. them 650-700?
Sponsored Links
Advertisement
 
post #2 of (permalink) Old 10-17-2002, 06:12 PM
 
Join Date: Jun 2001
Posts: 1,705
Thanks: 0
Thanked 0 Times in 0 Posts
 
Re: 87 4.2 vacuum advance

Are you using ported or manifold vacuum for your advance? Make sure you use manifold vacuum.
AllLockedUp is offline  
post #3 of (permalink) Old 10-17-2002, 06:21 PM
bchiker
Guest
 
Posts: n/a
Re: 87 4.2 vacuum advance

I'm hooked up to the same place as my brake power boost. Into the manifold.
Sponsored Links
Advertisement
 
post #4 of (permalink) Old 10-17-2002, 06:28 PM
 
Join Date: Jun 2001
Posts: 1,705
Thanks: 0
Thanked 0 Times in 0 Posts
 
Re: 87 4.2 vacuum advance

Grrr...I meant to say you DON'T want manifold vacuum. See what a cold and a few Sam Adams does to my typing???? [img]images/graemlins/blush.gif[/img]

At idle, there is no vacuum at the carb, the vacuum hookups are above the throttle plates. Having it hooked to the manifold vacuum means that at idle, you're drawing vacuum and thus explains why your advance is moving at 700 or so RPMs.
AllLockedUp is offline  
post #5 of (permalink) Old 10-17-2002, 07:13 PM
bchiker
Guest
 
Posts: n/a
Re: 87 4.2 vacuum advance

That sounds great! But when I look at the drawing it's showing the line coming out of the manifold going to the dis., 10" sw, and the res. for FWD? I'm I reading it wrong?
post #6 of (permalink) Old 10-18-2002, 02:40 PM
bchiker
Guest
 
Posts: n/a
Re: 87 4.2 vacuum advance

Well I hooked it up to a port on the carb today. It works I guess? The linkage only moves maybe 3/8" while going to full throttle, is that right? Also I just plugged the port in the manifold above where the brake boost hooks up, should something be hooked up there? Thanks for the help.
post #7 of (permalink) Old 10-21-2002, 11:32 AM
bchiker
Guest
 
Posts: n/a
Re: 87 4.2 vacuum advance

Can someone please go look at theirs? How is hooked up? How far is moving? Thanks
post #8 of (permalink) Old 10-21-2002, 01:40 PM
 
Join Date: Dec 2001
Posts: 965
Thanks: 0
Thanked 0 Times in 0 Posts
 
Re: 87 4.2 vacuum advance

try this link

http://home.sprynet.com/~dale02/home.htm
I'm sorry I didn't respond earlier. My 89 YJ 4.2 is connected to manifold vacuum. I'm a little hesitant to tell you how it should be connected, cause mine don't run so great. But, mine came connected to the manifold vac. I tried it on the ported vac. It ran worse. The vacuum diagram under my hood shows the Vac advance to manifold vac. And the vac diagrams on dale’s site shows the same thing. ( FYI, I remember it was back wards on my old mustang.)

And as for your original question about the vacuum advance kicking in at a low RPM. The answer is yes. It is connected to the vacuum to respond with the throttle. The distributor also has a mechanical advance built inside with, weights and springs that activates with RPM.

speed_buggy is offline  
post #9 of (permalink) Old 10-21-2002, 08:09 PM
bchiker
Guest
 
Posts: n/a
Re: 87 4.2 vacuum advance

Call me hard headed if you'd like, but I still don't understand. At idle with the vacuum advance line hooked to the manifold my diaphrapm seems to pull the linkage all the way. Should it do that?
post #10 of (permalink) Old 10-21-2002, 09:44 PM
 
Join Date: Apr 2002
Posts: 333
Thanks: 0
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.
Iron_Weasel is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Off-Road Forums & Discussion Groups forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome