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Unread 07-18-2005, 09:19 PM   #1
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Join Date: Jun 2005
Posts: 13
Vacuum Advance

I was setting the timing on my son's 90 YJ (4.2) today and noticed something peculiar. I set the timing to 9 degrees BTDC with the vacuum advance plugged. It idled really smooth...sounded better than it has in a while.

When I hooked up the vacuum advance, it started to pop and idle rough. I checked the timing with the dialback timing light and it appears that I've got about 42 degrees of timing with the vacuum advance connected. It's almost like the vacuum advance is pulling full open at idle. When you increase the rpms above idle speed, the timing advances slowly - I'm guessing that it's the mechanical advance operating.

I've got the vacuum advance connected to what I believe is manifold vacuum off of the fitting going to the power brakes. Does this sound right to anyone? Or possibly could I have a bad vacuum advance can?


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Unread 07-18-2005, 10:06 PM   #2
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2005 TJ Wrangler 
Join Date: Jun 2005
Location: Ellicott City Maryland
Posts: 102
I believe that the vacuum advance for the distributor should be coming from a "ported" vacuum port on the carb and not straight manifold vacuum. I'm not sure what a "ported" vacuum port does differently than manifold vacuum but that may be your problem(anyone know the diff?).
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Unread 07-19-2005, 07:31 AM   #3
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2008 JK Wrangler 
Join Date: Jul 2005
Location: Whitby, Ontario
Posts: 96
The factory set up for the 4.2 with Carter BBD carb ran the distributor vacuum advance off manifold vacuum. Try for the hose routing diagram.
If you want to try running off ported vacuum to see if it makes a difference there is a port on the Carter carb on the passenger side halfway up the carb near the valve cover. The hose connected there runs to the EGR valve, you'll need to put a tee into it to connect the distributor vacuum advance line.
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Unread 07-19-2005, 12:01 PM   #4
4x4 zeppelin
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Join Date: Jan 2005
Posts: 104
Well all I have to say Is check ALL vacume lines, some have cracks and will hinder the jeeps performance, I spent like 30 min looking over the vacume lines, I changed them and put the all in there right spot. But I do need to adjust my timing and lean out the carb. Keep us posted, and If I find out any more info I will let ya know
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