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Unread 04-04-2005, 06:57 PM   #1
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Join Date: Sep 2004
Posts: 158
Vacuum Advance

What's the difference between ported and manifold vacuum? Which do I use with a 'nutter bypass' and which would I use as a stock setup in 1984? How many degrees timing advance at what vac. pressure and what RPM should I run?

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Unread 04-04-2005, 07:59 PM   #2
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1986 CJ7 
Join Date: Jun 2004
Location: Monroe, WA
Posts: 1,595
The difference is ported vacuum is above the throttle plates, is zero at idle, will increase as the throttle is opened (to a max of about 12inHg) and will slowly return to near zero when wide open throttle.

Manifold is below the throttle plate, is a max at idle and will drop with the throttle opening. For 99% of vehicles, ported is prefered but feel free to experiment.

Start the timing at about 8-10* BTDC. Engine vacuum can vary from person to person, but at idle should pull about 20 inHg (steady reading). Since it is Nuttered, once the vacuum advance hose is removed from the distributor, it won't make much difference in RPM's as the mechanical advance won't kick in until about 1600 RPM's. You can also time it using engine vacuum, adjust dizzy until highest vacuum reading is obtained, then back off 1-2 inches. Mine pings when I go that route but yours might be different.
1986 CJ7, 4.2 w/4.0 head, TFI-HEI hybrid ignition, Clifford manifold w/Holley 390 w/cold air intake, OBA, 4.5" lift, Woody CV shaft and Tattons in front, 4.10 gears - lunchbox in front, Truetrac in the back, twin-sticked, blower upgrade for running topless, trying to keep it simple.

It's just a Jeep, and if you don't wheel it once in a while, it's not even that.
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Unread 04-05-2005, 08:48 AM   #3
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Join Date: Sep 2004
Posts: 158
Is the 8-10 BTDC when the engine is off and I point the rotor tip at cylinder #1, or is it when the motor is running and I use a timing light? If it is when the motor is off, what is the advance when the motor is running w/ and w/o vacuum?
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Unread 04-05-2005, 10:19 AM   #4
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1971 CJ5 
Join Date: Jan 2002
Location: Eagle Point, Oregon
Posts: 6,750
1) Start it. 2) Warm it up. 3) Check RPM's, needs to be below 700. 4) Unplug vac adv. 5) Time it. 6) Reconnect vac adv 7) Drive. 8) Readjust as nessary.
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