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post #1 of 4 Old 04-04-2005, 05:57 PM Thread Starter
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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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post #2 of 4 Old 04-04-2005, 06:59 PM
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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.

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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post #3 of 4 Old 04-05-2005, 07:48 AM Thread Starter
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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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post #4 of 4 Old 04-05-2005, 09:19 AM
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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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