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post #1 of (permalink) Old 03-05-2001, 08:35 PM
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Need help understanding vacuum advance

I need to get a better understanding of how the distributor vacuum advance works. I have been trying to get my timing set correctly and the advance seems to be giving me problems.

When I disconnect the vacuum advance line from the distributor the idle drops by 400 RPM. I use a golf tee to plug the lines and the idle does not go back up. I believe it should go up at least some. I then flip my high speed idle to the highest setting and adjust the idle screw to get the jeep to idle at 1600 RPM. I loosen the distributor bolt and adjust the timing. I currently have it set at 4 degrees BTDC. When I reconnect the vacuum advance line, my timing goes to 7 degrees BTDC.

Does all of this sound correct? If not what is correct? Is my vacuum advance stuck open when it should be closing at some point? If it were stuck open, how would I correct the problem? What should I look at?

Thanks,


The Missouri Mule
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post #2 of (permalink) Old 03-05-2001, 11:30 PM
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Re: Need help understanding vacuum advance

To the top.

The Missouri Mule
post #3 of (permalink) Old 03-05-2001, 11:41 PM
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Re: Need help understanding vacuum advance

Someone else was setting theirs at 1600, I think it was wrong info. It's usually around 600-800 , is the emission/tune-up sticker gone ? Thats the best starting point.

If it comes with instructions, then it's not what the pros use.
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post #4 of (permalink) Old 03-06-2001, 12:01 AM
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Re: Need help understanding vacuum advance

My sticker is there and states to set the timing at 1600 RPM. Set at 6 degree's +/- 2 degree's BTDC. I have the TR upgrade and am trying to get rid of pinging at high speeds.

The Missouri Mule
post #5 of (permalink) Old 03-06-2001, 12:07 AM
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Re: Need help understanding vacuum advance

How far is it advancing ? Pinging all the time at high speeds ,or just when accelerating ? I'm a chevy guy, I don't know what the upgrade does, but I'd check the total advance against the specs.

If it comes with instructions, then it's not what the pros use.
post #6 of (permalink) Old 03-06-2001, 07:10 AM
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Re: Need help understanding vacuum advance

I think there is some confusion on how to set timing on a 258. Below is a copy of a post from the AMC list for an 83. The alternative method is what you would use on a non- computer controlled unit (obviously makeing allowances for what you have hooked up). If you have no sticker, no computer, and no emmissions, (a vac line from ported to dist and nothing else) warm it up, set the low idle to whatever you use, remove and plug the vacuum at the dist., set timing between 4-8 BTDC (stock, I use 9 with the TR), and drive, checking for pinging. You can see how the difference in models, lack ior addition of computer, emmisions etc can cause confusion, strange results and conflicting advice.

From the AMC-list
MShumaker
Matt in AZ.

Yesterday, I offered to scan the appropriate 1983 TSM pages for the
peculiar method of setting the timing on the 1983 (and probably later)
258 engines. I got enough requests to warrant posting it. (Besides, I'll
be away for a while after surgery in the morning.) Rather than scan it,
the text that follows is transcribed from the 1983 TSM Supplement. Note
that the text refers to some supproting devices in the engine management
system, like the 4 inch Hg switch.


The TSM Supplement text:
Ignition timing -- Six Cylinder Engine
Primary Timing Procedure
1. Set parking brake. Shift automatic transmission to Park, manual
transmission to Neutral.
2. Start engine and allow to attain normal operating temperature. Ensure
that A/C is turned Off (if equipped).
3. With ignition switch off, connect ignition timing light and properly
calibrated tachometer.

NOTE: If the timing light has an adjustable advance control feature,
turn the control to the OFF position.

4. Disconnect vacuum switch assembly wire connector.
5. Disconnect and plug distributor vacuum advance hose.

WARNING: Use extreme caution when engine is operating. Do not stand in
direct line with fan. Do not put hands near pulleys, belts, or fan. Do
not wear loose clothing.

6. Start engine.
7. Increase engine speed to 1600 rpm and check timing. If necessary,
adjust timing to specification listed on emission control information label.
8. Tighten distributor holddown clamp and verify that ignition timing is correct.
9. Turn off engine and remove timing light and tachometer. Connect No. 1
spark plug wire, if disconnected. Connect hose to distributor vacuum
advance mechanism. Connect wire connector to vacuum switch assembly.

Alternate Timing Procedure

This procedure does not require the engine speed to be increased to 1600 rpm.
1. Set parking brake. Shift automatic transmission to Park, manual
transmission to Neutral.
2. Start engine and allow to attain normal operating temperature.
3. With ignition switch off, connect ignition timing light and properly
calibrated tachometer.
NOTE: If the timing light has an adjustable advance control feature,
turn the control to the OFF position.
4. Disconnect the 4 in. Hg vacuum switch hose and plug opening.

NOTE: The 4 in. Hg vacuum switch has black and red wires connected to it.

5. Disconnect distributor vacuum hose and connect hose to 4 in. Hg
vacuum switch.
6. Disconnect knock sensor wire connector and connect wire to cylinder
block (ground) with jumper wire.

NOTE: Grounding the knock sensor wire prevents electromagnetic
interference (EMI) from causing erroneous reactions by the
microprocessor unit (MCU).

7. Start engine.
WARNING: Use extreme caution when engine is operating. Do not stand in
direct line with fan. Do not put hands near pulleys, belts, or fan. Do
not wear loose clothing.
8. With engine at idle speed check timing. If necessary adjust timing
one (1) degree higher than specification listed on emission control
information label.

NOTE: With the alternate timing procedure the basic timing must be one
(1) engine degree higher than the specification listed on the emission
control information label. For example, if the timing specification is
listed as 6 deg. +/- 2 deg. at 1600 rpm, the alternate timing procedure
requires 7 deg. +/- 2 deg. at idle speed.

+++++++++++++++++++

As well this link is to a jpg of the Jeep Service Manual page where you can see the first method documented (Its a J-10 so make allowances):
http://www.wagoneers.com/XJ/tech/FSJ...8-vac-diag.jpg

Lots of stuff here if you browes around :-)


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