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post #1 of (permalink) Old 04-19-2003, 04:39 PM Thread Starter
 
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vacuum advance question

Why does my 258 run better with my vacuum advance unplugged?
With the line disconnected, it doesn't misfire anymore...
Shouldn't it run worse?

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Tim
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post #2 of (permalink) Old 04-19-2003, 08:39 PM
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Re: vacuum advance question

When everything is set and working properly, the vacuum advance only gives additional advance at light to moderate throttle opening, which increases the fuel efficiency of the engine. With more throttle opening, the engine can't tolerate as much advance without pinging, but that's where you start to lose vacuum, and so you get less advance.

In the '60s some high-performance engines didn't have a vacuum advance on the theory that they were extra complexity on an engine where fuel economy wasn't an issue.

If you have the basic timing too far advanced, the vacuum coming in could be giving you too much total advance. That will raise the voltage requirement to fire the plugs. If everything in the ignition system isn't up to snuff, that can cause a misfire.

If everything else is in good condition and set properly, the vacuum advance will only increase your gas mileage - mainly the highway mileage.
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post #3 of (permalink) Old 04-19-2003, 10:18 PM Thread Starter
 
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Re: vacuum advance question

Thanks for the info. I had already thought about my timing being too far advanced, so it checked it, it was set at 9btdc. I set it to 6 btdc, which is what the my Chilton's book said. Still, with the advance hooked up, it runs rough between 1500-2500rpm. This is after I installed new plugs. My cap, rotor and wires are only a month old. Any other ideas how to get my rig running right?

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Tim
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post #4 of (permalink) Old 04-20-2003, 07:05 AM
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Re: vacuum advance question

Maybe the vacuum advance diaphragm is broken, so when you connect it, you get a bad leak causing lean mixture. That seems pretty far-fetched, but I can't think of anything else.

I don't think that it's possible mechanically to get too much advance out of it. If you can get a timing light with the dial-in advance feature you can see just how much advance you really are getting and compare that to the spec.
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