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Old 05-01-2011, 12:05 PM   #11
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Re: How to use vacuum gauge for FE?

Yes- the trans modulator vacuum fitting should be a good one to use for the distributor and for your gauge.

Perhaps that funky manifold/ported vacuum mixing valve bled off manifold vacuum through the line that went to the carb and since you rerouted it, the vacuum is no longer bled off.

This explains why your car is spark knocking under heavy load- the vacuum advance is present even when load is high.
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Old 05-01-2011, 03:42 PM   #12
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Re: How to use vacuum gauge for FE?

I'll have to get some fittings and hook that all up to the transmission modulator fitting.
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Old 05-01-2011, 04:20 PM   #13
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Re: How to use vacuum gauge for FE?

This is where it's currently hooked up. This thing has 3 nipples and a rubber boot that fits all 3, combines the outer two into one, and sends the middle one to the distributor (formerly, to the thingy that went to the distributor). Doesn't look like all the other vacuum check valves but could be.
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Old 05-01-2011, 09:06 PM   #14
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Re: How to use vacuum gauge for FE?

THey may be venting some air flow thought the lines to prevent fuel from backing up the line into the vacuum diagram on the distributor and moisture too. In my Geo there was a recall for the map sensor line to allow some air flow through it to prevent water from backing up and freezing in it.
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Old 05-02-2011, 03:29 AM   #15
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Re: How to use vacuum gauge for FE?

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Originally Posted by theholycow View Post
If it continues behaving with the high, non-fluctuating numbers, what would that mean? Something wrong with engine, or it's just a design that runs abnormally high vacuum?

Could I potentially have hooked up to the wrong vacuum source? Is there any vacuum source that would do that? It comes out of the intake manifold so it must be manifold vacuum, right? It's the same vacuum my distributor advance is hooked up to.
I run vacuum gauges connected to the distributor vacuum advance line on both my cars.
For me, vacuum reads zero at idle, rising to between 2/3 - 3/4 manifold vacuum at light engine loads, then falling as manifold vacuum falls (engine load increasing).
I find cruising at the highest vacuum reading I can hold on the freeway gives me my best FE. This is about -42 Kpa in the 1.8L Mitsubishi and about -60 Kpa in the 2.8L Datsun.
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Old 05-02-2011, 05:44 AM   #16
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Re: How to use vacuum gauge for FE?

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Originally Posted by D.O.G. View Post
I run vacuum gauges connected to the distributor vacuum advance line on both my cars.
For me, vacuum reads zero at idle, rising to between 2/3 - 3/4 manifold vacuum at light engine loads, then falling as manifold vacuum falls (engine load increasing).
Your distributor isn't hooked up to manifold vacuum? What is it hooked up to instead?
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Old 05-02-2011, 08:26 AM   #17
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Re: How to use vacuum gauge for FE?

Normally the distributor vacuum advance is connected to the throttle body at the butterfly so at ide there is very little or no vacuum so the timing is retarded at idie. Keeps it from stalling when cold and burns more gas for the rpm. Then as you apply throttle the vacuum advance gets some as the butterfly valve is opening.
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Old 05-02-2011, 04:37 PM   #18
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Re: How to use vacuum gauge for FE?

I think you're on the right track by looking for another vacuum source. I've seen one way valves that look like what you have pictured above. If it's not a one way valve I think you have something restricting the vacuum line going to the gauge.
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Old 05-02-2011, 07:08 PM   #19
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Re: How to use vacuum gauge for FE?

I don't think it's a restriction, it jumps up quickly it just doesn't go down quickly.
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Old 05-03-2011, 01:50 AM   #20
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Re: How to use vacuum gauge for FE?

[QUOTE=theholycow;160422]Your distributor isn't hooked up to manifold vacuum? What is it hooked up to instead?[/QUOTE]

Although I haven't seen an exploded diagram of this model Mikuni carby, I believe this line comes from a small port just above the throttle butterfly (air cleaner side).
I've heard that some vacuum advance actuators do run on manifold vacuum, but I don't know how your Buick is meant to be connected.
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