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Old 04-13-2009, 09:08 AM   #1 (permalink)
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Vacuum gauge: all or nothing!

Hey folks,

Sunday afternoon (after church, so don't flame me) I went outside and really quick stuck the new vacuum gauge on my truck. I plumbed its vacuum feed into the vacuum supply on the distributor, thinking that would be a good place for a vacuum; the vacuum from the engine tells the distro when to advance and when to back off.

Well, maybe not. It seems that the vacuum supply to the distro is really more of a switch. At any throttle opening at all, the vacuum bottoms out. Let off the gas to nothing and the vacuum goes all the way to 29+inches, which is great. But any - and I mean any throttle is just about too much. I can feather-foot it to get variable readings, but the openings are too small to be of any use.

I'm driving a carbureted 22R, an old one. Just puttering along at 35mph, barely off-idle in 5th gear, I'm still showing no vacuum. I'm no rocket scientist but I'm pretty smart, and I'm really, really sure that can't be right. I mean, no vacuum even when it's going slightly downhill? C'mon.

So I'm looking for suggestions and hopefully helpful pictures. I can spend an afternoon cutting and splicing vacuum lines - I've got one that's going to need splicing already - but I'm hoping someone can point me to exactly the right line to patch into. There's a rat's nest of vacuum tubes under there for all the emissions hardware (nothing like my even older, much-missed Civic, but still), and some do their things according to when various switched valves say to, etc. So, anyone want to weigh in on this?

I wanted to be a throttle-feathering fool on the way to work this morning, I was disappointed.

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Old 04-13-2009, 09:41 AM   #2 (permalink)
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Distributor vacuum advance uses vacuum produced by the carburetor venturi, not manifold vacuum.
(Carburetor Vacuum Ports: Manifold, Ported and Venturi Vacuum Explained.pdf)
You need to locate a vacuum port coming from the intake manifold.
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Old 04-23-2009, 08:57 PM   #3 (permalink)
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Okay, this afternoon right after work I plumbed into the line feeding the brake booster. I know people say don't mess with that line, but there's so many vacuum lines coming off the carb and manifold and I can't begin to be sure which is switched and which isn't. And Chilton's is no help.

So to heck with it. And WOOHOO I do love having that instant feedback. I'm getting great vibes on how this tankful's gonna play out.

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