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Measuring manifold vacuum

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General_Failure Use your head, don't abuse it!

1,036 posts · joined Jun 2004 ·

#640038
Hello all.

I have been measuring the inlet manifold vacuum on my type IV 1800 van from the middle of the balance tube (where the nonworking warm air switch vacuum connector should be). It seemed like the logical place to connect it as it provides fairly even feedback on all four cylinders.

I have two questions:
Is that a good place to connect it? Logically to me it seems perfect.

What is a standard vacuum for idling? I get about 10" Hg. or 250mm Hg. Whichever you prefer.
I have no idea what the vacuum is meant to be, but it seems a bit low to me.
If at first you don't succeed. Build, build again.
Vehicle: 1975 Special order delivery walkthrough panel based LCA pop-top camper. Motor: Nippon 1.8L Single port Wasserboxer, Transmission: 3 rib 002.

mnsKmobi Wolfsburg Wizard

589 posts · joined Aug 2002 ·

#640118
General_Failure wrote:Hello all.

I have been measuring the inlet manifold vacuum on my type IV 1800 van from the middle of the balance tube (where the nonworking warm air switch vacuum connector should be). It seemed like the logical place to connect it as it provides fairly even feedback on all four cylinders.

I have two questions:
Is that a good place to connect it? Logically to me it seems perfect.
Sounds like a good spot but not having a stock setup I can't be sure.
What is a standard vacuum for idling? I get about 10" Hg. or 250mm Hg. Whichever you prefer.
I have no idea what the vacuum is meant to be, but it seems a bit low to me.
Should be more like 20"

General_Failure Use your head, don't abuse it!

1,036 posts · joined Jun 2004 ·

#640420
That's what I thought. Weird. In all my attempts to get it running properly, the closer I got the lower the vacuum. The surging problem is practically solved.
However... it will not start without a lot of cranking and constant accelerator pumping when the idle throat inlet tube isn't connected. Very touchy. When it is connected it's a pretty easy start except the first start when cold.

I should say that the vacuum is relatively even. Ie. less than one gradation mark of variance between cylinders. Also compression is pretty stock on all four.

I'm also pretty sure the issue isn't with the throttle shaft holes as this issue didn't exist after it was pulled off the road a few years ago.

One of my suspects is something along the brake booster line. I'd be happy if I could replace that horrid black plastic tube.
If at first you don't succeed. Build, build again.
Vehicle: 1975 Special order delivery walkthrough panel based LCA pop-top camper. Motor: Nippon 1.8L Single port Wasserboxer, Transmission: 3 rib 002.

greedy53 Bishop of Volkswagenism

3,062 posts · joined Mar 2006 ·

#640453
i suspect the booster is leaking try it with the booster vac blocked off
👍👍😂🙅🥷😬🤓

General_Failure Use your head, don't abuse it!

1,036 posts · joined Jun 2004 ·

#640584
Unfortunately I ran out of time today and didn't get to do the test on the booster, but I did start the van up and wiggle some stuff.
The right hand rubber U boot on the manifold responded to prodding on the vacuum gauge so I guess I need a new boot too. The variance wasn't massive, maybe 1"Hg. There was also a bit off wobbliness on the throttlebar mount / autochoke etc. on the right hand side that needs investigating. However it had no effect on vacuum.

I did this quick check while I was attacking its ancient exhaust system with muffler putty. It'll need a new one for registration, but with all the extra whistles etc.from the leaks it made listening for vacuum leaks very difficult. It's a lot easier on the ear now.

There is one thing I noticed which I logically expected to be the case anyway, but I'll mention it here for reference.

If the manifold vacuum is being measured from the middle of the balance tube, a leak in one of the L boots will make itself known by an increased level of oscillation in manifold vacuum.
Of course there are other factors which can cause the same thing like unbalanced carburettors or a burnt valve, but a stuffed boot is one extra thing to check.

.
If at first you don't succeed. Build, build again.
Vehicle: 1975 Special order delivery walkthrough panel based LCA pop-top camper. Motor: Nippon 1.8L Single port Wasserboxer, Transmission: 3 rib 002.