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Inches of mercury manifold vacuum...?
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Tinsnips
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PostPosted: Wed Jun 16, 2021 2:59 pm    Post subject: Inches of mercury manifold vacuum...? Reply with quote

Stock 1600 DP.

Vacuum measured 34 pict 3 retard port (which I THINK is manifold vacuum).

How many in. Hg at idle?

How many in. Hg at 3000 rpm unloaded?

I know American cars will give 17 inches either scenario, but have no idea what a beetle will do. My beetle is giving me 11 and 14 respectively.
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PostPosted: Wed Jun 16, 2021 3:04 pm    Post subject: Re: Inches of mercury manifold vacuum...? Reply with quote

From the "running the engine' part of this: https://www.ratwell.com/technical/EngineInspection.html

Ratwell wrote:
Vacuum Gauge - this test indicates how well the engine is breathing. The pumping action of the pistons produces the vacuum but only if there is a restriction between the combustion chamber and the air intake. When the engine is idling the gauge should read 15-17" of vacuum with 7.5 BTDC timing and 12" with 5 ATDC (retard) timing at the specified idle rpm. If the idle rpm if very high but the vacuum level is normal, then you have a leak that the PO tried to mask. Although these numbers seem low they are normal for a VW engine with it's low compression ratio. Low readings are usually indicative of a vacuum leak somewhere in the system and the problem can usually be traced to a bad hose or gasket. You must fix this problem because the leaks occur downstream from the FI/carb and result in the air/fuel mixture leaning out as I explained earlier.

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PostPosted: Wed Jun 16, 2021 3:10 pm    Post subject: Re: Inches of mercury manifold vacuum...? Reply with quote

Tinsnips wrote:
My beetle is giving me 11 and 14 respectively.

That's just about right.

If you want to make sure... using a vacuum hand pump, apply vacuum to the distributor can retard port at idle while you monitor the ignition timing under a strobe timing light. As you increase the vacuum at the retard port the ignition timing will retard. Measure how much vacuum is need to reach max retard. As long as your carb produces at least this much vacuum at idle (11in-Hg?) you are getting the max vacuum retard from the distributor.
You can also look up the model of your distributor here and confirm if the amount of retard is within the spec for that model distributor. Different models produced different amounts of ignition retard.
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PostPosted: Wed Jun 16, 2021 6:36 pm    Post subject: Re: Inches of mercury manifold vacuum...? Reply with quote

I’ll do the above prescribed vacuum can test soon as I can. But I already do know my DVDA dizzy behaves thusly; with initial timing at tdc, mechanical maxes out at 30 degrees, ported vacuum adds another 12 degrees for 42 total unloaded revved up degrees of advance....yet the manifold vac port doesn’t move (retard) the timing at all. Odd.
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PostPosted: Wed Jun 16, 2021 6:45 pm    Post subject: Re: Inches of mercury manifold vacuum...? Reply with quote

Hmm. You might want to put a mighty vac on each of the distributor's vacuum ports and make sure they both A) hold vacuum, and B) move the points plate.
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PostPosted: Wed Jun 16, 2021 6:50 pm    Post subject: Re: Inches of mercury manifold vacuum...? Reply with quote

Stock 1600DP 34PICT3 Bosch 034 - manifold vacuum after tuning carb per Bentley at 7.5BTDC and 900RPM, with valve lash at .006


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PostPosted: Wed Jun 16, 2021 9:35 pm    Post subject: Re: Inches of mercury manifold vacuum...? Reply with quote

ashman40 wrote:
Tinsnips wrote:
My beetle is giving me 11 and 14 respectively.

That's just about right.


Wut!? Please explain, then, why my same engine produces 17-18 in.Hg. at idle, and revving it makes the vacuum go
way down, not up.
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PostPosted: Thu Jun 17, 2021 12:06 am    Post subject: Re: Inches of mercury manifold vacuum...? Reply with quote

Tinsnips wrote:
I’ll do the above prescribed vacuum can test soon as I can. But I already do know my DVDA dizzy behaves thusly; with initial timing at tdc, mechanical maxes out at 30 degrees, ported vacuum adds another 12 degrees for 42 total unloaded revved up degrees of advance....yet the manifold vac port doesn’t move (retard) the timing at all. Odd.

It is common mistake to try measuring the different types one of the distributor's advance/retard system while multiple systems are connected. This gives a incorrect view of the advance/retard.
For example, you cannot accurately measure the amount of mechanical advance while the vacuum advance is connected. They work to complement each other and the vacuum advance is additive. To measure one advance/retard system you must do everything you can to isolate that system and measure its advance without influence from the others.

So when you say your mechanical advance maxes out at 30deg... was this measured with both vacuum advance and vacuum retard hoses disconnected and plugged, right? Doing this disables these subsystems and leaves you with just the initial timing + mechanical advance. 30deg of mechanical advance is a bit high but not unheard of on some distributors.

Also, when you say "initial timing at tdc" do you mean idle timing with hoses connected; static timing or idle timing with hoses disconnected? Typically, idle timing below 1000rpm with hoses disconnected should be the same as static timing. The mechanical advance does not begin until 1000-1200rpm. So idle timing below 1000rpm with vacuum hoses disconnected should be the same as static timing.

When you connect/disconnect the vacuum retard hose at idle you should see a significant change in the idle timing under the strobe timing light. Connecting the retard hose should cause the timing to retard by 8-12deg. This means with the vacuum retard hose connected and timing set to TDC... disconnecting the hose should result in the timing advancing to 8BTDC. If from here the mechanical advance adds another 22deg taking you to 30BTDC then your mechanical advance is working about normally (depending on your distributor model).

Even using a vacuum pump to manually advance the vacuum advance could cause the rpms to increase due to the increased timing advance. Once the rpms increase above 1000rpm you may be moving into the area where mechanical advance starts to add to the ignition timing. How will you know ho much of that additional timing advance is coming from mechanical or vacuum advance? You really can't unless you carefully map when the mechanical advance starts to come in and discount that amount based on the engine rpm.
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PostPosted: Thu Jun 17, 2021 9:24 am    Post subject: Re: Inches of mercury manifold vacuum...? Reply with quote

Here you go!

I like having this around to remind myself what the measured value means!

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PostPosted: Thu Jun 17, 2021 10:24 am    Post subject: Re: Inches of mercury manifold vacuum...? Reply with quote

Tinsnips wrote:
Stock 1600 DP.

Vacuum measured 34 pict 3 retard port (which I THINK is manifold vacuum).

How many in. Hg at idle?

How many in. Hg at 3000 rpm unloaded?

I know American cars will give 17 inches either scenario, but have no idea what a beetle will do. My beetle is giving me 11 and 14 respectively.

Normal vacuum readings as noted should be between 17 and 21 inches at sea level, and the vacuum reading should be pretty much the same both when holding constant revs at idle and at 3000rpm. For folks that live above sea level, the normal reading will be reduced by roughly one inch per 1000 feet in elevation.

For your situation where both readings are lower than normal and the reading is increased when held at higher revs, I would suspect an intake manifold vacuum leak, retarded ignition timing, or worn rings. A compression check with low readings would help point toward the rings as the cause. Be sure to recheck/readjust all your basic tuneup settings before making a determination.
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PostPosted: Thu Jun 17, 2021 2:48 pm    Post subject: Re: Inches of mercury manifold vacuum...? Reply with quote

kreemoweet wrote:
ashman40 wrote:
Tinsnips wrote:
My beetle is giving me 11 and 14 respectively.

That's just about right.


Wut!? Please explain, then, why my same engine produces 17-18 in.Hg. at idle, and revving it makes the vacuum go
way down, not up.

Just to clarify… much of the feedback here is about measured intake vacuum (at the intake manifold) but the OP’s comments were about the vacuum port on the carb where the vacuum retard is connected (upper left rear-facing port). I’m not certain this port is exactly the same as an actual intake manifold port? The distributor vacuum retard is designed to retard ignition timing at idle to increase the heat in the exhaust manifold to reduce emissions. Above idle, I’m not certain what the vacuum at the carb port is doing. More importantly, what the vacuum retard is doing above idle. I always believed the vacuum advance circuit in the distributor would overpower the vacuum retard circuit to disable it?

Intake vacuum is very strong (high vacuum) at idle. This is because the cylinders are drawing air-fuel from the intake manifold but the carb throttle butterfly is pretty much closed; restricting the flow of air. This creates high vacuum in the intake manifold.
Imagine lightly sucking on one end of a hose while covering the other end with your finger leaving a small crack for air to flow into the hose. You will feel resistance as you suck and high vacuum will form inside the hose because your finger is restricting the flow of air into the hose.

From there, imagine stepping the throttle pedal down to the floor (wide open throttle : WOT)… the intake now has a large opening open to air (atmospheric pressure) the vacuum in the intake will drop off to zero. This happens anytime the throttle is opened (acceleration) but to a lesser amount.
Imagine you remove your finger from end of the hose. Without the restriction you can draw in as much air into the hose as you want but can’t create vacuum in the hose. Depending how much you restrict the air flow and how much air you are trying to draw thru the hose you may have some vacuum or no vacuum.

Lastly, when you are running at higher rpms with the throttle plate mostly closed (cruising) the vacuum goes high again. The flow of air-fuel past the throttle plate together with the high rpm/high volume draw from the cylinders makes for high vacuum. Here the vacuum is created by the high volume of air (high rpms) being pulled thru the intake and some restriction to the air flow (mostly closed throttle plate). This high vacuum also indicates low engine load.

From the above you can see why intake vacuum is used as a measure of engine load.
    Low intake vacuum = engine under load
    High intake vacuum = engine not loaded

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Tinsnips
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PostPosted: Sat Jun 19, 2021 4:13 pm    Post subject: Re: Inches of mercury manifold vacuum...? Reply with quote

Ash,

The timing statistics I gave are correct. I segregated the influencing factors as required (as you described).

This engine is new. 120 pounds at each cylinder. No vacuum leak discovered by visual inspection or liberal berkbile 2+2 dousing at all the connections.

Performance is terrible. Weaker than granny. Poor, variable idle. Has a Ching Chang carb (that it came with). A Volksbitz is on the way. (Will be weeks from now).

What do y’all think of this; if it was a vacuum leak, the vacuum (inches Hg) would increase to the 17+ range once revved up (unloaded) as the throttle flow to leak flow ratio would increase drastically. Am I right?
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Tinsnips
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PostPosted: Sat Jun 19, 2021 4:41 pm    Post subject: Re: Inches of mercury manifold vacuum...? Reply with quote

Holy crap! Ground spiking at the distributor cap has just revealed that cylinder number one is contributing nothing!

Stay tuned…
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PostPosted: Sat Jun 19, 2021 5:17 pm    Post subject: Re: Inches of mercury manifold vacuum...? Reply with quote

That will cause a vacuum problem, and a major drivability issue.
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PostPosted: Sat Jun 19, 2021 5:33 pm    Post subject: Re: Inches of mercury manifold vacuum...? Reply with quote

Tinsnips wrote:
....yet the manifold vac port doesn’t move (retard) the timing at all.

The question is whether the carb produces enough vacuum to fully retard the distributor? Or is the distributor retard system broken?

Did you test the vacuum produced by the rear facing retard port on the carb and compare it with how much vacuum is needed for your vacuum retard to reach max retard? The later is tested with vacuum advance disconnected and using a hand vacuum pump to apply vacuum manually to the vacuum retard on the distributor can. Watch the timing change at idle as you manually increase vacuum to the distributor vacuum retard. As long as the carb port produces enough vacuum at idle to fully retard the distributor you are good. If the vacuum only partially retards the distributor or the distributor does not retard at all... you have a problem.


Tinsnips wrote:
Holy crap! Ground spiking at the distributor cap has just revealed that cylinder number one is contributing nothing!

Check if the sparks are making it to the end of the #1 plug wire. Better yet, remove the #1 spark plug and while grounding the body of the spark plug confirm the spark is passing thru the plug wire and the spark plug and jumping the electrode gap.
Another way to check this is to remove a spark plug wire from the distributor cap and listen for a change in the engine rpms as your disable one of the cylinders. Do this test at idle as well as at 2000rpms. Sometimes cylinders do not seem to be firing at idle but respond at higher rpms. This is still a problem but the firing at higher rpms shows the cylinder is actually working.
If a single cylinder is not working, look for cylinder specific issues like a bad/fouled spark plug; bad plug wire; etc.
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PostPosted: Fri Jun 25, 2021 6:41 pm    Post subject: Re: Inches of mercury manifold vacuum...? Reply with quote

Cause of # 1’s absence: the spark plug wire had backed off. The terminal at the plug end of the wire had an ID greater than .250… the plug post measures .250, so I crimped all four wire ends to .234….now the terminal has a firmer grip on the little screw-on barrel on the spark plug. Will keep an eye on it.

Regarding the dizzy: the vac can leaks on the retard side, but not on the advance side. So I plugged the retard/manifold port at the carb. Idling better now, but not as good as it should. Was getting 11 inches Hg at idle, now with all four jugs firing I get 13. Not nearly as good as Otto’s 19. But to answer my own question (kinda), when I rev to about 3000, I get 17 inches Hg…

More weirdness: even if the retard diaphragm didn’t leak, the retard still wouldn’t happen. Pop the cap, pull the rotor, and try to rotate the breaker plate with direct rotational force ( not vacuum), and the breaker plate only can rotate in the advance direction, not the retard… as if the relaxed/neutral position for the can is running up against a hard stop that won’t let it rotate in the retard direction past the neutral ( no vacuum on either side of can) position. To be investigated further…
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PostPosted: Sat Jun 26, 2021 4:31 pm    Post subject: Re: Inches of mercury manifold vacuum...? Reply with quote

Quick read and i don’t see where you have ID the actual model# of your distributor. Please post this.
Any chance the PO installed a DVDA vacuum can on a SVDA distributor? It wouldn’t be the strangest thing I have heard of.

You can look up the specs for your distributor model# here:
http://www.type2.com/~keen/ignition.html
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