need more suck

[cecil]

running dual 40 hpmx's with svda, Not getting a lot of vacuum at the dist. About 5" at most. Canister works.Was suggested I put a couple vacuum ports in my manifolds. Question is:which end, carb or head? and how huch vacuum should I expect.
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I may not be that funny or athletic or good looking or smart or talented....I forgot where I was going with this
I have this wierd desease called "must tear everything up and put it back together when there's nothing wrong with it...Itus."

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Get

http://vwparts.aircooled.net/Anti-Pulse-Valve-for-Vacuum-Advance-Distributors-p/anti-pulse-valve.htm

If that doesn't solve it, your engine has an anemic compression ratio, and that's why the vacuum is horrible. Either the rings are shot and the compression is low, or the compression is too low for the cam you are using. This is a very common problem with guys that just bolt the heads on, and don't do the CR math before hand to make sure it's right.

This one mistake will make pretty much any engine a total pig.
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[Alstrup]

What John said. BUT, I have experienced a couple of HPMXés where the vacum port was not functioning too well. I had to put a small drill through the vacum hole to clear it at the barrel end. I kinda forgot the size, but 2,2 mm comes to mind. You should check before attemping this. When I did this the vacum signal came to life.

T

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I have also seen a clogged port, but this is so simple you'd think that people would blow it out first thing.
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[mark tucker]

timing and valve adjustment can also contribute to low vacume.as can dry roted hoses and carbs/engine not tuned good.

[buggnuttz]

Vacuum advance works with ported vacuum not manifold
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owned 39 different VWs so far

[cecil]

well, lets go back to plan A. Did the math ( was a math teacher at one time) 7.5cr, 2056cc type 4, 86 cam, blew air thru vacuum passages. Changed to 32 vents,from 28. 135 jets. Did not check vacuum on each carb but both together is 5psig.
In the hospital right now with pneumonia (what fun) but plan on correcting this problem when I get out.
What size should the vacuum port be drilled to, by the way?
_________________
I may not be that funny or athletic or good looking or smart or talented....I forgot where I was going with this
I have this wierd desease called "must tear everything up and put it back together when there's nothing wrong with it...Itus."

[ps2375]

Vacuum is not measured in psi.

[cecil]

how about HgM
_________________
I may not be that funny or athletic or good looking or smart or talented....I forgot where I was going with this
I have this wierd desease called "must tear everything up and put it back together when there's nothing wrong with it...Itus."

[mark tucker]

[ps2375]

I would tend to think that the 28 vents would create more vacuum in most driving situations than the 32's would.

[cecil]

Mark, I enjoy your down home wit. When I worked in vacuum systems, we used molecules / cm as there is no true vacuum. Not even in outer space.
back to topic.
I switched to 32's for more mid top end. Worked great except for slight stumble around 2.8 k. That is when I started looking at vacuum. And found none, will very little. Right now, all I need to know what size is the vacuum port hole in a 40 hpmx.
_________________
I may not be that funny or athletic or good looking or smart or talented....I forgot where I was going with this
I have this wierd desease called "must tear everything up and put it back together when there's nothing wrong with it...Itus."

[[email protected]]

it won't matter, because it's measuring the MAP below the throttle plate. Above the plate the pressure is much higher.

Just make sure the hole is clear. Anti pulse valve helps a lot.

If the engine has low compression there is no solution other than raising compression to get the vacuum up.

[ekacpuc]

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but the venturi is bigger than the gap in the throttle plate. The venturis would have to be PUNY (like 20mm or less) to have this affect you are envisioning.

Just understand that Ported Vacuum is the exact same thing as Manifold Vacuum at all conditions EXCEPT IDLE (where ported vacuum is off).

If the engine has low vacuum it's a poor engine combo, OR the rings/valves are leaky.

"Poor Engine Combo" is something like this

W110 cam and 7.5:1 compression
W120 and 8.5:1
Stock cam and 6.5:1

You can get away with these mistakes a lot more with FI, but with a carburetor(s) you have to do things right or you will have problems. The entire idle and progression circuit rely on manifold vacuum to operate properly. Poor vacuum and the engine absolutely cannot run "well" with a carburetor(s).
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