HS-4 vacuum port : MGB & GT Forum : MG Experience Forums : The MG Experience

HS-4 vacuum port #1
grn78rd Jon Clark Clemmons, NC, USA 1978 MG MGB 1978 MG MGB	Posted: July 28, 2012 04:08PM Registered: 4 years ago Posts: 1,038

At the risk of pissing Lloyd off, I am going to ask a question. Since I am liable to get an good answer on the MGE forum in short order, I am choosing not to spend several hours researching it everywhere else.

I am preparing a set of AUD-52 HS-4 carbs for installation on my 1978 B. There is a manifold port on the back carb. I plan to send my distributor to Jeff S. for re-furb, but should I plan to use the manifold vacuum or carb port for dizzy advance?

Thanks for your answers even if it maybe a silly question.

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Re: HS-4 vacuum port #2
balloonfoot Lloyd Faust New Mexico, USA 1964 Porsche C Coupe 1967 MG MGB "MAX" 1985 Chevrolet Corvette "EASY 1ST" 1986 Chevrolet Corvette "Gas Mileage Special" 1989 Chevrolet Camaro ""I Rock""	Posted: July 28, 2012 04:29PM Registered: 4 years ago Posts: 8,269

Jon....the port on the carb is "ported" vacuum and takes a convention vacuum advance. Manifold vacuum connects to the intake manifold and uses a vacuum advance that works in the opposite manner. What you have is the best option. Tell Jeff what you have and he'll fix you up.

“Better to remain silent and be thought a fool than to speak out and remove all doubt”.
- Abraham Lincoln

"Anyone with an intense emotional interest in a subject loses the ability to observe it objectively: You selectively perceive events. You ignore data and facts that disagree with your main philosophy. Even your memory works to fool you, as you selectively retain what you believe in, and subtly mask any memories that might conflict."

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Re: HS-4 vacuum port #3
grn78rd Jon Clark Clemmons, NC, USA 1978 MG MGB 1978 MG MGB	Posted: July 28, 2012 05:28PM Registered: 4 years ago Posts: 1,038

Perfect! Thanks for the help. I will art up that way.

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Re: HS-4 vacuum port #4
rocannon Frank L Oklahoma, USA 1980 MG MGB "Restored By Photoshop Inc."	Posted: July 28, 2012 07:49PM Registered: 3 years ago Posts: 2,126

"... thus spake the walrus." drinking smiley

Procrastinate... later.

TANSTAAFL !

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Re: HS-4 vacuum port #5
gooser Drake Myers Danville Va, USA	Posted: July 28, 2012 10:20PM Registered: 5 years ago Posts: 6,919

use whichever one the distributor was designed to use.

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Re: HS-4 vacuum port #6
davester Dave Diamond Berkeley, California, USA 1971 MG MGB GT	Posted: July 29, 2012 12:04AM Registered: 2 years ago Posts: 1,311

What LLoyd said is almost, but not 100% right. Manifold vacuum and ported vacuum are opposite at idle, but anything other than idle or trailing throttle they are exactly the same. Ported is better, but it's not that big a deal.

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Re: HS-4 vacuum port #7
mrbarry michael barry dover tn, USA 1979 MG MGB "Moanin' Joan" 1991 Ford Bronco	Posted: July 29, 2012 12:19AM Registered: 5 years ago Posts: 2,759

Vacuum Advance - Carb vs Manifold
Hammer and Spanner
The pages of 'Bee' and 'Vee'
http://www.mgb-stuff.org.uk/ignitiontext.htm#vac

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Re: HS-4 vacuum port #8
gclifft Greg Clifft Dardenne Prairie, MO, USA 1974 MG MGB	Posted: July 29, 2012 12:21AM Registered: 10 months ago Posts: 81

In reply to # 2132366 by davester What LLoyd said is almost, but not 100% right. Manifold vacuum and ported vacuum are opposite at idle, but anything other than idle or trailing throttle they are exactly the same. Ported is better, but it's not that big a deal.

Agree 100% on your ported vs manifold statement. Once off idle they are the same.

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Re: HS-4 vacuum port #9
B-racer Jeff Schlemmer Minnesota, USA 1958 MG magnette 1971 MG MGB "POS" 1973 MG Midget MkII 2006 Dodge Charger "Daytona" 2008 Ford Super Duty	Posted: July 29, 2012 07:55AM Registered: 6 years ago Posts: 12,509

Ported will never allow vacuum at idle.
You really never need vacuum at idle unless you have a 400+ cubic inch engine and the massive reciprocating mass needs more advance to help create momentum.

If you have a port for ported vacuum, utilize it! Manifold vacuum was a way to build the car cheaper and it DOES require a special vacuum advance unit. Ported vacuum works with almost any vacuum unit.

jeff@advanceddistributors.com

How to advertise on this website

Advanced Distributors Lucas distributor repair, restoration and parts supplier.

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Re: HS-4 vacuum port #10
balloonfoot Lloyd Faust New Mexico, USA 1964 Porsche C Coupe 1967 MG MGB "MAX" 1985 Chevrolet Corvette "EASY 1ST" 1986 Chevrolet Corvette "Gas Mileage Special" 1989 Chevrolet Camaro ""I Rock""	Posted: July 29, 2012 09:45AM Registered: 4 years ago Posts: 8,269

In reply to # 2132366 by davester What LLoyd said is almost, but not 100% right. Manifold vacuum and ported vacuum are opposite at idle, but anything other than idle or trailing throttle they are exactly the same. Ported is better, but it's not that big a deal.

I tend to be very brief.....should have been more wordy.

It of course goes without saying the end result is the spark gets advanced. However, how that occurs is completely opposite with the two systems.

Ported vacuum: At idle there is no vacuum at the port and the advance system is not functioning. When you open the throttle, there is high vacuum at the port and it pulls the points plate clockwise and advances the spark.

Manifold vacuum: At idle there is high vacuum at the manifold and the advance system is always functioning....it is holding the points plate in the RETARD position by pushing it counterclockwise and retarding it. When you open the throttle, there is low vacuum in the manifold and this releases the vacuum retard at the points plate and thereby causes the spark to be "advanced".

How you time the engine depends on which "advance" system your engine uses.

Perhaps Jeff can explain it better.

“Better to remain silent and be thought a fool than to speak out and remove all doubt”.
- Abraham Lincoln

"Anyone with an intense emotional interest in a subject loses the ability to observe it objectively: You selectively perceive events. You ignore data and facts that disagree with your main philosophy. Even your memory works to fool you, as you selectively retain what you believe in, and subtly mask any memories that might conflict."

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Re: HS-4 vacuum port #11
gclifft Greg Clifft Dardenne Prairie, MO, USA 1974 MG MGB	Posted: July 29, 2012 12:36PM Registered: 10 months ago Posts: 81

In reply to # 2132508 by balloonfoot

In reply to # 2132366 by davester What LLoyd said is almost, but not 100% right. Manifold vacuum and ported vacuum are opposite at idle, but anything other than idle or trailing throttle they are exactly the same. Ported is better, but it's not that big a deal.

I tend to be very brief.....should have been more wordy.

It of course goes without saying the end result is the spark gets advanced. However, how that occurs is completely opposite with the two systems.

Ported vacuum: At idle there is no vacuum at the port and the advance system is not functioning. When you open the throttle, there is high vacuum at the port and it pulls the points plate clockwise and advances the spark.

Manifold vacuum: At idle there is high vacuum at the manifold and the advance system is always functioning....it is holding the points plate in the RETARD position by pushing it counterclockwise and retarding it. When you open the throttle, there is low vacuum in the manifold and this releases the vacuum retard at the points plate and thereby causes the spark to be "advanced".

How you time the engine depends on which "advance" system your engine uses.

Perhaps Jeff can explain it better.

Not true. I have a 10-15-5 advance unit my 1974 MGB which is factory set up for manifold vacuum. When I apply vacuum the plate is pulled clockwise to advance the timing. This just means at idle the vacuum advance is fully advanced.

I believe the engine is always dynamically (stroboscopic) timed with the advance disconnected, at least that is how I have always done it.

Respectfully,

Greg

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Re: HS-4 vacuum port #12
mrbarry michael barry dover tn, USA 1979 MG MGB "Moanin' Joan" 1991 Ford Bronco	Posted: July 29, 2012 02:00PM Registered: 5 years ago Posts: 2,759

http://www.mgb-stuff.org.uk/curvestext.htm#41814

to me the logical extension of the OP question is what happens in the dizzy , thus the ignition and engine timing with the application of vacuum [and rpm] .
i have been looking at and trying to understand the info at the curvestxt link above ..
which has centrifugal and vacuum advance #'s for a list of dizzy PN's. well realizing those #'s are for new units an old one would be ,, slack and wore.
however my dizzy is a custom curve from jeff and i now wonder what he set for centrifugal and vacuum , if i assume he set the values for my engine number and original dizzy type , as i read this stuff at my normal cruise of 3200 RPM and 15 INHG [manifold] that dizzy would wind in about 55 degrees of advance [30 centrifugal and 24 vacuum], and since i set 20 degrees at idle means im running down the road at 75 advance , more or less.
""Because centrifugal advance moves the cam position, and vacuum advance moves the points position, it can be seen that both advance mechanisms work completely independently, and total advance will be the sum of static advance plus centrifugal advance plus vacuum advance.""
quote from link

whatever the true case is it runs real good , a reconditioned distributor with a *custom* curve will make even a 120K miles 18V883 with a smog cam very happy.

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Re: HS-4 vacuum port #13
gooser Drake Myers Danville Va, USA	Posted: July 29, 2012 02:16PM Registered: 5 years ago Posts: 6,919

michael, in that case ask jeff. he may keep these kinds of records and if it's one that he re-curved only he can recommend you.

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