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Manifold or ported vacuum

Posted by mgholland 
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mgholland Avatar
Torsten Faruhn
Limburg, Netherlands   nld

Hello there,

My friend and I were working on the ignition of my BGT, because it ran very rough. As static and dynamic timing didn't work out well, we started to take a closer look on the distributor. It is a Lucas 25D with the vacuum advance on the inlet manifold of the dual SU HS4 carbs. We reset the contact breaker points, swapped the rotor, igniton coils and spark plugs, but still the car runs a little rough when idling and driving. We suspect that there might be a problem with the vacuum advance, which starts already at 1000 rpm idling.
My question now: is it better to have the vacuum on the inlet manifold or on the carbs (ported vacuum)? If so, where does the hose from the distributor connect to on (one of) the carbs?

Some extra info (and an extra question): my car is a US-spec model year 1971, with a rebuilt engine without the emission control stuff and the distributor has the year 72 stamped on it. Is it possible I got the wrong vacuum unit? Where can I check if the distributor and vacuum unit match or not?

Thanks in advance!

Regards,
Torsten

alex77 Avatar
alex alcoser
deep south texas, USA   usa
1969 Triumph TR6
1977 MG MGB "Harley Quinn"
1979 Triumph TR7

Its possible it wrong.
The '72 had HIF4 carbs, which were manifold vacuum
The '71 had HS4 carbs, which were ported vacuum
If they are mismatched, it will not run as good.

Here is almost the same question I asked 2 years ago.
http://www.mgexperience.net/phorum/read.php?1,795301,795333#msg-795333

Here is a little more info on the numbers can can be found on the vac. unit.
http://www.mgexperience.net/phorum/read.php?1,820955,821022#msg-821022

Hope this helps.

ingoldsb Avatar
Terry Ingoldsby
Calgary, Alberta, Canada   can
1971 MG MGB

Do you have an 18GK or an 18V engine (early or late 1971)?

Going from memory (and the Moss catalog), my 18GK has the 41339 distributor (manifold vacuum).

If no one jumps in to clarify or correct me I'll try to get out to the car later and take a look.



Terry Ingoldsby
terry.ingoldsby@DCExperts.com



Edited 1 time(s). Last edit at 2010-10-26 09:09 AM by ingoldsb.

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mgholland Avatar
Torsten Faruhn
Limburg, Netherlands   nld

My car is a 1970 built GT, but to the specs of the '71 model year. The engine is a rebuilt one from Ivor Searle. I already asked these guys which spec engine this is, but the couldn't answer this question.

Where on my HS4's can I find the pipe to which I have to attach the vacuum hose? I'll try to get the numbers of the carbs and vacuum unit tomorrow and post them here too. Maybe it helps a little more.

Greetings,
Torsten

gooser Avatar
Drake Myers
Danville Va, USA   usa

rear carb. on top next to the intake manifold. i don't believe all the hs4's had ported advance. may be wrong on that.

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Jim K Avatar
James A. Krasnansky
Liberty, KY, USA   usa
1970 MG MGB GT "Chloe"
1971 MG MGB GT "Roscoe"
1972 MG MGB "Camilla"

In reply to a post by gooser rear carb. on top next to the intake manifold. i don't believe all the hs4's had ported advance. may be wrong on that.

Torsten - that's where mine is.



Jim K is a grease-stained wretch

mac townsend Avatar
Fairfield, CA, USA   usa

it is very simple to change from one to the other...swap the vac hose and plug the open port.

whether you want ported or manifold depends on the advance unit in the distributor. You might want to look for the 6 digit part number stamped into the distributor body in small figures...about 1/8" (3mm) high. With this number it is possible to determine which year that distributor was used and from there whether the vac advance is manifold or ported.



1973 Pale Primrose Roadster. A nice 10-footer!
SUs, Datsun 5-speed
MGB Tips and Tricks: www.mgrescue.com

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Redwind Avatar
Charlie Hopek
So. Cal, USA   usa

When you get the numbers from your distributor and vacuum advance the chart for the 25D on this link may help you.

http://www.teglerizer.com/mgstuff/advance_curves.htm

ingoldsb Avatar
Terry Ingoldsby
Calgary, Alberta, Canada   can
1971 MG MGB

I'm trying to think of obvious external clues as to whether an engine is 18V or 18GK. The 18GK was maroon and the 18V was black - but that is no guarantee since they could have been painted (either way).

You really need to match the distributor number (which will identify the centrifugal advance) and get a matching vacuum unit. Then, hook it up the way that the particular distributor was supposed to be configured.

Jeff S. is the wizard on this - I would PM him and I'm sure he can tell you what matches what.



Terry Ingoldsby
terry.ingoldsby@DCExperts.com

DB Wood Avatar
Daniel Wood
Tumalo, OR, USA   usa
1969 MG MGB GT "Clyde"
1970 MG MGB GT

The 71 was the ONLY year where the vacuum advance was manifold instead of ported. All other years for the HS-4 carburetors had ported which would be indicated by a port on the rear carb. If yours doesn't have it then manifold is the way to go. The vacuum advance units have very different specifications so you need to use a vacuum pump and check to see which you have.

The ported distributors have more vacuum advance and less mechanical advance than the manifold types so you have to make sure you know what you have. Check this chart out, it will give you the proper info. You will have to take the distributor out to read the 41xxx number on the side of the body. http://www.mgbmga.com/tech/mgb21.htm

If you take the distributor apart, you can check out the mechanical advance. It typically will be sticky, frozen, or the springs will be old and weak, all which cause drivability problems.

The weak springs will also cause the timing to be retarded at 3000 rpm when set to stock idle specs. The engine can run hot as a result or just run poorly at high rpms. 32-35 degrees at maximum advance is the most important place to be. If the advance springs are good then the idle spec will be OK when you set the timing at maximum advance or vise versa



Dan Wood
70BGT driver, OD, Pertronix, HS4's, Peco, .060 over, Elgin cam, Superlite wheels, poly bushings, panhard rod, rear tube shocks, 1" lowered front end, HD shock valves, etc, etc.
69BGT project (V-6?)
88 Saab SPG Turbo
86 Vanagon Westy (South African conversion engine 2.0 OHC 135HP)

Lucas= Loose
Unsoldered
Connections
And
Splices

mgholland Avatar
Torsten Faruhn
Limburg, Netherlands   nld

Well, I've been under the hood and checked some things:
First, I can't find a port on the carbs, so the setup should be (correctly) manifold vacuum.

The I looked for the serial numbers of the distributor and the vacuum unit and the specs of the vac unit. I found the following numbers:
Distributor serial: 41234 with an H added after it and under this serial are the numbers 39 72
Vacuum unit serial: 54411985
Vacuum specs: 5.13.10

This doesn't correspond with the data I found on Teglerizer, which was for a 1971 18GK engine:
Distributor serial: 41339 (LC)
Vacuum unit serial: 54414868
Vacuum specs: 7.13.5

According to Teglerizer, this distributor is from a 18V engine with ported vacuum. There's no info (just a *) on the vacuum unit. The list shows also that the vacuum unit is from a 65-67 18V engine with ported vacuum and a 41288 serial distributor.

Does this mean:
a. the distributor and vacuum unit can work together, but only on a ported vacuum setup?
b. the distributor and vacuum unit are uncompatible with each other (they are listed in separate rows, but with the same engine type) and don't work on a manifold vacuum unit?

So I would have to:
a. get a distributor that suits a manifold vacuum setup and get a fitting vacuum unit?
b. change the carb setup to ported vacuum? This seems the more expensive choice to me.

Regards,
Torsten

gooser Avatar
Drake Myers
Danville Va, USA   usa

so i wpuld have to:



option c. send the distributor to jeff and see what he recommends.

alex77 Avatar
alex alcoser
deep south texas, USA   usa
1969 Triumph TR6
1977 MG MGB "Harley Quinn"
1979 Triumph TR7

I would PM Jeff for this one. The vac unit is for ported, no question. Could be the vac unit was changed on the distributor. Either way, does not look like the distributor is original to the engine.

Dan, you have to reword your statement- '71 was NOT the only manifold vacumm. my setup from a 74.5 is manifold vac. It could be the '71 HS4s were, but the '72-74.5 were also. Not sure as the chart shows some, but not all '71s as ported.

DB Wood Avatar
Daniel Wood
Tumalo, OR, USA   usa
1969 MG MGB GT "Clyde"
1970 MG MGB GT

I meant that the only HS4 carb with manifold vacuum was the 71. HIF4's were all manifold.
The cheapest way to go would be to drill and install a vacuum port in the rear carb. You already have a distributor that will work with. It needs to go in the same place relative to the throttle plate as the earlier years. Get a hold of an earlier carb and measure carefully before drilling.



Dan Wood
70BGT driver, OD, Pertronix, HS4's, Peco, .060 over, Elgin cam, Superlite wheels, poly bushings, panhard rod, rear tube shocks, 1" lowered front end, HD shock valves, etc, etc.
69BGT project (V-6?)
88 Saab SPG Turbo
86 Vanagon Westy (South African conversion engine 2.0 OHC 135HP)

Lucas= Loose
Unsoldered
Connections
And
Splices

mgholland Avatar
Torsten Faruhn
Limburg, Netherlands   nld

Thanks guys! I PM'ed Jeff on this one. I'll report what his findings are.

It is correct that the 1971 model has a manifold vacuum with HS4 carburetors. It seems to be an odd model year, since it also had an engine that was only used for one year (18GK),

Regards,
Torsten

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