• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Manifold or ported vacuum?

Share this page

Torsten

Torsten

Freshman Member
Offline
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
 
The vacuum advance value is marked on the vacuum unit in three numbers. I'd leave the hose attached to the intake manifold, personally.
 
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.com (https://www.teglerizer.com/mgstuff/advance_curves.htm), 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
 
hi torsten-
after years of trying to sort out my distributor problems, i bought a new pertronix electronic distributor, with a vacuum advance, for my 74 b with twin hif's.
The instructions reccomended using port vacuum, but stated that manifold vacuum would work.sounded fishy to me, but i remembered how strong my tr3 ran with ported vacuum, so i decided to change it.
i removed the rear carb., flipped it over and drilled a small hole just before the throttle plate, almost touching the plate. inserted a one inch piece of tubing that i sawed off an old jap carb that i had in my junk bin. stuck it in the hole with some jb weld.
the results were amazing--it idles perfectly, accelerates like it never did before, and i thought my gas guage had broken due to the increased gas milage.

i guess what i saying is don't be afraid to tinker with it. if your not happy with the ported vacuum just plug it up.
good luck, emmett
 
Thanks for posting emmett1010 :cheers:
,I have a 74B chrome bumper with the hifs. I was looking at the Hs's as an alternative but I may try your trick before totally giving up on my hif's. Any other tricks you may know about concerning the Hif carbs would be very much appreciated as well as they are becoming my nemesis :hammer:.
Where's that guy who invented bi-metalic jet supports? :hammer:

I have a 23D distibutor Torsten , with points etc Non vacuum advance (sport Cam)and it's only my budget (and being a full fledged luddite)keeping me from doing the pertronix switch.
Good luck Mark
 
Hi,

I found a cheap vacuum unit on Ebay and a friend of mine happens to have 'my' distributor unit in his pile of... distributors! :smile: The only thing the distributor needs is a little cleaning and care, so it makes a good project for the winter.

Thanks for your info and comments!

Regards,
Torsten
 
25D dizzy and HS4's, there oughta be several unused (plugged) ports on the stock intake manifold. Pick one and done.

In the dizz itself, check to be sure the advance plate and the stage it mounts to are in good order. They can be either badly worn and slop around or bound up to non-function. Rotor "slop" is another issue.
 
This is the 72 factory setup on the wifes car. PJ

72manifoldvac.jpg
 
Hi Paul,

That is a nice looking engine bay! It's the same setup as mine, except the emissions control stuff.

I've got the vacuum connection on the exact same spot on the manifold. Now I'm waiting for the right vacuum advance unit and the rebuilt distributor.

Although the car runs nice now, I'm curious about the difference the right distributor and vacuum unit makes. I'm told that the car will have more 'bite' then.

Regards,
Torsten
 
Torsten, The only thing I'd change is the butterfly's in the HIF carbs. They have the valves in them and it makes it hard to adjust the idle down to where it should be, which in turn causes a minute delay in the shifting. If I have to pull the carbs, I'll put the solid butterfly's in. PJ
 
Back
Top