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MG MGB Technical - Vacuum hose connection

Under the hood of my 72/3 MGB there is a black plastic vacuum hose.

One end is connected to the distributor advance unit, but the other is floating. Where should it connect to - I cant see anything logical, but that doesnt mean much!

Thanks

Peter
Peter Green

Peter. I am not that familiar with your model car as, I believe, it has HIF carbs rather than the HS series that I own. But, on my HS-4 carbs, the connection between the dizzy vacuum advance and the carb is on the top of the rear carb, at the rear. Other carbs have the connection at the rear side of the rear carb. But, take a look at the rear carb and see if there is a brass nipple that looks lonely.

Les
Les Bengtson

hi Les

If HIFs are the ones with the float chamber underneath them, yes. There is a small hole on the rear of the rear carb that does not appear on the front carb. From what you say, it would appear that there should be something in it maybe?

Peter
Peter Green

Peter. I am not sure. I own a 68 BGT and a pair of 79 roadsters. Of them, I am fairly confident. About yours, I am less so.

The cars having the HS series carbs had the vacuum advance hooked up to the rear carb.

At some point, the vacuum take off point was transferred from the rear carb (called "ported vacuum") to the intake manifold (called "manifold vacuum).

At some point, perhaps with the introduction of the HIF carbs, the vacuum take off was changed from the carb to the intake manifold.

I have been looking at the Moss Motors catalog and at Clausager's "Original MGB" and have not found when the change over took place.

It may well be that your car, as a 72/73 had the vacuum advance take off from the manifold rather than from the rear carb.

Hopefully, someone owning a car of your specific year will be able to provide some guidance.

In the mean time, what does the top of the intake manifold look like? Is there an unused nipple on the top, towards the rear?

Les
Les Bengtson

Les

Nothing of the right size. There is a blanked off one that was something to do with the anti-run on valve, then there is a really big one in the middle that has also been blanked off, then there is one on the front end too - also blnked off - but the little hose wont reach either of the last 2.

I too looked at Moss pages but couldnt even see the nipple shown seperately unless I missed it. What Moss part number was it if i did please?

Thanks

Peter
Peter Green

On a 73 MGB the distributor advance vacuum is supplied off a nipple on the intake manifold just behind the rear carb.

Here is a picture.

warm regards,
dave

Dave Braun

Do I have the wrong pipe then I wonder as it wont fit over the nipple on the rear of the inlet manifold. The end of the pipe is a 90 degree bend.
Peter Green

North American spec cars had manifold vacuum from the 18GK engine in 1971. Originally the vacuum pipes were copper with brass screw unions at either end. Later vacuum pipes were plastic and used a straight rubber connector to the vacuum capsule and a right-angle connector at the carb or manifold. Maybe you do have the later capsule and pipe. Get a bolt that its one of the blanks (or drill a blanking bolt) and solder or braze a short piece of brake pipe into it i.e. something the rubber connector will slide over. Whatever you do no ports on the inlet manifold must be left disconnected - all connected or plugged.
Paul Hunt

I would just connect to whichever port results in the neatest looking job. Since this is apressure connection with no flow taking place any one of the ports will work. and yes blank the others off or you will never get the mixture right over the rev range.
Stan Best

Solved the problem - thank you everyone for your help.

The pipe did go over the nipple at the rear of the inlet manifold ... after some help. I removed the 90 bit from the pipe and heated it in boiling water for half a minute. This made it suble enough to go over the nipple and I then reconnected the pipe.

Thank you all

Peter
Peter Green

This thread was discussed between 28/07/2008 and 29/07/2008

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