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MG MGB Technical - Correct vacuum advance connection - HIF

OK, so I just finished reading Peter Burgess's Speedpro book. I made myself read ALL of it - even the sections where I thought I already knew enough, only to discover that of course I didn't. And it's not Peter's fault that I now know less than I thought I did before. Such is the nature of learning. It's a great book!

Among the things I learned (or think I learned) was that on my '73 model, the vacuum advance ought to be connected to one of the carbs - just slightly upstream of the throttle plate. And of course on this car that I've owned for 20 years, it has always been plumbed to the manifold.

But here's the rub - my HIFs don't have these vacuum advance inlet ports. What's more, I have about three sets of spare HIFs and none of them have it either. I've seen HIFs that do have them, but on these I only see what looks like a brass plug just about where I think the port should be.

I see that Peter has responded to the thread on K&N air filters, I'm hoping he or somebody equally knowledgeable will respond to my minor question. Drilling out, or otherwise removing, the brass plug does not look difficult to me, but is it worthwhile?

I'm anxious to get on with a few other performance geegaws, but it seems that I ought first to make sure that what I've got is working right.

Allen Bachelder

In daily driving, there is virtually no difference in vacuum from either source. My '73 also uses the manifold as its vacuum source, and I've left it alone.

See the thread, "R 45D4 euro spec dizzys manifold vacuum?" - specifically the note by Paul Hunt. He refers you to a real-world test he performed testing out each method (also here: - look for the link on "vacuum advance"). Good reading, and not nearly so long as Burgess's book. :-)

Rick Stevens

I also think there may be an error in the book with regard to that one point - My GK series has the port off of the manifold and this corresponds with the information found on the following chart.

There were so many variations - I think Peter may have just overlooked a few. His book is excellent however.

check out this chart on the following site

it indicates port or manifold vacuum source for most years.

J Delk

Whilst certain engines and years had an *original* connection of *either* carb or manifold (basically everything before 71 was carb, after that North American spec was manifold. UK kept carb until September 76), any can be run with either. If your carbs don't have the port then use manifold, although I do know of people who have drilled and tapped one. My right-hand V8 HIF has one on the bottom, and the 73 roadster rear HS has one on the top. This *may* be because the butterflies open differently, the butterfly should move to place the port on its inlet manifold side as soon as possible after opening, not keep it on the piston side. The problem with the bottom port is that fuel runs down and rots the very expensive vacuum capsule. I changed mine twice before making a minature seperation chamber mounted *above* the port on the HIF, so that only a minimum amount of fuel can gather in the chamber before it runs or is sucked back into the carb. So far so good. Carb vaumm *is* preferable, it puts the vacuum capsule under less strain, and unlike manifold gives no advance during cranking with the choke out, which makes starting a bit easier.
Paul Hunt 2

Many thanks to all three of you. Judging by your responses, I think I'll restore my other six MGs before I pursue this further! 8^)

Allen Bachelder

This thread was discussed between 02/10/2006 and 03/10/2006

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