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MG MGB Technical - Vacuum advance for SU HIF-4 Carbs

Hello,
I am restoring an early 1974 MGB that has the SU HIF-4 carbs. The engine has a vacuum advance distributor but I dont know where the vacuum advance vacuum tubing would connect. On the earlier MGB engines with the SU HS-4 carbs the vacuum advance tubing would connect to the side of one of the carbs but the SU HIF-4 carbs dont have a vacuum port.
Can anyone help me?
Thanks for your time.
Jeffrey Palya

Jeff:
The distributor vacuum hose should be attached to a fitting screwed into one of the small ports on top the intake manifold.
willie L.

Jeffrey
You are best off not running with the vacuum advance connected. Because it is in the manifold, this gives a lot of advance at idle/low revs and consequently pinking when accelerating from lower revs. With the HS carbs, the vacuum advance was u/s of the throttle disc so at idle or low revs, the advance was inoperative until conditions required it.
Martin
Martin Williamson

Martin, unfair on the manifold! Not running the vacuum will ruin your fuel consumption, and it is not responsible for any pinking as you accelerate from low revs: the vacuum will drop to near zero as soon as you prod the throttle from idle. If you have pinking there it's down to something else. The manifold vacuum response is similar to that of a carb (ported) take-off except at idle, when it is high and reduces emmisions.
Steve Postins

Steve is correct on the use of the vacuum advance system with manifold vacuum. The distributor, on cars originally delivered with manifold vacuum advance, was specifically designed to work with that system and the mechanical and vacuum advance curves differ from those designed to be used with the earlier, ported vacuum advance, distributor systems. As Willie notes, the 74 was designed to be used with a distributor having the vacuum advance connected to the intake manifold. If the distributor has been replaced with another distributor, you need to verify the specification number of the distributor and determine where the vacuum advance was designed to be connected.

One item of interest is the later model UK spec distributors which were designed to be used with either ported or manifold vacuum. This may be the distributor being sold as the "Euro Spec" distributor here in the US. Two of my "Euro Spec" distributors have the specification number of the last UK spec distributor and two of them have no specification number listed on the distributor body. Les
Les Bengtson

I added a port to my rear HIF carb so I could move the vac avance pick up from the manifold. My car is a bit of a Frankenstein though. Originally US but with all the emissions stuff removed and HIF carbs and what looks to be an early distributor fitted. I figure with the port added I can try both the carb pickup and manifold and see which does what. I borrowed a HIF carb off my mechanic that was already ported to measure and figure out where to port mine.

There should be some stuff in the archives about "drilling a HIF for vac advance" I think.

I haven't actually run the engine yet so no idea if I got it all right!

Simon
Simon Jansen

Simon,
I am not sure, but it would be really surprising that SU ltd. had made different carbs of the same type and for different applications. In my thinking, for a manifold ported HIF carb, there should be a brass plugg in the place of vacuum advance take-up ( right carb ). If so, just drill it off to reverse it to a carb ported installation and press fit in the vac. advance connector!
Renou

Guys, Thanks for the information.
I took good look at the distributor while rebuilding it and installing a pertronix kit in it and it is a 1974 model. So it should have an advance curve that would go with the SU HIF carbs with an intake manifold port for the vacuum advance.
Thanks for all of the help.
Jeff
Jeff

Renou, someone else told me they did that on their carb and they removed the brass plug but I am sure on mine there wasn't any such plug. There is though a plug for where the bypass vent is (idle bypass I think?) which looks similar though. I think people might get these confused. On mine I drilled carefully from the underside of the carb so the vacuum hole ends upo right near the throttle butterfly. I will see if I have any pics and put them on my web site perhaps.

Simon
Simon Jansen

This has been covered before, the difference between ported and manifold vacumn is not great, use either but use one to get the good fuel mileage these cars are capable of.
S Best

This thread was discussed between 04/08/2005 and 06/08/2005

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