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MG MGB Technical - Vac take-off

My distributor has a vac advance unit on the side of it but no pipe connected to it. The car runs fine but why would there be no pipe on this? Would it make a difference to have one? Where is the vac take-off on the carbs? Or is it on the intake manifold? I also have a 123 electronic distributor ordered which has the vac advance unit in it. Will it make a difference to leave this disconnected?
R Kelly

Ross,

The vacuum advance will deliver better fuel economy by delivering more advance under steady state cruising load which would be inappropriate for acceleration. So I would say, yes hook it up. The take off should be on the rear carb, close to the mounting flange, on a home market car (I think)

good luck,
dave
Dave Braun

Excellent. Ill connect it up, so! I have a length of braided rubber hose which will fit nicely. Is this sufficient or do I need to use a solid pipe?
R Kelly

From September 1976 UK cars moved the vacuum take-off from the rear carb to a port on the inlet manifold (export cars moved it much earlier, 1972 in the case of USA). Originally (on your car) it would have been a plastic pipe pushed into rubber connectors at each end. More or less as Dave has said, vacuum advance gives improved cruising economy but also aids part-throttle acceleration, so is definitely an advantage. If the port on either the carb or inlet manifold is left open it will seriously upset carb balance. If there is no obviously unused port on the inlet manifold then it raises questions about the engine and its ancilliaries.
Paul Hunt 2

My 74B has a vacuum advance on its dizzy as well. A friend in my Mg club did some tuning on the car (he's an MG mechanic) and disconnected it without my knowing. The car ran well this way for a couple of years til I noticed it. Since I was going to take a long trip I put a hose on it and reconnected to the intake manifold. What I discovered was that the car had a much more stable idle to it when disconnected! Also when I started from a stop It acted a lot smoother and with a bit more power than when it was connected. Since I returned from my trip I again disconnected it and again things improved. Of course
the intake manifold cannot leak air so I saw that that was done too. One other thing I had done a long time ago was to remove the air pump. I really don't think they help anything anyway. Bob
Bob Ekstrand

Maybe the timing was out on yours when you reconnected the vac unit though. I'd say if you reset the timing by the book it would have ran better. This may be somethign Ill need to do with mine too.
Ross Kelly

My car (73B) idles very poorly without the vacuum advance attached. Would have to turn the idle screws way in for it to even run at idle. This to me suggests a real fuel savings when it is connected.

Erick
Erick Vesterback

I cracked open the bonnet today after I got home from work just briefly and looked for the place the pipe is supposed to attach to. There was a pipe connected to it and the other end was hanging loose down the bottom of the engine bay! No wonder the car was drinking petrol! Anyway I reconnected it while the engine was running and the difference was instant! The idle sped up, was a lot less lumpy and when I took the car for a drive the acceleration was much smoother and faster. Its still idling a bit rough though and I reckon the valves need to be adjusted. Well Im only after getting the car so these are all things that will be taken care of over the next few weekends. It needs a bit of TLC and a damn good tune-up. Ill wait until I have the new distibutor before I tune it though.
Ross Kelly

This thread was discussed on 14/03/2008

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