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MG MGB Technical - Ignition Advance

Bare with me. This may seem pretty dumb.

'75 MGB stock setup with manual choke and pancake air filter.

The dizzy has a cap on the advance mechanism. I've tried to trace the location of a vacuum line but can't figure out where it was connected.

Did the '75 untilize the vacuum advance? If so where was it routed?

Brad Batchelor

think those went to the intake manifold. Should be a fitting there..capped off somehow also.
J.T. Bamford

I am not too sure that it should be on the mainfold as I believe that way would provide constant advance. Look more on the carburetor for a capped port near the throttle plate.
D Maples

I looked all over the carb thinking that's where it would be but found nothing. Unless it is possibly on the very bottom and somehow overlooked.

The only plugs I see are from the waterways that fit the old choke unit.

Brad Batchelor

Checked the Bentley's. On a '75 it goes to the manifold. '77 and later to the tcs valve, then to the manifold. Lots of the car's emissions control systems have been so bodged over the years that sometimes it's best to just get out the book and start with square one, to restore things back to their original configuration. Usually it starts with an innocent valve job. Somehow, the hoses never were marked before removal and it was put back together using the best guess method. Never ran particualrly good as originally, so a few more hoses are moved around a bit, hoping to hit the magic combo. Someone suggests de-smogging it to restore it's pep. A H.A. attempt is made, usually involving taking off the pump belts and plugging off the gulp valve. Not much difference. Eventually it runs OK so we leave it alone, at whatever state of dissassembly gives adequate performance. Took me about a week to get all the stuff on my '80 hooked up correctly and functioning..
R. L Carleen

Thanks, I'll check the manifold for a possible connection.
Brad Batchelor

Brad, not a dumb question at all -- the ignition
system went through all kinds of modifications
to meet emissions requirements in the 70's.

The original US spec 75 ad 76B's had the vacuum
advance disabled. This assumes that you have
the original distributor however.

The 77-on B's used intake manifold
vacuum advance only in fourth gear,
through the TCSA switch. This was as I understand
to reduce surging and improve gas mileage, but
doesn't affect performance much.

I would check the number stamped on the distributor
to determine if it needs vacuum advance, and
if so whether from the carb or intake manifold via
TCSA. The following chart has all the details on
dist. number and vacuum requirements.

http://www.mgbmga.com/tech/mgb21.htm
Ronald

Thanks Ronald. That was extremely helpful. I'm thinking it was disabled. The plug looks like something the factory would have installed.
Brad Batchelor

This thread was discussed on 04/06/2004

MG MGB Technical index

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