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MG MGB Technical - VACUUM ADVANC SPRING

How can I reposition the spring on my vacuum advance. I found the spring loose in the advance housing. I could epoxy it back in place but do not know how deep to insert it.
Sandy
st

What distributor?

The vacuum capsule has a spring inside it, but it is a sealed unit. Is that the one you mean? If so how are you going to seal the capsule again? That spring should just sit between the upper half of the case and the diaphragm, over the stop-rod as in the attached which is a 45D4 capsule.

The only other spring is on 25D4 capsules and fits up inside the body of the distributor where the adjuster rod exits, and bears against the knurled adjuster wheel. That is supposed to come free when you unscrew the adjuster, but with the adjuster in place it presses on the adjuster to give friction to help hold it still.

Paul Hunt

PAUL.
It's a 24D distributer.The spring that I referred to is the one with a small loop on the end and slips over the pin on the advance plate.
While I was cleaning the unit the small 1" long spring fell out.1Z9V2V520312669448ring diameter is about 1/8".
Sandy
st

The distributer is marked 25D--49491-A with 32-74 stamped below the casting number.
The spring just fell out of the vacuum housing.
The long list of numbers beginning with1Z9V was not sent by me. I do not know what it represents.
Sandy

st

That being the spring that has fallen out the vacuum capsule is almost certainly scrap. It should be attached to the diaphragm at the inner end, so that as the diaphragm moves with changing vacuum the spring moves in and out to move the points plate and alter the total advance. If you glue it in you won't have any vacuum advance, but at least it will hold the points plate in position, except for being able to alter the knurled wheel for fine timing adjustments. If you did away with the capsule altogether the points plate will be free to twist by quite a bit, so altering the timing in a random and unpredictable fashion, unless you come up with some other method of fixing the points plate in place.

49491 would be the distributor spec, except that 25D4 never got anywhere near that high. Sure it isn't 41491? That was used in the USA and Canada 1972-74 on the 18V 672/673 engine. As such it would have had a vacuum capsule which should be stamped 10-15-5, and represents the starting vacuum, ending vacuum, and maximum advance in distributor degrees.

'A' probably represents a change in component design that didn't affect timing. 32-74 is week 32 in 1974 i.e. the date of manufacture.
Paul Hunt

Incidentally, even though there is a spring between the vacuum capsule diaphragm and the points plate, it only expands by exception and not in normal use. The 45D4 capsule has a flat metal strip instead of a spring.

I think the 25D4 only has a spring to make things easier when connecting it to and disconnecting it from the points plate, as the lower body of the diaphragm slides in and out of a tube in the distributor body. Because the 45D4 doesn't have the timing fine tuning feature it attaches to the side of the distributor body with two screws, and with those removed it is easy to manoeuvre the capsule to disconnect and reconnect it from and to the points plate.
Paul Hunt

Paul. The casting shows just 25D
one more photo to follow

st

41491 A plus 32-74 are the only other ID marks on the distributer. There are no markings at all on the vacuum advance unit. It is relativly clean and on my desk under a large lamp.The spring appears to have just pulled out of it's tube. It has not broken off as the inner end would show had it been broken..
Sandy

st

Sandy,
Either get a new vacuum capsule or lock the base plate and forget about vacuum advance. Trying to repair by braille and hoping it will last should not be an option. It will come apart again, the timing will oscillate and be lost and the car will run like a bag of nails!!!
Allan Reeling

Maybe the capsule had already been replaced with a 'generic' item then. The attached image shows typical 25D vacuum capsule markings on the left (45D on the right).

As I say the internal end of the spring is supposed to be attached to the diaphragm, and the whole spring moves in and out with changing vacuum to alter the timing.

As Allen says, either replace it or fix the points plate in place. Personally I'd always replace as vacuum advance give improved cruising economy and sharper part-throttle acceleration.

You could glue the spring into the tube so it doesn't move at all, and that would then hold the points plate in a fixed position, but you would still have the facility to fine-tune the base timing with the knurled wheel.

Paul Hunt

No need for all that nonsense. Road tune the timing.
R Cintas

This thread was discussed between 21/04/2016 and 10/05/2016

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