Morris Minor Owners Club

I've discovered while fine tuning the rebuilt engine (and struggling!) that the vacuum advance on my 25D is caput, and the most likely culprit.

New bits on order so no big deal - however, when I come to play with the timing with the engine warm, do I move the dissy with the vacuum attached, or set it with it off then reconnect it? Obviously it affects idle speed too, so was thinking not to connect it until set properly?

Remove the vacuum line and plug it off so the engine isn't drawing extra air in through the manifold vac connection. Set the timing, replace the vac line. This will give you a starting point to work from.
Go for a spin and accelerate hard in top and see if the engine pinks, if it does retard the timing slightly until it stops. If it doesn't advance the timing until it does then back off again until it just stops, this will give you the optimum setting allowing for wear and modern fuels.

If, as you say, your vacuum advance is kaput - repair/replace it. The engine is designed to be ‘tuned’ (whatever that means to you) using the timing advance curve of the particular distributor fitted.

Testing the part is far better than just guessing it is faulty. Very easily carried out with a liming light. The engine idle speed is adjusted after the timing is set properly.

I personally would not remove the vacuum advance while ‘tuning’ the timing. No point, really, unless you wish to check the mechanical advance curve.

.. as for the vacuum advance unit, there are quite a few new ones that are faulty ,from a batch of five, four were faulty , a quick test is to suck on the inlet and cover it with your tongue, feeling the pressure from the vacuum , it should not leak out.

John ;-)

Ha, John,

As an aside, a vacuum can never ever leak - there is, by definition nothing to leak anywhere! It will always be air (or something else) that actually leaks towards the vacuum.

It is a leak , a negative leak......

John