Tuning question....vacuum related

Many cars suggest that when setting the timing that you remove and plug the vacuum source to the distributer. Does this mean that you plug the line from the distributor to the manifold or do you plug the manifold source…or both??

The timing is set without any vacuum advance or retard being activated. This is why they advise having not vacuum source connected to the vacuum pot on the distributor. Does your distributor even have a vacuum pot for advance or retard? You didn’t give any details about the year and type (automatic or manual, which carb setup and etc) of car you have

Plug the line to the manifold, or in our case from the carb, so there is no vacuum leak.

I’ve never seen the point of bothering with the vacuum connection on the Jag, since it’s ported vacuum. That’s different from 'murican cars, which always had manifold connections.

You disconnect the vacuum hose so that the distributor does not receive a vacuum signal. You then plug the line to the vaccum source (carb or manifold) so that the engine does not have a vacuum leak which will affect running. There’s no need to plug the disconnected line into the distributor - it will just see a constant atmospheric pressure - ie no vacuum signal.

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What David said. I always kept a couple of golf tees with my ignition tune up tools (timing light, vacuum gauge and dwell tach) just for this purpose.

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What you actually meant to say was that there is no need to plug the line into the distributor because you want there to be no pressure differential across the distributor diaphragm. Both sides are seeing the same (in this case, atmospheric) pressure, hence the diaphragm doesn’t move.

There are some fancy distributor capsules (not on this particular car) which had both a retard connection on one side and an advance connection on the other side. These would need both lines disconnected and plugged at their sources.

kind regards

Thanks all…a simple answer to a vague sentence in the manual. This happens to be for a Sprite BTW