During its outing at Goodwood the Spencer Javelin developed all manner of farts and pops and it was not Lord March's petrol. After much investigation, a lot in the rain, we gave up at the campsite and trailered it home on Richard Gane's trailer and then put Richard's Javelin back on the trailer.
Anyhow this weekend I went to investigate with the timing light to discover no mechanical advance, so suspected a weight had come off. Sure enough, a weight had somehow become detached from the rotor pivot bush. I really could not see how the weight had found its way out as there did not seem to be excessive wear.
It seemed a simple fix to unscrew the set screw under the rotor arm, but it was not to be. Chris sent me and the distributor outside the paint shop as he did not want any grease in the shop. I promptly lost the screw in the gravel outside the garage. A replacement was found but the dizzie failed to work after at least an hour of trying.
I put my spare in the ailing Javelin and the original problem went away and the engine was back to normal. Except this dizzie seemed to have a wandering advance and made a noise at higher revs. So that too came home to go on the dizzie tester.
The original dizzie showed a problem on the tester which was quickly fixed by putting a proper screw from an old dizzie into it. Its advance curve, dwell and vacuum all now worked perfectly.
The new dizzie showed a peculiar fault whereby the vacuum advance also extended the dwell considerably. It did make a noise, so the baseplate was removed and the weights were examined up to 5000rpm. Nothing untoward was heard or seen. I replaced the baseplate, it seemed the noise came from it. But I cannot figure out where from or how the dwell changes. Pictures follow.
See the tight initial advance spring and the slack secondary advance spring.

View showing the set screw that holds the rotor spindle which must allow it to turn easily.
Also see the welded extension to reduce maximum advance on this stock MG 25D.
This show the advance and dwell. Double the reading for crank degrees. Showing 65deg which is a little over recommended of 60.

Max advance which occurs at 1400rpm . Equates to 22 deg at crankshaft.

Vacuum advance added to give max of 32 deg at crank.

This shows the spring attached to the diaphragm of the vacuum unit that pulls the baseplate around to add the advance.

The rotor arm can be turned to check the mechanical (centrifugal) advance is free to move.
