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| Joined: | 03-12-2005 |
| Location: | Kingman, Arizona USA |
| Posts: | 532 |
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Brass Tube.
This is the manifold vacuum spigot. It's pressed in, and if it fits loosely you'll have a hard-to-find vacuum leak. In a street car this is the manifold vacuum port for the vacuum switch and the distributor. If you won't be hooking up vacuum to either of these two devices, the spigot should be capped off or replaced with a threaded plug.
Distributor.
If you look at the General Specs in the shop manual you'll see the centrifugal advance specs. Note that the curves for the 23D (Del'lorto) and 25D (Stromberg) distributors are the same. The 45D distributor is intended for smog cars with greatly retarded static timing and thus gives a lot of advance just above idle -- not a good choice for performance.
The stock distributor provides 0 crankshaft degrees of centrifugal advance at 1000 rpm and 17 crankshaft degrees at 2500 rpm. Bear in mind that centrifugal advance is in addition to whatever you use for static advance, so you'll have to add the two together to see what the actual advance is at any particular rpm.
Given the odd things previous owners do, once the engine is running, it is prudent to verify by actual test that you have the correct centrifugal advance at each of the rpm's stated in the manual. If there are doubts or problems, open up the distributor and verify that you have the correct advance plate (AKA 'action plate'). The one for a JH with a 23D/25D distributor is marked 11^ which means 11 distributor degrees or 22 crankshaft degrees of centrifugal advance. This is the maximum possible centrifugal advance available, and (guessing) will occur around 3200-3500 rpm. See http://www.mgcars.org.uk for further information.
For a stock or near-stock 907 engine, the factory advance curve is generally appropriate. However, you might be able to improve things a bit if you put the car on a dynamometer, determine what advance gives max power for full throttle at various engine rpm's, then modify the distributor to suit. Any racing shop with a distributor testing machine should be able to do that, assuming of course that they can get or make the appropriate parts.
If you are not going to be using the vacuum retard function of the distributor, and can not remove the thing for some reason, a bit of epoxy in the right place will keep the capsule end of the coupling spring from moving due to vibration of the diaphragm. This may result in more stable ignition timing under some conditions.
Can't help with the driveshaft issue but would be interested in hearing about whatever you discover. Do you expect an improvement in acceleration with an aluminum driveshaft, or would this be done only to reduce overall car weight? And as it sounds like you've got all the other issues under control, I'll wish you good luck in your endeavors.
Update: the link above is incomplete. The correct one is:
http://www.mgcars.org.uk/electrical/25ddistributor.html
Last edited on 07-17-2005 09:27 pm by Mark Rosenbaum