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MG MGB Technical - Rough Running with New Distributor

I posted this under a reply for a different link, but I don't think that it was noticed, so I am trying a separate link. My car is a '73 NA spec B.

My original distributor was missing part of the collar that was used to hold the distributor in place. I purchased a used one from someone to replace it with.
I made sure that it was for an 18V engine and then swapped my pertronix, rotor button and cap from my distributor. I then set the static timing to 10 degrees as I had done with the previous distributor.
The idle jumped to 1200 instead of the old ~850 that it was set before. So I tried turning the distributor body (with the vacuum advance unplugged and the intake port plugged) to get the rpms more reasonable. Then it starts shaking as if there was a vacuum leak. I turned it back so that it idles at 1200 and took it out for a test drive (a little anxious to drive it again). On deceleration in gear, the car backfires horribly (no backfire when not in gear). I think that this means that there is too much timing. It also was coming very close to overheating. I pulled the plugs and the electrodes were slightly tan indicating good mixture.

In general it just runs too rough (car shakes when idling). I am not sure what I could have done that would have changed anything? I have no had a chance to check for vacuum leaks, but that will be on my list to check this weekend, could there be a vacuum leak at the distributor (ie would it matter if there was)?

A little confused and anxious to drive. Thanks again.
Zach

Zach,
Sorry to hear of your woes.

Sounds like the replacement may be very , very tired.

The rapid jump in revs at idle and timing causing backfiring (high vacuum on decelration) sounds to me like weak / worn springs on the weights or a faulty vacuum capsule or both.

There is also a remote possibility the distributor has been modified or altered at some stage in it's life and therefore is set up for a different angine tuning but is mechanically sound.
Either way it does need looking at.

Can you get it to a distributor place or maybe an mg specialist for a second opinion?

HTH
Cheers, Pete.
Peter Thomas

Zach,
I don't know you mechanical experience with carbs, but I think I know what you are talking about. Is the aluminum collar at the bottom of the dist. housing broke? If so you can use some of the old parts and new parts to make a good dist. If you have a moss, ect. catalogue you can see the dist. apart. If you take the points, condensor ect., out you will have a base plate held in by two philips screws. If you remove these the base plate comes off. In side you will see two weights held in by two springs. The weights are marked, 16 degrees, 18, ect. You want these to match. Most springs wear out and they should have different tensions as you rotate the shaft in a locked position. IE, the rotor button should turn slightly, only about 30 or so degree's with the bottom shaft held tight in between fingers. It should spring back quickly if it is correct. Anyway, I don't know the story of the old or new dist, but if you got a new used one and cannot return, I would take it apart and remove everything. If the shaft does not have a lot of play/wear, you can use the weight's, base plate, ect. from your other dist. You should them have something similar to original.
Also, to check vacuum advance you can suck on the hook up on distributor. It should be very,very hard and sometimes you can actually see the advance plate inside the distributor move. Even if it does leak it should not cause the massive tuning problem you are having.
Also, 18V engine's differed a lot over the years, and you might have you timing way off. You probibally want to use a timing gun to see what you are at. Check out this link and you will see what I mean.

http://www.mgbmga.com/tech/mgb21.htm

J Arthurs

You shouldn't be turning the distributor to adjust the idle. You set the timing on the distributor with the vacuum advance disconnected and plugged, then reconnect vacuum and adjust the carbs for idle.
Paul Hunt

This thread was discussed between 06/09/2002 and 07/09/2002

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