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MG MGB Technical - 25D or 45D or electric style?

Ok, I've been told by my garage a couple of months ago that my distributor was worn and should be replaced at some point, though not urgently.

Car died yesterday (loss of power, no ignition light during loss of power, unable to restart) and the breakdown man suggested I needed a new distributor too.

Have a 1973 B with a 45d distributor (it used to be a 25d, but I wasn't able to get a like for like replacement).

Should I go back to the 25d (I found the points and condenser easier to replace /adjust than on the 45) or consider one of those electronic ignition jobbies?

I have a standard twin SU carb setup, standard 1798cc engine, and do mainly about town driving.


No ignition light suggests another problem, not related to the dizzy. More than likely the ignition switch. If you have a meter or a test bulb check whether you have 12volts at the white to green fuse in the main fuse box with ignition on.
Nothing wrong with the 25d, depending on the age of you B, the advance curve is probably better than the 45 for your engine, but doubt whether you would notice. Also I think the 45 vacuum advance unit used manifold vacuum as opposed to carb vacuum, slightly changing the vacuum advance profile, some say for the worse.
What exactly are the mechanics saying is wrong with the current dizzy? Apart from wear on the main shaft or it's bearings, a seized shaft and/or bob weights stopping the advance working or a failed vacuum unit there is not a lot to go drastically wrong. Worn bearings on the main shaft is not easily fixed but the others are. Up to date I've never needed to condemn a dizzy, and the only time they are likely to grind you to a halt is through a faulty rotor arm, condenser or points and a detached or broken earth between the base plate and dizzy body. Quick fixes all!
Fitting an optically triggered electronic system will compensate for wear on the dizzy shaft.
Allan Reeling

Thanks Allan, the garage said worn main shaft. The breakdown man was a bit more vague(!)

By getting 12 v at the green-end fuse, do you mean measuring with a meter probe at each end across the fuse or measuring from each end to earth (I get no volts going across the green wire and white wire but get 12 volts from green to earth and white to earth.)

The white feeds battery voltage to ignition light, coil, fuel pump, overdrive as well as through the fuse to the green circuits. 12, or so, volts to earth, at either end of the fuse is fine. But try "wiggling" the key with ignition on and test lamp connected to earth, to see if there is a dodgy contact in the switch. Also follow the white wire through the bulkhead to it's bullet connectors and clean them.
If the "main shaft" is worn, grasp the rotor arm and see how much sideways movement there is. A few thou is acceptable but If there is a reasonable amount of wear it will affect the dwell and hence the ignition timing.
Allan Reeling

Will try that this evening. The ignition light comes on though when I turn the key, and it seems to turn over but not fire up.

What did the tacho do when the engine cut? What's it do when you try to fire it up? The tacho gets it's power via the green outlet on the fuse box and also it's operating pulses from the coil which gets it's power from the white circuit. When there is an ignition problem causing an engine to die, the tachometer also dies. With a fuel problem the tacho will fluctuate as the engine mis-fires.
Check the condition of the points as well as the gap.
Allan Reeling

I'm not 100% sure, but I think it fell away to zero as I lost power. When I tried to restart, the needle move ever so slightly, but that was all.

However, I have managed to get it going!

Steps I took-
1- checked voltage from green to earth with key wiggled- 12v

2-cleaned white bullet connector at mass of connectors near the bulkhead

3-followed white wire through to ignition switch, whilst pulling the plastic sleeve thing back to look at all the spade connectors attached to the ignition switch, I noticed that the white one had disconnected. I'm not sure in the process of pulling the sleeve back I had disconnected it or if it was just not fully in. Reconnected that.

4- checked points gap (ok at 0.016"), cleaned points and rotor arm with a bit of Emery paper, including inside the cap.

5-the rotor arm and inside of the distributor was a little bit oily, so gave it a wipe. Waggled the rotor arm, and whilst there was a bit of play, wasn't sure if it was more than a few thou

Tried starting, and success! Not quite sure which step had solved it though.

Thanks Allan!

Nat, did the car just drop dead?

1- Meaning the car died and the tach dropped to Zero and you were NOT able to get the car to crank at all even though you had lights, directionals, horn.

2- or did the car crank and not start?

Scenario 1- would make me suspect of ignition switch. I've had scenario 1- only to come back 20 minutes later and the car started right up and die another day.


79 MGB
gary hansen


The car lost power and didn't respond to any press of the accelerator, and revs went to zero (headlights and indicators were still working). It was a smooth loss of power rather than anything intermittent, though in the week before I occasionally noticed a very slight 'missing of a beat' if that makes sense. I pulled over and then tried to restart it. Ignition light came on when I turned the key, I can't quite remember if the car cranked at that point. I think it did, and it failed to start.

Nat. " Tried starting, and success! Not quite sure which step had solved it though. "

I would monitor this closely. I too thought I had it solved, only to have the car die another day. in my case, it was the ignition switch.


79 MGB
gary hansen

This thread was discussed between 17/08/2017 and 19/08/2017

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