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MG MGB Technical - Dizzy problem

Hi,
Hi,

Have a bit of a puzzler with my other '72 Gt, type 25d4 distributors.

Been having problems with starting; eventually found the problem to be a bad wire (the soft cotton covered type wire) between the LT (white/black wire to coil) terminal and contact set. Have replaced this.

Also while searching for the problem I replaced the contact set with a new one (adjusted the gap) and replaced the condenser/capacitor with a new one.

I also replaced the low tension lead and terminal (i.e. the cotton wire thing to the white black terminal) on my spare dizzy too, this already has a new contact set and condenser on it.

Checked the earth leads (i.e. the other 'cotton' wire) from the moving plate to the dizzy body on both dizzies for a good circuit.

Still having problems whichever dizzy I'm using.

Have even put on a new contact set and condenser to one of the two dizzies and still nothing.

I have placed the plastic insulating washer/bush on the base of the contact post, round end of contact breaker arm on this bush, condenser and terminal lead above this followed by insulating bush and nut above this.

Still no spark. what's going on? Have checked white/black lead. when touching the dizzy body and pulling it away, there is a spark from the coil.

Any help appreciated.

Iwan Jones

It seems you have a meter as you have checked that some wires are good. Can you measure volts to ground when the points are open, then 0v when they are closed? I imagine the dissy cap is in good shape and the carbon bruzh contacting the rotor arm. There have been lots of reports of defective rotor arms, the rivet is different on the duds, also anyone who is a regular on here will know that capacitors on sale now are of "variable" quality. I would try one of the old ones that used to work.
Stan Best

Were did you put the wires on. The pigtail from the coil connection and the condenser lead go between the top insulator and the points spring. Not under the nut as thats a dead short. If thats what you have done your not the first. Denis
Denis4

Hi,

Thanks for your answers.

No Denis, the condenser and LT lead to the terminal are both under the plastic insulating bush above which is the nut.

Stan, I might have time to do this tomorrow.
I know this causes problems i.e. dizzy won't work at all.
Have checked both dizzies and both the touch the contact breaker arm..
Iwan Jones

Iwan
Hi

You say you have a plastic insulator/bush on the bottom then the arm, wires from condensor and dist.wire then insulating bush and nut.
What shape is that top insulator
It needs to be like a bush and flat washer all in one
If it is just flat the wiring is probably touching the post and shorting out making your coil think the contacts are shut all the time
The top insulator's job is to keep the spring arm and wiring from contacting the post and also insulate the nut from the wiring
Most of the contact sets available here have a flat fibre or plastic washer on the bottom then the arm and then wiring is located against the arm by the top insulator/bush thingy down over the post into the arm then the nut on top
Hope this helps Willy
WilliamRevit

Exactly as you said. Top insulator (top hat shape) keeps everything away from the post and the nut above and compresses it down onto the contact breaker arm. I've checked on both that nothing untoward is going on here.
Iwan Jones

OK
When you say you have no spark ,I take it you mean when winding over on the starter. Do you mean you have no spark at the plug leeds or out of the coil
Willy
WilliamRevit

You say you are getting a spark when you touch the wire from the coil on the distributor body - that proves that wire back through to coil and the ignition switch is OK.

What about when you touch that wire on the dizzie spade with the points closed? If no spark then, then the circuit through the cotton-covered wire to the points, through the points themselves, and what should be another cotton-covered wire from the points plate to the distributor body is open circuit. With the coil wire on the distributor spade, get another wire connected to earth and tap it on the points spring. If you don't get a spark now then the first cotton-covered wire is open-circuit. If you get a spark there tap it on the points plate. If you get a spark there then the second cotton-covered wire is open-circuit - if indeed it is present at all.

If you *do* get a spark from the distributor spade with the points closed, then try again with the points open. If you still get a spark then the points are short-circuited by something, either incorrect assembly, a short-circuit condenser, or a fault in the points at the pivot. Remove the condenser wire from the points post, and if the points now spark as they open then it is the condenser. Otherwise you will have to recheck the assembly of components on the points plate (yes, even if you are certain sure you have done it right) and if that really is correct then try another set of points. Points and condenser may be new, but that is no guarantee of anything these days.
PaulH Solihull

I had what I think was the same problem - after reading Paul's "B&V" Spanner pages I came on the issue of the rotor button and replaced it with a red epoxy one. Problem solved.
B Dubuc

Hi Paul and others,

thanks for the info.,

Will hopefully have time to look this weekend, it is a bit away from my home in another garage without electrical light.

Will report back.

Ta,

Iwan
Iwan Jones

Hi Paul

Sorry have not been able to get to grips with the problem till today.

The engine tries to start sometimes other times it doesn't, just turns on the starter.

So followed your advice.

So as you said, I am: 'getting a spark when you touch the wire from the coil on the distributor body - that proves that wire back through to coil and the ignition switch is OK.'

Then I followed your: 1. 'What about when you touch that wire on the dizzie spade with the points closed? If no spark then, then the circuit through the cotton-covered wire to the points, through the points themselves, and what should be another cotton-covered wire from the points plate to the distributor body is open circuit.'

I did the above and got a spark at the spade and moved onto your next paragraph

2. 'If you *do* get a spark from the distributor spade with the points closed, then try again with the points open. If you still get a spark then the points are short-circuited by something'

...... I was getting a spark at the distributor, but none with the points open - fine I said & tried to start the engine again. It tried a bit then died.

I decided to go back to test 1. - this time no spark at dizzie spade with points closed! This may explain the intermittent starts. The problem is not evident all the time, or so it seems. So I did the next test stage you described.

'With the coil wire on the distributor spade, get another wire connected to earth and tap it on the points spring. If you don't get a spark now then the first cotton-covered wire is open-circuit.'

....... I got a spark on the points spring - so the next stage .......

'If you get a spark there tap it on the points plate. If you get a spark there then the second cotton-covered wire is open-circuit - if indeed it is present at all.'

....... I got no spark touching the points base plate or the plate below, while still getting a spark off the points spring and on the dizzy body.

Hope all this helps.

What exactly needs doing now? New points (even though they are new) and new base/moving plate (to which lower 'cotton' wire is attached? Is there something rubbing catching I can do? Funny how both dizzies are doing the same thing.
Iwan Jones

Iwan

A quick reading of your last post suggests either you've got the same problem with both distributors or the problem is else where. Any chance of a decent photo of the dizzy, a picture's worth a thousand words.

Bob Davis

Intermittent as you say, and I probably should have also said that when you get a failure condition, go back to the successful test you did previously and check if it is still successful. As well as redoing test 1 redo the test of tapping the wire on the distributor body. With erratic test results and two distributors I suspect the problem lies back through the coil and its ignition feed, which the aforementioned 'going back to a previously successful test' should reveal.

The only thing you can do with things like that is to hook up a semi-permanent 'monitor' such as a light bulb or voltmeter to various points in the circuit and see which ones show a loss of the correct condition when the engine cuts out or won't start and which ones still show the correct condition, and the fault is between them.
PaulH Solihull

This thread was discussed between 31/10/2010 and 28/11/2010

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