MG Car Repair: ignition fault, ballast resistor, rotor arm

Question
QUESTION: Hi,
I have a 1972 MGB GT i cannot get a spark from the coil. I have replaced the coil, condenser, and points. The coil has a white/black wire to the negative side and a plain white wire to the positive side.
With the ignition on i get a feed to the plain white wire to the coil, the positive side also has a wire attached which goes to a "Dubilier" electrical component that looks similar to a condensor only with a plastic body. Not sure if this is some sort of ballast resistor os radio suppressor,either way i can disconnect this and it make no difference to the coil output.

Thanks,
Mark.

ANSWER: Hi Mark

The small cylindrical component attached to the coil + terminal is a radio suppressor.  You can safelv remove it to ensure that it is not causing your problems.

I suggest that you remove the distributor cap, and check all the following:

1.   You haven’t forgotten to re-fit the rotor arm, have you?  (I have done it).
2.   Inspect the carbon brush for excess wear & lack of spring tension.
3.   Visually inspect the white wire with a black stripe that goes from coil + to the distributor.
4.   Check that the wire to the condenser is not earthing.  
5.   Check that the thin flexible wire inside the distributor has not snapped.  This earths the action plate and its failure can cause a lack of spark.
6.   Turn the engine by hand until the points are fully open and then check the points gap.
7.   Now turn the engine until the points are shut.  Switch the ignition on, open the points with your fingernail  or a toothpick, and look for a small spark between the points.  (This is easier to see inside a darkened garage).  If there is a spark, then the coil is working OK.
8.   Pull the central HT lead from the distributor cap, and carefully hold it one millimetre above the rotor arm (beware of shock).  Get someone to turn the engine over with the starter while you check for a spark just above the rotor arm.   There should be no spark.  If you can see a spark, then the rotor arm has failed.

If all this fails, try replacing the spark plug leads.  They only have a service life of a few years - after that, they become unreliable.  


---------- FOLLOW-UP ----------

QUESTION: hi, earth wire in distributor was broken, i have replaced it and also the feed from the + to the distributor as it looked very brittle.

Should the white wire from the ignition go to the - terminal on the coil? it does currently and i get 12.65v to it with the ignition on.

Thanks,

Mark

Answer
Hi Mark.  You have the wires to the coil the wrong way round.  The 1972 MGB is wired for negative earth, so the power (white wire) should go to the (+) terminal, and the wire to the distributor (white+black wire) should be attached to the (-) terminal.  12.65 volts is a good voltage.

Make sure that the earth wire inside the distributor is not too stiff, it must not stop the action plate from moving freely.  You can check it by sucking on the vacuum advance pipe.  Use a clean length of screenwash tubing for hygiene reasons.  Suck on it and you should be able to see the action plate move.