MG Car Repair: No Spark on my 1971 MGB, secondary windings, distributor shaft

Question
I'm not getting spark on my 71 MG. Did the basic spark check   (Pull coil wire) Got nothing.  Checked point operation and they are opening and closing ...I have 13.25 points coming to coil...Replaced coil....nothing..Would bad condenser cause this??

Answer
Hi Jeff,
It could be bad but so can a dozen other things be wrong too. You are not making the correct tests to know what is wrong and never replace a part until you know what is wrong.

The coil is powered all the time that the key is on. The other small wire from the coil (primary)goes to the distributor and in to the points which ground the coil when the points are closed. As the engine rotates the distributor shaft rotates and opens the contact points. A electromagnetic field has built up in the coil while the points were closed. When the contact points open the field has no place to go so it transfers to the secondary windings in the coil. Because a coil is just a transformer, the amperage drops and the voltage increases from the original battery voltage to about 35,000 volts. This high voltage charge seeks the closest place to jump to ground and it finds that at the spark plug. But to get there it must travel down the coil wire to the cap and in to the rotor and out to a peg in the cap that is lined up with the rotor so it jumps that small gap and travels down the plug wire and into the spark plug where it can jump the plug gap to ground. (ignition spark)
When this high voltage/low amperage charge transfers to the secondary in the coil and is discharged at the spark plug, all of the charge can't make the jump so some of the charge is left in the secondary windings in the coil and is still seeking a ground so that charge transfers back to the primary windings at about 300 volts and heads towards the contact points to try to jump that gap to ground (which would burn the points) So a condenser is added to bleed off that charge.

You looked at the points opening and closing but you didn't go far enough with the test. You checked it mechanically but not electrically. You should have used a test light or volt meter to confirm that when the points were open and the key on, you should see battery voltage on the points arm and spring. Then rotate the engine until the points are closed and check again and you should see no voltage on the points arm or spring. This is the test of the primary system.
A simple and rough test of the coil would have been to open the points (key on) and confirm power at the points arm and spring and take a ground of any kind (most mechanics use a screwdriver) and short out the points spring or arm while holding the end of the coil wire about a quarter of an inch from any ground.

If you have that but nothing at the plug wire then You need to look closely at the inside and out side of the cap for carbon tracking or dirt and then hold the end of the coil wire close to the center of the rotor and spin the engine or use the screwdriver to make the coil fire. If you can jump a spark to the center of the rotor at all, it is shorted internally.

Howard