NickinSussex

Thanks for your replies. I am investigating the problem without trying to fire up the engine - just to confirm, when turning over with the starter the strobe light on the #1 ignition flashes at the same frequency as when I put the strobe light on the coil lead (ie 3 times per revolution). As I said, if I then take the cap off the spark leaps from the central rotor contact in the cap to the #1 ignition contact inside the cap - about 2 cm or so ! However, the sparking returns to normal when the 12v wire from the starter solenoid to the ballast resistor is removed ie when the coil is operating in its normal 6v mode. I have checked the resistance across the coil high and low voltage sides - it is about 1.6 ohms and about 5.4 kilo ohms. The ballast resistor also has 1.6 ohms resistance. The battery produces 12v, the voltage dropping a volt or 2 while the starter is engaged. I have tried an old set of ignition leads and still the problem exists. As I said previously, one of the first things I did was replace the distributor cap with a new one. I am trying to get my head around the physics of the situation which suggests that much more than double the energy is being produced by the coil when the coil voltage is increased from 6v to 12v ! I guess the next thing will be to buy a replacement 6v coil and see if that cures the problem - there can't be many other things to try changing! Any more thoughts would be appreciated.

Hi there I have a newly rebuilt GT6 mk3 with a 6V coil with a ballast resistor. An Accuspark unit is installed. When the starter is engaged the spark plug to #1 cylinder fires every time the coil fires (ie not once every 2 revolutions, but 3 times every revolution) - checked using the strobe tester. If I move the strobe tester to other ignition leads, the same problem occurs on the other cylinders. You can hear sparking inside the cap. When I took the distributor cap off , with spark plug #1 connected, the same thing happens - you can actually see sparking from the central rotor contact in the cap to the #1 ignition contact. I replaced the cap with a new cap, and the same problem occurs. The system has the standard feature where 12v is switched to the coil when the starter is engaged, but reverts to 6v when the starter is disengaged. I removed the 12v connector on the starter solenoid and engaged the starter. This time the #1 spark plug sparked normally (ie once every 2 seconds), so it appears that when 12 volts is supplied to the coil an immense amount of energy is being produced causing the "crossfire" inside the cap. With 6V everything is okay. I have been racking my brains as to how this could happen. The car ignition system was perfectly okay last year and the only thing I have done over the winter was rebuilding the carbs. I could try replacing the coil, but I though it worth asking the community first about this problem before committing to the purchase. Any ideas as to what could cause the problem would be appreciated. As an engineer I am flummoxed !