What could it be?

Don't forget, a 12v coil is actualy a 6v coil with a ballast resistor in series with it. While you are starting the engine, the battery voltage drops to 10v ish and the ballast resister is bypassed (with a relay) to compensate for the low battery voltage. Once started, the ballast resister is put back in series.
If you have a standard coil and a standard ballast resistor then feed it with 24v, you will be cooking both coil and ballast resistor.