[Felix]

My 2 cents.

I think the later tachos, such as those on the 916 and SSie bikes, are electronic ones using a stepper motor. These are powered by a 5 Volt signal from the ECU. The carbed bikes, I believe use a analogue 12 Volt driven tacho. If you simply try to hook up the 916 type to a carbed bike on the primary of the coil (or the output of the ignition relay), it will over drive the tacho, which in turn will sap the output to the ignition coil. This would explain why Alan's bike only ran on one cylinder. As a simply, and I think safe experiment, you could try to put a simply resistor divider between the relay output and the 916 tacho input, say a 7.5kOhm resistor from the relay output to the tacho input and a 4.7kOhm resistor from the tacho input to ground.

Might be worth a try.