Further to my thread "Ignition Problem", thought we had found the problem and replaced the ignition sensor but still no luck! The coil,distributor cap and amplifier are all ok, the only odd thing is that the gaffrig tacho on the good engine sits on zero when switched off but the tacho on the bad engine sits about 1" below zero. Does this point to another problem?

We really need more info to help diagnose what it could be0. I would suggest checking ground wires. The tach's should "jump" when the ignition is turned on. As long as they zero-out when the ignition is turned on they're OK.

Also, check the actual ignitions as well.

Thanks for your suggestions. Discovered that the culprit is the rev limiter (Overspeed Assembly) - the little black box that hides behind the module marked Thunderbolt IV. Managed to find a new one here in the UK so hope to get to sea in a day or so. Have to say that I could not find this on any of the wiring diagrams for my set up or no mention in the workshop manual.

Glad you found it!