Derek;

I don't believe that rev counter connection on a 240 ignition amplifier will work with the GT Tach because it is probably a (single wire) voltage pulse intended to work with later voltage sensing Tach circuits.

Kai;

...but a GT Tach might still be made to function with a 240 ignition amp...the GT Tach uses the same Smiths, current sensing, magnetic coupling ONLY, circuit as the 1800, so its' two white wires must be in series with Ignition Coil primary (and in the correct polarity...direction of current-flow matters!) I would advise to wire it in series with Ign Coil primary and try it (in both directions).

Cheers