[Paul Thompson 'Hooligan']

you're not running a wasted spark system are you? [img]/ubbthreads/images/graemlins/smirk.gif[/img]

if so - each cylinder will see a spark on each rev - compression and exhaust, which would double the tach output signal...

If you have a multimeter with an rpm reading available, clip it on the same wire feeding the tacho and see what it reads.

Let us know how you get on. [img]/ubbthreads/images/graemlins/smile.gif[/img]