Rev counter misbehaving with uprated electronic ignition

[Green411]

This is interesting. I'm in the process of uprating my 411 S3's Pertronix Ignitor 1 to an Ignitor 2 which senses dwell and more importantly doesn't get upset or fry if I leave the ignition on. In other words it's got more electronics in the module.
I fitted the Ignitor 2, connected red and black leads to the coil, fired it up and re-timed it, and hey presto: a misbehaving rev-counter. The motor ran fine but the tacho read 1500-2000 rpm at a 750 rpm idle, ok and about right but with some flickering on a test drive, and shot up to 3-4000 rpm whenever I put the brakes on. Weird or what?
I note in the Ignitor 2 instructions that the red doesn't have to be connected to the coil +ve, it can also go to a switched 'live when cranking' feed from the fusebox, which should get rid of the problem. Haven't tried it yet as I put the Ignitor 1 back on in a huff, but will update when the Ignitor 2 is fitted again.
BTW the Ignitor 1 was a big improvement over the points, I had quick, helpful responses when I emailed Pertronix Europe, initially about how long I could leave the ignition on without starting before Ignitor 1 damage occurred - answer was only 15-20 seconds, which I hadn't realised when I first bought it.
I must do my research thoroughly, I must do my research thoroughly, I must do my research thoroughly.....