Last weekend I removed my instruments to glue up a crack in the case and paint the cover (matt black, of course). Since then I've noticed that the tacho doesn't always fire when I start the engine, but will jump up and start working along the way - this could be anywhere between idling at warmup, to 20-odd K's down the road. I don't know what causes it to start working, since I've tried taking it to redline (in and out of gear), cruising on constant RPM, riding over bumps and even turning it all off and back on again. It's happened once before last winter, when I'd also had the instruments off. But after about a month it just started working again and I forgot about it. I don't really "use" the tacho apart from setting the choke on a cold morning, but it just annoys me - any ideas on the cause?