2 cylider = 1 fire/ ignition pulse per revolution
4 cylinder = 2 fires/ ignition pulses per revolution

If you're unsure you should test it against a known frequency signal. My oscilloscope has a square wave output. Another way I've done it (requires great care) is to use mains supply over a voltage divider (20:1) to get a 50Hz 12V signal, on a 4 cylinder that's equivalent to 1500rpm, and on a 2 cylinder 3000rpm. If you're not absolutely sure what you're doing don't do that cause it can kill you if you bugger it up.

If it's not a static factor of 2 then there's something wrong with the tach.

Hope this helps and good luck with it.

I think you're onto something with the 2cyl/4cyl factor, but it might be the other way 'round i.e. out by a factor of 0.5. Thinking about it, the tacho reads about 3,000rpm when I'm doing about 60km/h in 2nd gear (4:37 final drive), but the engine sounds more like about 6,000rpm.

Cool thanks for that, I'll have a read. I'm definitely no electronics engineer, but I've got a pretty sound understanding of electronics. If it does prove to be over my head, I have an old dash-top tacho that has the "4-6-8" cyl switch on the back. I can always pull that apart and see how it works.

If it's for a 2 cylinder it would be out by a factor of 2, i.e. ready twice as many revs as it's actually doing assuming it's accurate.

How good are you at electronics? I made a circuit using a D-type flip flop to half or quarter the ignition signal to drive my cruise control unit. If you're pulling off the coil voltage signal you'll need to condition the signal first. The Megasquirt build guide has some notes on setting up the input circuit for tach signal. Basically you need to get the signal into a form the flip-flop will accept, use that to divide the signal by 2 and switch the tach with that output.

If that's all a bit over your head I can't think of anywhere you can buy one, mine cost about 3 bucks in components and several hours diicking around.

I have a spare tacho out of my Honda Z360 (it's a Nippon Seiki electrically operated) that I've put into my Wag, but of course due to it being for a 2cyl engine, the reading is exponentially inaccurate. That is, at idle the reading is accurate (enough) but the higher the rpm, the more inaccurate it reads.

I'd really like to keep it for it's 9000rpm redline and max. reading of 10,000rpm. Plus, it's a nice 100mm/4" (approx.) diameter which makes it easy to read at a glance, but not as ostentatious as those common aftermarket "monster" ones. My question is, would it be possible to "rewire" or recalibrate it to suit a 4cyl?