I may be missing the point but surely electronic rev counters are designed for a specific number of cylinders? If the original rev counter was from a motorbike, it may have been designed for anything else but four cylinders. This could explain the calibration problem perhaps?
Good luck.

Location: Beautiful North Hampshire

I don't think it's safe to make any assumptions about the stickers. It could be simply that had some pre-stamped cases and made use of them.
In a PM JonE sent me a photo which showed one transistor. It would be good to know if there is another, as without two all conventional design ideas are out. I don't think it would be possible to make one with only one transistor that has a linear scale but I may be wrong.
Martin's point about the number of cylinders is valid of course but many aftermarket tachs have jumpers or suchlike to take account..In a conventional circuit these amount to what is a "coarse" alignment adjustment and there will be a fine one somewhere to tweak the indicated reading..which hopefully is linear across the range. The latter will normally be a factory adjustment.The Yazaki instrument does not not appear to have a jumper or wired alternative so may be designed for a specific number of cylinders ..or perhaps one of the pots seen in the photo might be for it...But that's a wild guess..
As JohnC says the circuit almost certainly requires three wires...B+...B- and the contact breaker feed. Working at all with the the later connection the only feed of power is difficult to comprehend.
But the 330 ohm being part of a crude conversion ..i'm not convinced..The ohms law calc would only be true for full scale deflection and it's anyone's guess what the rest of the range would look like (unless there's a shunt stabilising zener bodged in as well).
JonE error report based on a feeling could be improved upon if the car was taken for a run and the indicated revs noted at 10.20,30 40 and as high as possible above that (if it's safe to look and write at the same time :-)) Perhaps a tweak of one of those presets might be all that's needed.

@ John C. I triggered my 555 tach from a few turns on a ferrite ring through which the CB > coil wire passes. No problem with interference at all.. maybe because of the low Z seen by the 555?

Location: Nr Penzance

Martin - the back casing indicates "46CYL..." Could that be pertaining to the two variable controls under the rubber plug?

John, Roger - cant get it running yet, but managed to get better pics of the innards and the transistor. The two things on the back are variable resistors, or rather variable somethings.
I've put a whole load of pics in here, and the wording on the little cans and innards:
http://aus7in.wordpress.com?p=1425&shareadraft=571b823d3df79

Presumably if I have a signal that follows revs (of sorts), those variable thingummys are going to trim SOMETHING!

Hi JonE

I have had a look at all the pictures you posted. The Hitachi 2SC282 is a Silicon NPN switching transistor. The Sanyo SG005 is a medium power rectifier diode. The stripy things are resistors and the green things are capacitors. There are also a couple of smaller diodes. So you have a one-transistor circuit.

I an 99% certain that the two screwdriver adjustments are preset variable resistors. Since the blue wire connects to one of them, I suspect that the green wire connects to the other. You might like to check this as I can't tell from the photos. If this is the case, I am wondering if one wire is for 4 cylinders and the other one is for 6 - you just connect whichever is appropriate to the breaker points and ignore the other. The variable resistors allow separate calibration for the 4 and 6 cylinder modes. Maybe this setup was cheaper or smaller than providing a switch or links ?

The insulated studs are for the battery positive and negative. I suggest you try it out with the three connections, experimenting with blue or green to the contact breaker and trying the effect of tweaking the relevant variable resistor.

Cheers, John C.

Location: New Forest

great, thanks John. Will report back.
I think you are right that the green goes to the other variator.

What is interesting is that that blue wire is short, and on BOTH my unit and that of the chap on the Commer forum it appears to have been connected to the negative post (i.e. one didnt even see it when in it's cowl)

Not sure if this helps...

I have a Crypton revcounter on a Morris Minor sidevalve and the back has 4 terminals, earth, signal and 2 terminals that are bridged with a small strip of copper. The instructions state that you remove the bridge to turn it from 4 to 6 cylinders.

Location: Cardiff