Here is a thread related specifically to tachometer repairs.

gt orphanage

otto instruments did mine in brissie. for some reason the are back to front when you refit them if the car will not start swap the 2 wires.

And fro Gerry;

I had my Tacho's guts pulled out,replaced with modern,same front,by Geoff Wright 0755206509
works from home Palm Beach,100 or 150$ left it with him one weekend pickup the next,very happy
with the results Gerry

What was the deal Gunna? My tacho is slow...haven't investigated the issue too closely at this stage though.

I found this on the web. You can adjust the Tacho at a fixed speed using a Fleuro tube light (read right through it though, for our 50Hz mains.

Bellett Tachos have two adjustments on them. I don't know what they both do but judging from the spare one i have here on the table, one of the adjusters will be close to one end of its range. The other one, somewhere in the middle. I would think the latter is the one to adjust and see what happens. You will need access to the adjustment holes though so the Tacho would need to be removed but still connected of course. Its only a single point (rpm) adjustment though but might suffice.
My Sport Tacho is fast so i need to do this also !

There is a way to check the accuracy of a tachometer using the highly accurate and stable frequency of AC power supplied from any source of commercial shore power. The concept is to use the AC line frequency as the measuring standard. The only tools needed for this calibration check are a

piece of tape and a fluorescent lamp which operates from commercial AC line power.

A fluorescent lamp is actually a gas-discharge lamp with the interior of the glass envelope coated with a light-emitting phosphor. When the gas within the lamp is ionized by alternating current it emits pulses of energy. One pulse occurs for each of the voltage excursions of the AC waveform. For the 6O-Hz power common in North America, there will be 120 such flashes per second, 60 positive and 60 negative. The pulses of energy created within the

lamp excite the phosphor coating, which in turn emits visible light. Because the energy driving the lamp is not continuous, the light emitted is not continuous. The fluorescent lamp emits 120 pulses of light per second, but the human eye's persistence of vision makes us think the light is always on.

We can use the pulsing light output of the fluorescent light as a very accurate measuring tool with which to check the calibration of the engine tachometer.

First, obtain access to the front of the engine. Place one piece of white tape on the face of the large pulley mounted on the engine's crankshaft

(usually this is the largest pulley in sight). Illuminate the front of the engine with light from the fluorescent lamp. Run the engine at 1,800 rpm, as shown on the tachometer. If the tachometer is accurate, four stationery, or very slowly moving, white marks will appear on the face of the pulley where the tape was placed. If the tachometer is inaccurate, the tape marks may be rotating in either direction. Adjust the throttle until the four tape marks

appear to stand still. Note the tachometer reading. If the difference between the reading and 1,800 rpm is at all significant, look for a small

adjustment screw on the back or within the body of the tachometer. Turning this screw slightly should make the indicator needle move to exactly 1,800rpm. Increase the engine speed to 2400 rpm. At this speed, only three tape marks should be visible on the crankshaft pulley. Repeat the check of the tachometer reading and, if necessary, readjust the tach.

The basis of this stroboscopic speed calibration is quite simple. At 1,800 rpm, the engine is turning at 30 revolutions per second. The lamp is

flashing at 120 flashes per second, or four flashes per engine revolution. Therefore, if the engine is turning at exactly 30 revolutions per second the tape mark will appear four times, with each apparent tape position 1/120 of a second or 1/4 revolution apart. When the engine runs at 2,400 rpm there will be only three light flashes per engine revolution. If the boat is in a country where the standard AC power frequency is 50 Hz, the check speeds would have to be 1,500 and 2,250 rpm since the light would flash 100 times per second.