I wanted to see how fast I was actually going - and the RPM of the engine. I was not helped by the inaccurate rev counter and speedo. So for anyone else wondering...

 

First up I needed to calibrate the rev counters. The MERA units are factory calibrated at 1000, 4000 and 6000 RPM. They have internal capacitors that age and need replacing. Then they need adjusting via an internal potentiometer. Once set, they are accurate within 100 RPM and can be spot on at 1000/4000 as specified. The PAL units are the same but suffer with a little 'rise and fall' lag - so MERA are more accurate.

 

The speedometers are calibrated by adjusting the pointer spring pre-tension. Setting for this means removing the needle so I added some small dots to the glass face. Pretty much +10% out across the range. 

 

For the 5 speed 130, 4th is 15.90 mph/1000 RPM and 5th is 20.70 mph/1000 RPM

For the 4 speed 105/120 4th is 16.65 mph/1000 RPM

 

Just sharing, as I could not find this info out there anywhere and the contemporary road tests show the 4 speed as 15.5 mph/1000 RPM, which is wrong.

 

Photo is of PAL rev counter accuracy after calibration. Before it was about 200 RPM fast.

Well, FWIW I know of some tuned 130 5-speeds that were revving out at 120mph in 5th, so that was near enough 20mph/1_000rom.

13 minutes ago, KenONeill said:

Well, FWIW I know of some tuned 130 5-speeds that were revving out at 120mph in 5th, so that was near enough 20mph/1_000rom.

 

Heard that too, but hard to believe! 

 

Couple of other things. If you use the earlier 200 RPM divided rev counters in your 4 speed you can use them for accurate speed measurement - if you remember:

 

1800 = 30

2400 = 40

3000 = 50

3600 = 60

4200 = 70

 

Also. Test your temperature gauge. With the engine running (13.8 V) short the sender wire to ground through:

 

137 Ohms = 40C

23.5 Ohms = 90C

10.2 Ohms = 120C

 

Out of 5 units I only found one accurate one! some were 10C out. Worn out from use and heat.

This is inside the MERA rev counter. The components marked with a dot need replacing. They wear with age.

 

You can drill a hole in the plastic case to adjust the setting whilst assembled. PAL unit already has the hole (metal case).

 

Note that this is the rev counter especially produced for the 105 (GL) - has different yellow and red bands. Rare item!