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MG Midget and Sprite Technical - Midget Mk2 Tacho

Good evening learned scholars!

I am helping someone out with their new acquisition, a '64 Mk 2 midget. Some aspects of which are a little out of my comfort zone.

For a start, what on earth is going on with the engine, why are the carbs on the wrong side? Actually... hang on... carbs? Where is the ECU?

I digress...

The car is a +ve earth (or at least that is how the battery is attached and what it says on the tach! Who knows... the wiring is a real mess). The tacho is the style with the loop-de-loop of wire on the back. All of which I think is normal and correct.

Whilst driving, the tacho goes a bit mad, swinging wildly and obviously greatly over reading. Does anyone know what might be wrong?

Cheers,
Malc.
Malcolm

There is a capacitor in the tacho which causes those symptoms, or is there electronic ignition fitted?
Rob
MG Moneypit

I believe it is the one arrowed.
Rob


MG Moneypit

Rob, agreed. If changing C2 it's also worth changing C3 while you're in there. RV4 may have an intermittent "slider". Spray it with switch cleaner. Also, if you have a signal/ function generator, you can then calibrate the tach.
Bill Bretherton

Thanks both. The external wiring is so simple I had assumed it was something internal, but didn't know where to start.

Rob, do they not work with electronic ignition systems? Even one designed specifically for a positive earth car?

Malcolm

Malcolm, some electronics upset RVI type especially if the current drawn on the primary is reduced. It's an age thing, as we all know.
Rob
MG Moneypit

I managed to get hold of the tach' and replaced C2 with a new 0.22uF cap.

It is still wildly over reading and fluctuating (3500 rpm at idle).

Bummer.

I didn't have any of the other bits in my electronics spares so didn't bother changed anything else.

Looks like I need to have another go.

At least I did the hard bit... unstick ing the bezel!
Malcolm

Malcolm
It's worth checking RV4 for continuity through the "slider" (centre connection) as this will affect readings and is also used for calibration. "Twiddling" it i.e. moving adjustment rapidly backwards and forwards may have some effect.
Bill Bretherton

Thanks Bill. Will try that next time I get chance.
Malcolm

This thread was discussed between 29/08/2020 and 06/09/2020

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