Erratic speedo

[daveg]

As Jamie says the arm drives the oddometer and does not affect the speedometer function so if you aren't bothered about measuring distance traveled you can use it with the arm disconnected.

What I think happens is that over the years dirt, corrosion or possibly dried up lubricant between the oddometer number wheels makes the mechanism stiff and causes the problem. I took mine apart, cleaned all the bits and re-assembled with a light oil which cured the problem.

Tony Press mentioned a local instrument maker who could repair it which is probably the easy option.

[Ron Hayhurst]

Have a look at the article on Magmo Speedos on the Bristol Club site at ba7c.org In the technical section scroll down to Speedometers and see how to free up the slot through which the ratchet arm moves- plus a few other aspects
Ron Hayhurst

[Michael Aldred]

Here is a photo of the ratchet arm bearing against the cog. Every rotation of the cog produces the “yawing” of the speedo needle from its correct speed indication.

Thank you to the additional replies. I think I will try the professional instrument maker option first but if all else fails I will strip the mechanism down to clean it and reassemble.

   

[Henry Harris]

The mazak casting degrades and swells with age and tends to tighten on the ratchet arm.
Remove the ratchet arm and ease the side clearance with a needle file or a nail emery board.

[Michael Aldred]

Hello again,

We have a diagnosis! The problem was the cable as suggested by Chris KC. Len Kershaw who is the instrument maker of repute mentioned by Tony Press (and who is also President of the AUSTIN 7 Club in Melbourne) was able to establish this so further work on the speedo was not required. I have a cable on order so once that arrives I will be able to see that I am speeding. 

Thank you to all those who helped. 

Michael