SOHC.co.uk Forums > CB750

Speedometer

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davefirestorm:
Would consider a rebuild/recalibrate but would want to keep original clock

Nurse Julie:

--- Quote from: davefirestorm on March 02, 2020, 08:21:42 PM ---Would consider a rebuild/recalibrate but would want to keep original clock

--- End quote ---
Kent 400 (Peter) refurb and overhauls your own clocks, so you will still have the originals. He's done loads of sets for us, always does a great job.

Lobo:
Dave, you could consider re-datum’ing the needle to the spindle; I did this with my old Kombi - with success. Basically, you want to decrease the tension in your return hairspring, so instead of reading 60mph, the needle goes to 65mph.
It’s a significant job, whether you feel it’s worth it is another thing.
(1) disassembled the speedo, and move the needle to 60mph.
(2) Either (somehow) clamp the spindle here, or mark / tag the spindle. (or see next para below as an option)
(3) remove the needle - two opposing teaspoons under the head and prised up together (with something under them to protect the face) (it’ll pop off with a ping, don’t do this near open drains 😱)
(4) keeping the shaft in the same spot replace the needle, but now set to 65mph.

The problem is it’s a real pain to redo if still not right. An option is an electric drill to drive the speedometer at a REPEATABLE constant rpm. (1) BEFORE you disassemble check the indicated mph, after you have re-datum’d (2) double check you’ve ‘added’ an indicated 5mph. Reposition the needle if necessary, check again... and reassemble the speedo.

As a final comment, your speedo may not not under-read linearly, and so choose a datum mph that is most important to you. eg I daftly recalibrated my Kombi to read correctly at 80kph.... giving large errors at town speeds which I guess, are more important. (meaning .. I had to tear it all down again, aargh)
Good luck if you decide to go ahead,
Simon.

Addendum... is your speedo constantly under reading 5 mph, or only at 60? The point is, if it (eg) overreads 5mph at 30, and reads correctly at 45mph, you probably shouldn’t be doing anything to it!

davefirestorm:
Lobo,my dexterity is not too good ,arthritis and  dyspraxic ( according to my wife lol )I’m hamfisted and very clumsy😂😂so wouldn’t risk tampering with it myself but thanks for your input.
Julie,May entrust to Kent400 thanks

Bryanj:
The big book(constuction and use manual) for ALL road vehicles used to say speedometers had to be acurate tp +or- 10% but on my recent naughty boys course they told it is now only +10% so it will red between dead on and +10% or between 60 and 66 for a real 60.

That is what plod have to work to

Trucks by the way have to be +or- 2kph at speeds over 20kph

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