Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: davefirestorm on March 02, 2020, 06:45:19 PM
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Speedometer on my 1975 750F USA spec reads incorrect speed,example it shows 60mph but actual speed shown on Sat nav is 65mph.Would like it to read accurately,wonder if it’s been fitted with wrong clock? What numbers should it be showing on dial to identify it ( I can’t look now as Bike stored 20 miles away ) Thanks in advance😃
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My 76 750F speedo is fast by 7 mph at all speeds, I just work around it, after I rebuilt it I was following my wife through Liverpool and though she was taking a chance with all the speed cameras but she said that she was keeping to the limits, so I did some checking against the car and realised why keeping to the 30 mph limit was so difficult.
Cheers
Dennis
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I keep to speedlimits these days ( sign of getting old lol ) would hate to get a ticket If I was to forget lol.Incidently my 42 year old Hondamatic read dead on accurate.
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My 400/4 speedo seems very accurate. It gets checked every time I ride past the speed cameras on the Perimeter road on the way to work at Heathrow but then my clocks were refurbished by Kent400 of this forum.
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I have never on any vehicle owned seen the correct speed, alwaysup to 5mph in it , luckily when they show 70 your actually doing about 65-67
Sent from my SM-G950F using Tapatalk
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Would consider a rebuild/recalibrate but would want to keep original clock
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Would consider a rebuild/recalibrate but would want to keep original clock
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.
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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!
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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
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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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Having spent what seemed like an age learning Construction and Use, as I recall it, there is a specific tolerance for over reading, but none at all for under reading......that is, your speedo should never indicate you are travelling slower than your actual speed.
That is why your sat nav will invariably show your true speed as a few mph slower than the vehicles speedo......I've never actually come across a speedo that under reads.....bit of a risky thing to have these days....so that merits sorting out.
Mick.