SOHC.co.uk Forums > CB500/550
Partially Restored 74 CB550
McCabe-Thiele (Ted):
Would the sale of a 750 tacho not cover the cost of a 550 unit?
Might be Kent400 could swap some internals.
McCabe-Thiele (Ted):
Two wheel spares have loads listed.
https://www.ebay.co.uk/itm/334629151614?hash=item4de973a37e:g:3bwAAOSwjkRjc8PU
davidcumbria:
Thanks Ted I have contacted kent400 to see what he thinks. I suppose the overall ratio is a combination of the cam drive ratio and internal gear ratio in the instrument. If he can convert my 750 tacho saves having to do a buy and sell and my tacho is in good external condition.
McCabe-Thiele (Ted):
Useless factoid.
BLMC Mini Smiths Speedometers had a number in very small print on the clock face that we used to refer to as the Wey Speed. This digit was about 6 digits long it was some sort of standard for the calibration. If the Wey Speed was the same for two clocks they were interchangable even if the face markings differed in terms of the maximum speed.
Sometimes a silver faced clock from an old Morris Minor matched that of a Mini so you could switch them knowing they were accurate.
davidcumbria:
So Kent 400 has kindly got back to me and clarified there is no gearing inside the instrument just a spinning magnet . This means the 4:1 ratio 750 and 6.7 :1 ratio 550 is established at the camshaft take off. Looking at the parts list the tacho drive gear spindle is completely different between the 750 and 550 as you would expect so it is highly doubtful they are interchangeable. Looks like my only way forward is to get hold of a 550 tacho and have that refurbed which I will likely do at some point.
Fitted a seat trim with the prepared ends yesterday which involved drilling 12 holes in my seat pan. Very fiddly but got there in the end and happy with my seat after a shine up with bike sprits spray cleaner I heard about on here. So cosmetically that’s it for now 😎
Tested my plug caps and got 13k , 14k, 15k, and 35 k ! Got a replacement on order for the outlier and will see what that measures on my multimeter before replacing all of them. Bike fires up on the button and no indication of problems at higher revs. The plugless leads for both coils measured 15k., running Denso non restrictor plugs. Kind of shows that the system is pretty robust though some think that the high resistance will shorten coil life. Will play safe and have 5 k caps all round if my meter isn’t trying to fool me.
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