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'81 CB750C - Can a Feller Get It Running and Riding After 18 Years?

I like this topic. I keep searching for 6th year on my 750k. My speedo goes to 80mph but I've seen 750c speedometers go to 120. What is that about?
Secondly, if you decrease the rear sprocket by a tooth, 1) why shorten the chain? and 2) is the loss of lower torque noticeable?
 
Secondly, if you decrease the rear sprocket by a tooth, 1) why shorten the chain? and 2) is the loss of lower torque noticeable?
Shortening the chain.......That's done so that the chain can still be adjusted in the axle slot. I know what works with the CB750C, other models may be different, so......assemble with the chain length that you have and go from there, just so the chain can be assembled and still have the right movement in the slot for adjustment, which may mean that the chain needs to be shortened. If you have trouble taking off from a standing start then the sprocket gearing is unusable. One tooth from stock is usually usable on most bikes and unnoticable on takeoff. Now, changing the front sprocket can cause problems with clearance in that area, especially when using an O-ring chain on a bike designed for non-O-ring chains, like busted crankcase or busted oil pump on these bikes. So.......stock front sprocket size is best. Now, don't go shortening a stretched out chain, replace it and the sprockets.
 
Make sense, thanks.
Two follow ons: why the speedo difference between the two bikes, and does is the ideal DOHC chain with o-rings or without o-rings?
Thanks.
 
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