Changing my rear sprocket.

[xtopherburner]

I'm changing out my rear sprocket from a 46t to a 39t on my 82' CB750k to get better highway performance. The chain seems to be in good shape and both sprockets are fine. I have been told by a local shop that I need to replace the whole set but if I just take out a few links from the chain I think I will be fine. The old set is 530 pitch and so is the new 39t sprocket. Any thoughts?

[TOOLS1]

Chains and sprockets should always be chained together. If you change one component the wear pattern will be different, and cause the parts to try to wear together. This will cause them to wear even faster.
46 to 39? That is a really big jump. That bike is going to really SUCK in stop and go around town riding. I would try a 43 tooth sprocket.
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[xtopherburner]

Makes sense. I would never change a timing chain in my car without new gears. But as for the ratio, I'm trying to keep it under 4500rpms @ 70mph. Would an 18/43 tooth ratio do that? I don't want to make it undrivable around town but I would rather be slower off the line and not tax the engine so much on the open road.

[TOOLS1]

4500 RPM is not bothering that engine at all. What will bother it is bogging it down, and slipping the clutch to death trying to get rolling.
Just for reference here are the gear ratios for the different gear sets.
18-46=2.56
18-43=2.39
18-39=2.17
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[xtopherburner]

I am new to Japanese engines. At 70mph I am revving 5000+. Is it ok to run that engine like that for hours at a time?

[TOOLS1]

Not at all.
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[TOOLS1]

I just checked and the 46T came stock on your bike. A change to a 43T would slow it down a few hundred RPM. Check this site out http://www.vintagecb750.com/products/2/chassis-brakes/88/sprockets scroll down to the last sprocket #22-0315.
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[xtopherburner]

Cool. Everything you gave me is super helpful. I also found a great RPM calculator.  The last four fields are the only ones you need for a motorcycle and there is one flaw with the calculations: you have to plug in the tire RADIUS where is says to put in the tire hight.

http://www.csgnetwork.com/multirpmcalc.html

After playing around with that and checking out everything you said I decided to go with an 18/43 if I even change it at all.

[shinyribs]

On my '80 I had 18-39 gearing once. Only because of the Hurricane rim that was on the back. It was awful. And that was with a 17" sport bike tire.

Now, I'm at 18-48. 17" sport bike tire. I cruise 75 at 6500 and get 47-52 mpg. Rpms don't phase these machines.

[xtopherburner]

That settles that. I'm going to spend my time and money on better upgrades like a sissy bar and luggage rack and leave the drive train alone. Sounds it knew what they were doing when they built it.  Thanks.

[Rodgil]

I went from 18/46 to 18/40 on my 1982 CB750F. Fourth gear is now like the "old" fifth, so fifth is a good cruising highway gear. Anytime you like to ride it like it was before, go back to fourth. First is like the "old" second, only a problem if you regularly get stuck in very slow traffic. Take off, even uphill, is fine.