Topic: CB650 RC03 odd tach cable gear ratio

[Turkenberg]

Hi everyone,

I am almost done restoring my CB650 RC03 (pic below) but I found a buzzer....
My tach reads super low ; like almost half of engine speed guesstimate.

Tried this tach on another CB650 RC03 and read nice.
Measured tach ratio with a drill, has approx. 4:1 ratio (the one in the pic)
My Engine serial is RC03E-xxxx so it's the stock engine. But I can see it has been opened before by someone.

From the FAQ

CB750 Speedo  2240:60  Tach 4:1
CB650 Speedo  2240:60  Tach 4:1
CB550 Speedo  2240:60  Tach 20:3
CB500 Speedo  2240:60  Tach 20:3
CB400 Speedo  2240:60  Tach 20:3
CB350 Speedo  2240:60  Tach 20:3

Just measured tach cable gear ratio on my engine and I have 20/3 from crank to tach cable..!
So it should be the wrong camshaft right ?

Upon disassembly how could I identify the camshaft and now from which model it came ?
I am guessing it should be CB550 or even smaller displacement (bc matching ratio),
But is there any mark or serial on the camshaft that will let me know where is my camshaft from ?

Thanks a lot, I am a long-time lurker and learnt a LOT on my bike (it is quite rare in my country), kuddos to everyone here!

[RAFster122s]

Measuring lift and duration, the later is a challenge, can get you an idea of the camshaft based on the numbers when compared to aftermarket performance cams...most of which are cut for the cb550. Odds are it is a cb550 camshaft and the cam gear has the different ratio used by the 550.

[Turkenberg]

Thanks,

I found the following pictures from here : http://www.chinonthetank.com/2016/05/cb650-cam-in-a-cb550/

From the attached pics  :
- on closeups, CB650 is left CB550 is right
CB650 has R10 - D marking
CB550 has R10 - K marking

- On overall view, CB650 is top CB550 bottom
Notice the slightly different pitch from the tach drive gears.

- On tach gear drives pic, same pitch difference is seen obviously.

I am eager to open it now Will try to keep you posted but should not happen until october...

[Turkenberg]

I jumped in yesterday and in-place camshaft has R9 and D marking.
Seeing the tach gear pitch it is obviously neither CB650 nor CB750, so one of the previous owners should have dropped-in a smaller engine camshaft.

Anyone would know ?
According to every posts I read, those marking does not really help identifying. I might have to take it out and measure lobes

[RAFster122s]

Cb550 tach gear ratio is the only choice I have ever heard of. Only minor clearance needs doen in a cb550 to fit a cb650 camshaft, so therefore a cb550 or cb500 camshaft is a drop in replacement. You merely lose the tach unless you go electronic or fit the cb500/550 drive gear and cb500/550 mechanical tachometer.