Topic: question for the gear head experts: 650 cam in a 500 engine and tachometer ratio

[zenocchio]

When I bought my bike the PO told me that he put a 650 cam in it, the engine is a 72 cb500.
i never paid too much attention until today, while going on a highway. I was doing 100km/h (60mph) and i was running at 8000rpm. I was in 5th gear at about 1/3 throttle. I felt I could go way more, and i gave up at 9500rpm at about 110km/h with a lot of throttle still.
that made me think that my tacho is a bit off.
at first i thought it was the new aftermarket dime cycles tacho (1:7 ratio) but it should be good on the cb500, and then i remember the 650 cam.
I have no idea if the tacho gear is a 650 or original 500, so my question is:
what is the ratio in the 650 cam? what is my new red line? can I feel comfortable going up to 12000 rpm in 5th gear? if the cam is actually a 650 (how can i check it?), can i just change the tacho gear?

tough questions, i know, but I'm sure someone out there have been here before. 

[flatlander]

with the 650 cam, your tach will show about 2-3000 rpm more. maybe someone else knows the precise conversion but that's about it, as far as i remember. and they're not that accurate anyway

the tach drive gear is different. you could get that and a tach from a 650 or electronic tach.

[SOHC4 Cafe Racer Fan]

with the 650 cam, your tach will show about 2-3000 rpm more. maybe someone else knows the precise conversion but that's about it, as far as i remember. and they're not that accurate anyway

the tach drive gear is different. you could get that and a tach from a 650 or electronic tach.

+1. It's my understanding that you can get an accurate rev reading by swapping in a 650 tach drive or by using an electronic tach.

[dave500]

what sprockets have you got?17 and 34 is standard for 500s

[trueblue]

with the 650 cam, your tach will show about 2-3000 rpm more. maybe someone else knows the precise conversion but that's about it, as far as i remember. and they're not that accurate anyway

the tach drive gear is different. you could get that and a tach from a 650 or electronic tach.

+1. It's my understanding that you can get an accurate rev reading by swapping in a 650 tach drive or by using an electronic tach.

If you use an electronic tach, you don't need the tach drive.  On my 650 I removed the gear and made a bung to push in the top to seal it up.  I no longer needed the gear there spinning round so I threw it into the parts bin in my shed.  IIRC the 650 tach drive is 4:1, which would explain about 8000rpm at 100kph.  I think the 500's normally sit about 5000rpm at that speed, but I don't own one so I can't be certain.

[zenocchio]

thank you all for the reply,
i have the regular sprockets, 17 front for sure and 34 (i believe) in the back.
what is the behaviour of the bike at the max rpm and max speed in 5th gear?
can i safely say that with full throttle in 5th gear the risks of red lining are limited?

[SOHC4 Cafe Racer Fan]

thank you all for the reply,
i have the regular sprockets, 17 front for sure and 34 (i believe) in the back.
what is the behaviour of the bike at the max rpm and max speed in 5th gear?
can i safely say that with full throttle in 5th gear the risks of red lining are limited?

Assuming your bike is stock, it will likely feel wobbly at max speed.  I highly recommend that you replace your steering bearings and wheel bearings with sealed roller bearings (All Balls Racing and others), and that you replace the stock plastic swingarm bushings with brass ones.  You should also make sure that your brakes are properly serviced so you can stop the bugger.  I would advocate at a minimum servicing the master cylinder, replacing the brake fluid and bleading the lines.  I would also recommend upgrading your rubber lines to stainless steel braided lines. 

[zenocchio]

thank you all for the reply,
i have the regular sprockets, 17 front for sure and 34 (i believe) in the back.
what is the behaviour of the bike at the max rpm and max speed in 5th gear?
can i safely say that with full throttle in 5th gear the risks of red lining are limited?

Assuming your bike is stock, it will likely feel wobbly at max speed.  I highly recommend that you replace your steering bearings and wheel bearings with sealed roller bearings (All Balls Racing and others), and that you replace the stock plastic swingarm bushings with brass ones.  You should also make sure that your brakes are properly serviced so you can stop the bugger.  I would advocate at a minimum servicing the master cylinder, replacing the brake fluid and bleading the lines.  I would also recommend upgrading your rubber lines to stainless steel braided lines.

that is a good advice,
safety first. to make you feel better, I can tell you that the bike is fully serviced, new brake ss lines, new upgraded mc rotors and pads. the steering is as new, and the swing arm bushings have been replaced.

I'm riding an old mad'm and definitely i don't want to get hurt.

[Scott S]

 The 750 tach and the 650 tach are the are ratio. Depending on the year of your bike, you might be able to find a 750 tach that matches your speedo.

 The flash accentuates the slight color difference. In normal light, you'd never know that wasn't the stock 550 tach.

[zenocchio]

The 750 tach and the 650 tach are the are ratio. Depending on the year of your bike, you might be able to find a 750 tach that matches your speedo.

 The flash accentuates the slight color difference. In normal light, you'd never know that wasn't the stock 550 tach.

I have a set of newer gauges, so no red line issues (at least on paper)

If I am swapping out the tachometer gear with a 650 one will I get an accurate reading? Does someone have an extra one?

[zenocchio]

Forgot the pic

[SOHC4 Cafe Racer Fan]

I don't know.  I ditched the mechanical drive for a direct rpm output from my Dyna 2000 to my electronic tach/speedo gauge.

[trueblue]

The 550 tach drive won't fit with the 650 cam, if your tach is working you already have the 650 drive.  You need the guage to match the drive, either a 650 or 750 gear will do

If you're after something a little different I went with speedhut electronic guages, weren't that hard to fit up, I just used a set of cheap chinese guages as surrogates, there is a link to a full write up in my signature.  The only thing I would do different if I were to do it again I would go with the GPS option for the speedo.

[dave500]

ive always liked that set up of yours Andrew,its clean and looks almost factory,they are easily read aswell,i hate too much gizmo in instruments!you replaced those rusted screws in the fuse holder yet with stainless?

[trueblue]

ive always liked that set up of yours Andrew,its clean and looks almost factory,they are easily read aswell,i hate too much gizmo in instruments!you replaced those rusted screws in the fuse holder yet with stainless?

Not yet Dave, I'll get to it one day, probably when I get round to painting the Honda on the cover white, I've only been meaning to do it since I put the bike on the road.  The main thing when I did anything to the bike I wanted it to look, to the untrained eye, like it could have come from the factory like that. 

[zenocchio]

good suggestion about the gear.
I guess I should have got a different ratio tach then when i replaced my old instruments. What is the ration on the 650?
is anybody here able to tell me what my new red line is now on the 500 tach?

[alacrity]

+1 what true blue said.  Different spline pattern. Cb550 tach drive won't mesh with
Cb650 cam splines


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[CBGhia]

good suggestion about the gear.
I guess I should have got a different ratio tach then when i replaced my old instruments. What is the ration on the 650?
is anybody here able to tell me what my new red line is now on the 500 tach?

I have not swapped the gauges out on my bike yet, so I am still using the 550 tach with my 650 cam.  The redline is well north of 13000 (as indicated on that tach). 
Ok, just did the math.
If it is 4:1 for the 650 and 6.67:1 on the 550 (which I think it it is) then 13000 indicated would be 7800 rpm in real life.  14000 indicated would be 8400 in real life. 

Basic formula:  I / 6.67 x 4 = actual RPM    (I = Indicated RPM)

[zenocchio]

good suggestion about the gear.
I guess I should have got a different ratio tach then when i replaced my old instruments. What is the ration on the 650?
is anybody here able to tell me what my new red line is now on the 500 tach?

I have not swapped the gauges out on my bike yet, so I am still using the 550 tach with my 650 cam.  The redline is well north of 13000 (as indicated on that tach). 
Ok, just did the math.
If it is 4:1 for the 650 and 6.67:1 on the 550 (which I think it it is) then 13000 indicated would be 7800 rpm in real life.  14000 indicated would be 8400 in real life. 

Basic formula:  I / 6.67 x 4 = actual RPM    (I = Indicated RPM)

This is The answer I was looking for!
Thanks mate!

So when displaying 9500rpm at 110km/h in 5th gear, I am actually running at 5600rpm.

Thanks guys!

[flatlander]

well, actually it may be something different as these old tachs are not known for their accuracy. i'd take them as a ballpark indication.

[wowbagger]

I've got a spare 650 tach drive. PM me if you want it. Though, it sounds like the problem may be the wrong ratio in the new tach gauge.

[zenocchio]

I've got a spare 650 tach drive. PM me if you want it. Though, it sounds like the problem may be the wrong ratio in the new tach gauge.

Yes, good drive, bad gauge