Topic: CB750K8 stock tach reading 2x rpm?

[Pyroviking]

Greetings,

I've had my CB750K8 for about 2.5 years now, and have used it exclusively for cruising around town, stop light to stop light and have never paid close attention to the tach, other than that it moves up and down when it's supposed to. So, I took it on the highway for the first time last weekend and at 70mph/7500rpm, went for the next gear only to realize that I was already in 5th. I took the next off & on-ramp to count the gears and confirmed 5th gear.

All of my long distance riding in the last decade or so has been on twins, and the last Honda 4 I had was a 954, so I don't really have an ear for revs on SOHC4 bikes.

I set up a dwell/tach meter to compare, and was hoping some of you more experienced with the SOHC4 bikes could chime in to either tell me what I'm doing wrong, or confirm that my tach is reading WAY high.

If I'm calculating correctly:
Std. 8 cyl = 8 pulses/2 rev (4:1)
Std. 4 cyl = 4 pulses/2 rev (2:1)
SOHC4 = 2 pulses/2 rev (1:1)

So, if I'm comparing the RPM reading on the dwell meter to the stock tach, I need to multiply the Std. 8 cyl reading by 4, right?

This gives me 400 x 4 = 1600 rpm vs 2950 and 1025 x 4 = 4100 rpm vs 7200. If my logic is correct, then this would explain why it shows such high rpms on the highway, but I sure would appreciate being corrected if I'm missing something. Thanks in advance.

Cheers,
Andrew

[Yamahawk]

This is a cable driven tach, correct? If so, the magnetic cup drive may have some dirt and lube causing it to drive faster from friction, than it should from the magnetic force. Either that, it it may need lubrication, and the easiest way to do that, is to remove the tach, get a couple cotton balls dipped in kerosene, and stick them into the bulb sockets in the tach. Remove the lights when you take the tach off the bike, and stick the soaked cotton balls into the holes, and set in a Warm place, out in the sun is good on a warm day. Be sure the tach sits upright, so the kerosene doesn't get a chance to drip on your lens. In a day, the kerosene will evaporate and lubricate the tach mechanism. If this doesn't do it, time to disassemble the tach for thorough cleaning.
Charlie

[madmtnmotors]

Sounds like a slipping clutch at highway speed leading to a rise in RPMs, unless you're running some crazy sprocket ratios like 16/52 or something.

[rotortiller]

What bike did the tach come from?

[Pyroviking]

In a day, the kerosene will evaporate and lubricate the tach mechanism. If this doesn't do it, time to disassemble the tach for thorough cleaning.

Thank you. Yes, it's a cable driven tach. I will certainly try that immediately.

Sounds like a slipping clutch at highway speed leading to a rise in RPMs, unless you're running some crazy sprocket ratios like 16/52 or something.

Thank you. As far as I know, the sprocket ratio is stock. Would a slipping clutch account for the discrepancy with the dwell/tach meter in neutral, though?

What bike did the tach come from?

As far as I know, it's the original one, but I really have no idea. Is there an easy way to determine if it is or isn't?

[PeWe]

Is the tacho new on this bike or bike is a new purchase for you?
CB500 tacho runs much faster on a CB750. I know since I have tried it. 3-4000 instead of 2000 rpm something
Tacho gear ratio http://mostynindustries.com.au/speedo-tacho-ratios
CB750 4:1
CB500 7:1

[Pyroviking]

Is the tacho new on this bike or bike is a new purchase for you?
CB500 tacho runs much faster on a CB750. I know since I have tried it. 3-4000 instead of 2000 rpm something
Tacho gear ratio http://mostynindustries.com.au/speedo-tacho-ratios
CB750 4:1
CB500 7:1

Thank you. The bike is a relatively new purchase for me and I have not changed the tach.

This is very interesting. If my experimental data is correct, then my tach is reading high by almost exactly 7:4. That would be quite a coincidence. In addition to this, is there any easy way to confirm definitively that I do have the wrong tach? Are there any clues in the pics in my first post? Redline, maybe?

[madmtnmotors]

Are there any clues in the pics in my first post? Redline, maybe?

Possibly. My K8 CB750's redline at 8500 RPM.

[ekpent]

+ the faces are a different color. Speedo looks green and the tach blue as well as the redline markings being too high.

[Yamahawk]

+ the faces are a different color. Speedo looks green and the tach blue as well as the redline markings being too high.

Yep, just noticed the tach redline too, it's likely the wrong tach the PO substituted to sell it.
Charlie

[Pyroviking]

I think the mystery is solved. Thank you all very much. You guys are the best.

Cheers,
Andrew

[Yamahawk]

Andrew,
   Had the same thing on the K1 I have, the PO had substituted CB400 gauges and seat on the bike, and the tach had a redline of 9500rpm, seemed like it wasn't revving as high as the tach indicated, so I got a set of CB750K3 gauges on it and all is well. These guys here are really a great help for us all!
Charlie

[brewsky]

Just for reference, I'm at 4000 at 60mph, so that would make it 4666 at 70mph...(K8 with stock gearing and 130/90-17 rear)

[Pyroviking]

Again, thank you all. I purchased the correct tach on ebay without breaking my wallet. I'm really relieved that the issue wasn't something more serious.

Cheers,
Andrew