Topic: KO tach woes

[dhall57]

My 1970 750KO tach works fine up until it starts approaching 6 grand. Accelerating through the gears the needle than will start jumping back and forth and going crazy and bouncing back and forth between 5500 and 6500 grand and not go past the 6500 rpm mark no matter how hard I push it. I can run along all day on the open road at 70 mph taching 5 grand and the tach is perfect. Is the crazy needle just old age being 41 years old and the only cure is getting another tach or can I take the chrome cover off and try to do some cleaning and light oiling of the internal parts. And yes the tach cable is brand new OEM Honda. Has anybody had anything like this happen with there sohc4 bike?

[Rigid]

If I remember right, the tach is driven internally by a rotor spinning inside a housing and the needle is actually moved by magnetism as the rotor spins and drags the housing around against a calibrated spring.  I suspect your housing and rotor are bumping each other due to worn bushings.  Another possibility is grit or debris between the two that is pulling, then dropping, your needle.  You sure won't hurt anything with a clean and lube in there.  The parts are delicate but I believe common to later models in that part of the tach.  I would see no reason you couldn't swap parts over from a newer model once inside.

[ekpent]

Def no expert here but know some are but there is a dampening pot I believe inside the instrument with some type of a specific weight oil.silicone or some fluid that helps alleviate the needle bounce. But since yours does not go over 6500 we better leave that to an expert.

[CB750faces.com (Lecram)]

I have restored dozens of CB750 gauges and this is often caused by an internal defect. If the needle and/or internal mechanism don't run smoothly, the needle will get stuck.
It's almost always repairable. The internal mechanism is fragile, so be carefull for damaging this mechanism.

[Rigid]

Here is what is inside a CB750, SOHC Tachometer:

[CB750faces.com (Lecram)]

Here is what is inside a CB750, SOHC Tachometer:

The K0 gauges are different. This is a later gauge.

[Rigid]

Here is what is inside a CB750, SOHC Tachometer:

The K0 gauges are different. This is a later gauge.

The parts that drive the needle are different?  I am familiar with the indicator light differences. 

[CB750faces.com (Lecram)]

Here is what is inside a CB750, SOHC Tachometer:

The K0 gauges are different. This is a later gauge.

The parts that drive the needle are different?  I am familiar with the indicator light differences. 

This is how a K0 gauge mechanism looks like. The tach is almost the same, except the ODO wheels and some drive shafts