Topic: Tach Gauge Spikes to 12K RPM at Idle

[Montgomery]

Hey All-

Bought a 1971 CB500 for $300 a couple of months ago, and have been gradually been fixing the plethora of small things that are wrong with it. When I got the bike, the tach was stuck at 4K, and the cable was broken. I ordered a new cable from cycle-re-cycle, and tore apart the tach in the mean time. Didn't take hardly any effort to unstick the tach, graphite it, and reassemble. Before I put it on the bike, I tested it with a cordless drill, and it ran up to 8,000 RPM (Fastest the drill would turn) and back effortlessly dozens of times.

When I connected the new cable to the tach and started the bike, however, the tach immediately spiked to 12,000 RPM while the bike was at idle, and stayed there, even after shutting the bike down. After I pulled the cable and diddled the tach, it dropped back to zero. This happened several times. Watching the cable spin while disconnected from the gauge, it turns at a normal speed, at least slower than what the drill ran at when the gauge showed 8,000 RPM.

Any thoughts?

[Gorms]

Break open the gauge.  There is obviously something wrong inside (i.e stripped gear or something).

[Steve F]

A tight or sticky cable would do this. If it won't spin freely in the sheath, it may sometimes bind and then snap free causing the needle to suddenly "jump".  Take the inner cable out and check for kinks and that the end isn't rounded over and fits the tach's square drive correctly.  Lube it well before putting it back.
Steve F

[Montgomery]

Steve-

Thanks for the tip. I took the bike out for a ride, and noticed that the cable was jumping at higher engine speeds, which falls in line with the cable binding and then snapping back. Is there anything in particular I should lube the cable with, or anything to stay away from?

-Monte

[750goes]

Check the mounting plug on the inside of the tacho where the cable will sit - it could be rounded somewhat after you have used your drill and bit (what sort of drill did you use?)

I did a fix on a worn housing - put a small length of heat shrink around the end of the cable, then inserted into the tach - and presuming the cbale is kink free and lubricated -a ll should be well.
I think I even used two layers of heat shrink - and tacho was smooth as silk

[Hush]

As I understand it graphite (pencil lead) is the best lube for a tacho cable, oil or liquids will work their way up the cable and flood the tacho, pretty messy.

[Kframe]

Pencil lead is a great idea, thanks!

On the '74 550 I just got the tach only goes up to 2k right now, and then kinda hangs there even when you kill the bike.

I've had the tach open on my VT700, but doing surgery on this bike's tach is going to be a new one for me.
-K

[Montgomery]

Check the mounting plug on the inside of the tacho where the cable will sit - it could be rounded somewhat after you have used your drill and bit (what sort of drill did you use?)

It was a battery powered Dewalt, with a philips screw bit that fit perfectly in the housing, which is still square.

Dumped some graphite in the cable, didn't seem to help, so I pulled it all the way off again. Turns out that the new cable is already broken, the female connector that mates with the back of the tach gear had one of its teeth broken off already. Now I'm wondering if I got a faulty cable, or if the tach gear did something to it.

-Monte

[Steve F]

Pencil lead is a great idea, thanks!

On the '74 550 I just got the tach only goes up to 2k right now, and then kinda hangs there even when you kill the bike.

I've had the tach open on my VT700, but doing surgery on this bike's tach is going to be a new one for me.
-K

Actually, pencil lead is close to what you want (pure graphite) but not quite, since pencil lead is mixed with clay to formulate pencil lead "hardness".  Get graphite lube at the local hardware store, usually can find some in the locks section.

[Hush]

The tacho gears are pretty light, too light to damage the cable in my opinion.