Gah! Tachometer!

**Devil** · 25th August 2004, 09:02

Ok fired the bike up last night, warmed her up, hopped on and started to ride down the street, got into 4th gear around 60km/h and noticed that the tacho was reading about 8500rpm, where it should be at about 6000rpm.

After a few dirty words I pulled over, down through the gears. Noticed the tacho was very slow in responding, the bike seemed to be going fine. Started off again, back up to 4th, tacho reading around the same, checked that it wasnt the clutch slipping, everything felt fine.

How does a tacho work? I dont know where it reads from, what would make it go out?

1991 Kawasaki ZR250.

**Holy Roller** · 25th August 2004, 09:15

Not sure if the tacho is cable or electronic if cable check that both ends are connected and screwed up tight, mine is cable but its an older bike it started doing strange things like yours and I found that the cable had come loose but not fallen off, tightened it up and all was back to normal.

**Devil** · 25th August 2004, 09:43

Thanks ill have a better look tonight, it was a bit dark by the time I had the problem *hmmph*.
Cant wait till I have a full licence so I can have a late model bike (stupid 250 prices!)

**Blakamin** · 25th August 2004, 10:14

Mine does the same thing...about 2 weeks behind the actual revs... i just listen to the engine now

**Devil** · 25th August 2004, 10:26

I *would* do that, but im a picky bastard and I like things working

**pete376403** · 25th August 2004, 10:26

Mechanical tach (excluding the old Smiths Chronometrics) work just like a mechanical speedo - the cable drives a rotating bar magnet. This magnet is positioned inside the open end of a steel "cup". there is a small clearnace between the ends of the magnet and the sides of the cup. The cup connects to the speedo/tacho needle, and has a light spiral spring that returns the needle to the stop when nothing is turning. When the cable turns (ie bike moving / motor running) the magnetic field "drags" the cup around, and the tension of the spring balances the magnetic drag to get the needle pointing to the right place.
I've seen problems as you describe when oil gets up the cable and into the gap between the magnet tips and the cup, although this tends to shoot the needle right round to the end stop.

If the tacho is electronic then none of the foregoing applies - electronic tachos/speedos use a stepper motor to move the needle to a position determined by a sensor, either magnetic or photo-optical, and controlling electronics.

Mechanical tach is usually driven from the exhaust cam, electronic will be somewhere in the ignition circuit.

**Devil** · 25th August 2004, 10:29

Hrmph. Thanks for that. Ill have a closer look, but from what I saw I dont *think* I noticed the kind of cable that would lead me to believe it was mechnical, only really saw electronic type wires. However... I did notice a bit of oil/goo around a particular cable that was heading down to the engine.

**scumdog** · 25th August 2004, 10:31

My sporty did the same thing and it turned out I had a loose battery terminal, I wish I could have taken a pic. of the revs indicated at the time!
Also caused my indicators to cut out when least expected.

Most common cause of electrical/starting problems I have found is a loose terminal connection.

**Blakamin** · 25th August 2004, 11:43

Mines electonic and it gets about right when its over 10,000rpm so i'm lucky it redlines at 15. :
my tacho is stuffed tho, and i cant find a new one anywhere. when the bikes switched off it sits at 2,500 until you tap it a few times, then goes to the needle rest... oh well

**FROSTY:Me** · 25th August 2004, 12:58

devil--Im pretty sure its an electric tacho.
It might pay to open it up and spray a bit of electric cleaner in there.

**Posh Tourer :P** · 25th August 2004, 19:52

Devil: The beemer electronic tacho is driven off the coil... that is usually where a tacho comes off anyway if it isnt mechanical. Also, if you spray too much CRC into it, it'll lose resistance and flick up to max over certain revs - next door's Zephyr one did after he had a dashboard fire (badly installed radio). He sprayed too much CRC around after the fire and it dissolved the grease (we suspect). The beemer has the opposite problem. The tacho was bought new from Germany, probably sitting on a shelf for 20years, so was *very* sticky and slow to respond, especially in the lower revs. Slowly adding CRC drop by drop, week by week, and trying to get it right. It is almost there now, just a little bit slow on the drop down.

Blakamin: see above. However you might just have to clean it out a little - sounds like a dirty needle to me... perhaps not enough grease or lubrication of any sort

**Blakamin** · 25th August 2004, 19:59

Originally Posted by Posh Tourer :P

Blakamin: see above. However you might just have to clean it out a little - sounds like a dirty needle to me... perhaps not enough grease or lubrication of any sort

Thanks, had it apart last week and resoldered all the joints, but from what i can see, it seems that the spings (2 clock-like) seem to have stretched/lost tension.

**Devil** · 26th August 2004, 08:12

Thanks people.
Got to the wiring last night. Honestly! Who keeps all the wiring in the back of the headlight enclosure!!! Unplugged the thing, got an old toothbrush into the connections, cleaned them out, plugged it back in and it seems to be going ok again. *shrug*. In the meantime I noticed the headlight enclosure (the plastic thing the headlight screws into) is hella cracked

yay for duct tape!

**Hitcher** · 26th August 2004, 09:38

Originally Posted by Blakamin

Mines electonic and it gets about right when its over 10,000rpm so i'm lucky it redlines at 15. :
my tacho is stuffed tho, and i cant find a new one anywhere. when the bikes switched off it sits at 2,500 until you tap it a few times, then goes to the needle rest... oh well

When you email Victorian Motorcycle Wreckers about the pipes you can ask about a tach as well...