GT 500 tacho drive from oil pump

[Ian T]

Hi
I wonder if anyone can advise. My tacho gets to about 7k then it starts to play up - sometimes it drops then wildly increases. The bike runs well all through the rev range its just that this is not represented in the rev counter. I assumed the tacho was at fault so I bought a new one but it hasn't cured the problem. I've traced the cable down to the oil pump so I assume the fault lies here although it could be the cable. Any idea what could be wrong? Any guidance would be most welcome.

Ian

[karl pa]

I wouldn't be to concerned, my GT500 starts doing it around 3k rpm, and my speedo does it around 50 mph and starts jumping around.
The way I understand is the liquid in the meters between the cable and the needle are sealed in a sort of cup, if some liquid escapes it causes the meter needles to jump around and not spin correctly. You may want to oil the cable to see if it fixes it, but to fix the meters you need to open them and replace the liquid.

[Ian T]

Thanks Karl.

I bought a new meter and it still does it. Do u know the type of liquid within the meter itself - is it a thin oil perhaps ?

[karl pa]

If you look at oldjapanesebikes.com resources under new production GT750J gauge shells. He lists a special silicone fluid from radio control hobby car shop. but advises caution that the process if not done correctly can render gauges useless.
Many of my gauges don't work perfect, which I believe many from that era do not.

[joolstacho]

I don't think I've done a GT one, but others that I've worked on don't have any fluid in them apart from very small amounts for lubrication.
They work using magnetism- a spinning magnet attracts the needle assembly, and the faster it spins the more the magnetic attraction, so the needle rises. The clearances are close, quite possibly worn bushings could cause problems.
But you've tried a new (?) tacho, so assuming it is new the problem wouldn't be the tacho head itself.
The ones I've encountered were fixed by flushing out the mechanisms with Meths - the old oil gets sticky. I'd imagine loss of magnetism could be an issue also.

Is there an expert in the house?

[karl pa]

I don't think I've done a GT one, but others that I've worked on don't have any fluid in them apart from very small amounts for lubrication.
They work using magnetism- a spinning magnet attracts the needle assembly, and the faster it spins the more the magnetic attraction, so the needle rises. The clearances are close, quite possibly worn bushings could cause problems.
But you've tried a new (?) tacho, so assuming it is new the problem wouldn't be the tacho head itself.
The ones I've encountered were fixed by flushing out the mechanisms with Meths - the old oil gets sticky. I'd imagine loss of magnetism could be an issue also.

Is there an expert in the house?

I never attempted gauge repair, but like I said, according to Ian at oldjapanesebikes, the GT750 uses silicone fluid, and no mention of magnetism. If the GT750 uses fluid I would think the other models would be similar.
But it does say later models are different, so maybe later ones are magnetic. He does refer to a bell which may be a magnetic, but refers to the fluid damper to stop needle bounce.

[joolstacho]

Quote:
Working Principle of Tachometer:
An Electric Tachometer works on the principle of relative motion between the magnetic field and shaft of the coupled device. The motor of tachometer works as a generator, i.e. it produces the voltage based on the velocity of the shaft.
(The fluid is just the lubrication).

[gammakeith]

The oil is actually for damping. The drive to the (magnetic) speedo/rev counter heads is via a long flexible cable. Even in good condition this does not always provide a completely smooth feed to the gauge. At some rpm's this can spin a bit jerkily and the speed at the end varies slightly in rotation. If the mechanism is not properly damped then the needle responds accordingly. The damping is easy to check. If you rotate the needle it should fall back quite slowly. No oil and it slams back quickly. The oil is in a small cup on the lower down on the shaft so not obvious.

I could put some pics up if people are interested as I have two T20 clocks in bits, one of which has lost its oil.

Keith

[karl pa]

The drive to the (magnetic) speedo/rev counter heads

This is why I like this forum, there is plenty I can learn still.

The oil is in a small cup

How do you add fluid to the sealed cup, I have an old speedo that I may open and try to learn.

[gammakeith]

I have never done it, I'm afraid. However, I am aware it is possible. When rebuilding my T20 speedo/tach I just picked heads with good damping.

I'm sure others will advise

Keith

[Zunspec4]

Hi Ian T,

The first step I would take would be to remove the speedo cable inner. Flush the outer with a solvent, clean the inner of dried congealed oil. Re-lube with a light oil and re-install. The issue you describe can be caused by an inner cable "gripping and then slipping" due to the old lubrication getting a bit gummy.

Cheers Geoff

[Ian T]

Wow - great advice guys, thanks.

Before I delve into the tacho I’ve ordered a new tacho cable so will report back once it’s installed and tested.

Cheers all.

Ian

[Ian T]

The cable change hasn't made a difference - needle still bouncing all over the place. Keith - if you are able to post a few pics of where fluid is found within the 'cups' that would be great.

Many thanks

Ian

[gammakeith]

Hopefully clear on the attached. As I understand it, the silicone oil is contained in that small tapered cup above the coil spring which sits just below the needle and frame. Not to be confused with the main magnet drive bowl further down. As said before, I've never added any oil so cannot help with the process or the recommended oil. Lots of clocks show a damp looking area where the original oil has oozed out. Don't store them upside down when off the bike!

Getting the bezel off without damaging it is usually very hard. If I were in your position I would be tempted to send it to an expert. I only pulled mine to bits as it had a crushed bezel and I managed to get some dismantled spares to build a good one.

Keith

[Ian T]

Thanks Keith - Good advice. Do you know of any UK based specialists who could sort?
Cheers
Ian