Tachometer isn't steady.......always jumping.

[Henk7777]

I hope someone knows the cause of this problem...
I placed a new tachometer/odometer on my R65...a Koso RX2N.
Engine is running steady, but the tachometer is jumping between 500 and 1500 rmp.
When I am running 65 or 70 mls/h, the tachometer is jumping between 3000 and 9000 rmp while the engine is running good and has full power...
My stroboscoop has a rmp reader and is also no steady on the rmp's...   so it looks like some device is giving the wrong info to the tachometer.........but the engine is running steady and good...

What could be the problem and how do I test that..

Henk7777

[Barry]

I don't know a specific answer with the tacho being an after market device.

In general terms though the tacho obviously doesn't like the signal it's getting from your ignition system. Where did you connect the signal wire ?    It would usually connect to the +ve terminal on the ignition coil.

[Rob Valdez 79 R65]

How is your main harness earth wire?
Probably OK, since the bike is running strong.

And your stroboscope is doing the same thing.

Have you contacted Koso?

[georgesgiralt]

How is your strobe wired ? If it is only plugged to the battery and the ignition hot wire, this means that your ignition is doing a lot of misses...
Try to do the same test without the Koso plugged and report back.
The ICU is electronic as is the Koso. It is not uncommon for electronics to have non compatibility issues...
P.S. If the above test provide the same result at the strobo RPM, try to run the strobo on an external battery because some strobes do not like the way the BMW charging system regulate the current (spikes) and react to that..
Do not ask me how I know that ....

[nhmaf]

I am not familiar with that aftermarket tachometer, but thee are several general things to consider, as the other have pointed out already-
1.  Usually (*) the tachometer connection is from the wire on the ignition controller side of the coil which is <12V pulse.  But, some tachometers expect/use a coil inductive pickup from one of the high voltage sparkplug leads.
2) The bike may be running fine, but the tachometer may not have a good earth ground of its own - if it doesn't have a good ground, nothing it displays will be accurate.
3) Remember that the BMW airhead is a wasted spark system.  The ignition generates a spark for both cylinders each time it sparks, even though one cylinder is on on the compression stroke and the other is on the exhaust stroke.  With certain aftermarket tach configurations, it might interpret this as 2X RPM count

[mrclubike]

what ignition system do you have

[Tony Smith]

A couple of thoughts.

Can you try the Tach on another vehicle to see if in fact it is working OK?

If you have a standard BMW ignition module there is an unused tach trigger pin which you could possibly capture and use.

The BMW ignition does fire a "waste" spark, but that is hardly unique, many other multi-cylinder engines also do that.

Given that there is nothing in any way unusual in the BMW system I am inclined to think that there may be some electronic "noise" that the tach is unable to filter.

I took the trouble to download and read the install instructions for your tach and they do make the point in a number of places that the tach must have a good ground. I'd suggest that on the basis of past experience, anywhere forward of the steering head is not a good ground on just about any BMW. Try running your earth lead back to the main frame and see if that improves matter.

[Bob_Roller]

If you  repainted the frame, your main grounding, or earthing point, a bolt on the voltage regulator may have paint in the threads .

All the  brown wires, which are earthing wires for all the circuits at the front of the bike are ganged together at this point on a single bolt, or screw .

[Henk7777]

All,
Thanks for thinking with me..
There are some thinks I will check and modifi on the bike...and I will contact Koso (manifacture tachometer)
The main + wire is coming straight from the battery and grounding is also ok....I had checked that...
Give me some time and I will let you know my findings..

Thanks..

Henk

[Red_Hen]

Good Morning - I've got the exact same issue on my '82 R65LS - it all started when I upgraded the ignition system - Red Centre Bean Can and Ignition Module - bike runs better than ever but tach moves erratically - I do have a spare tach but have not put it in yet - I think it must be the voltage with new ignition system is so much more robust that it's not compatible with the electronics in the tach - I have a good tech person who's going to rebuild the front forks and adjust steering head bearings - if this gets figured out, I'll report back.

[Justin B.]

When I experimented with the Vapor "cluster" on the Junkyard Dawg I originally had a somewhat erratic tach.  After switching from the inductive type of coil pulse (wrapping a plug lead with a few turns of wire) I went to the alternate method of direct wiring to the coil and had a rock-solid tach afterwards.  I don't know if the Acewell would allow this, or not, but it couldn't hurt to check...

[nhmaf]

This tach will support picking up the rpm signal from a plug lead, or the primary high side of an ignition coil, or from the ignition module...... I think (along with verifying everything is well earthed) you also need to give us details on which method you have the thing wired up, as the manufacturer shows in the directions all the options, and suggests which ones are more preferred.

[wilcom]

After you get the tach fixed up.... tell us about the very nice exhaust system you have on the bike!!

[Red_Hen]

Good Morning - I've got the exact same issue on my '82 R65LS - it all started when I upgraded the ignition system - Red Centre Bean Can and Ignition Module - bike runs better than ever but tach moves erratically - I do have a spare tach but have not put it in yet - I think it must be the voltage with new ignition system is so much more robust that it's not compatible with the electronics in the tach - I have a good tech person who's going to rebuild the front forks and adjust steering head bearings - if this gets figured out, I'll report back.

Reporting Back:  Finally got around to replacing existing tach with another R65 electric tach - completely solved my jumpy tach problem. 

Is there a shop out there that overhauls these tachs?  I spoke to the gent who owns Overseas Speedo and he doesn't repair them and neither does Terry Vrla, aka wirespokes

[georgesgiralt]

We are lucky because these tachs give access to their electronics easily.
So if I where you, I'll open it (like willing to change the bulbs) and make a note of all condensers in it. Buy them and replace them.
It could/should well solve your problem.
If you can't solder properly or lack the tools, any electronic repair shop is able to do the job for you.
Just my 2¢