Tachometer is way off (doubles rpm?)

[RG68RG]

Anyone know a common reason why the tach on this bike would be so wrong? At idle, It appears to be a hair over 2K which isn't correct at all.. Then as you rev to what would be 4K it buries the needle.  It's an 83 Ls and both tach and speedo are in the same panel...  Any help would be appreciated. 

[Barry]

The tacho gets it's input from the ignition coil so basically it counts sparks.  While it's possible the tacho is faulty the first thing I'd do is go through the whole ignition system checking on all connections. 

[RG68RG]

I'll do that when i get a chance .Thanks!   I was just hoping there was something that is common with this particular bike. 

[mrclubike]

Anyone know a common reason why the tach on this bike would be so wrong? At idle, It appears to be a hair over 2K which isn't correct at all.. Then as you rev to what would be 4K it buries the needle.  It's an 83 Ls and both tach and speedo are in the same panel...  Any help would be appreciated. 

What do mean by in the same panel
Do you have the stock tach and speedo or a after market one
If it is after market it may need to be set for  4 cylinder  

[RG68RG]

No it's stock...the tach just has one multi prong plug and what looks like no adjustment.

[Bob_Roller]

Here's a link to Snowbums BMW Technical site .

http://www.bmwmotorcycletech.info/tach.htm

It will give you some idea where to look for issues .

[RG68RG]

Thanks for the link.. been reading it...!

[Bob_Roller]

It may be easier, to get a replacement tachometer .

[RG68RG]

I'm starting to think a replacement is probably the answer too..

[georgesgiralt]

Hello
The tacho is simple. A bunch of discrete components and an integrated circuit to convert the impulse count into a current for the galvanometer.
IMHO, the electrolytic condenser are dead (30 years of harsh conditions take its toll ) and replacing them may solve the problem entirely.
It is cheap and may return the device to it's proper status...
Just my 2¢

[RG68RG]

Hello
The tacho is simple. A bunch of discrete components and an integrated circuit to convert the impulse count into a current for the galvanometer.
IMHO, the electrolytic condenser are dead (30 years of harsh conditions take its toll ) and replacing them may solve the problem entirely.
It is cheap and may return the device to it's proper status...
Just my 2¢

Where is the (are the) condenser/s at? I have the tach off the bike and on the kitchen table..

[georgesgiralt]

Remove the screws (and sometimes the color tape) and remove the black plastic cover. You will see the printed circuit board and the indicator bulbs.
On this PCB are  all the components. Change the condensers and try it on the bike.

[Barry]

If there is another R65 owner nearby a simple swap of the tacho to another bike would confirm if the fault lies with  tacho or the bikes ignition system.  Actually any airhead would do or even any other twin cylinder machine with wasted spark ignition but that would be more work with the wiring.

[georgesgiralt]

You can actually test with a strobo light if you have one displaying the RPM. It should gives you twice the actual RPM if set on 4 stroke engine. (I don't think the strobo maker has though of wasted spark ignition...)
You can also use any electronic tacho for a 4 stroke engine and take it's reading with a pinch of salt given the origin of the tacho.
I do not think the ignition circuit is at fault. if you have points, the tacho just count the number of times the points open per second and divide by 2. If you have an electronic ignition, I doubt the ICU can give a false timing information and still deliver a spark at the right time to the cylinder... But it's just my thinking...

[RG68RG]

Now I just realized that (tonight)  there are no backlights to illuminate either gauge. the bulb in the speedo is just plain missing (didn't just fall out it's not anywhere to be found) and the ones on the board of the tach just don't work. I looked up a replacement instrument cluster setup on ebay, and they are way too pricey used.. So... Since I don't touch most electrical stuff.. that's probably that.  Last time I had a bike with this much wrong with the electrical...I was 17 and didn't care.