1991 R100RT tach issue

[b1620]

Hey guys;
I have an issue on my 91 bmw r100rt. I thought I had a speedo problem, thinking it was reading high but I took my GPS with me and the speedo is almost perfect. So now I'm thinking it must be a tach issue. At 70 MPH it reads 5000 RPM.
I have a 120/90 18 on the back with 33/11 finial drive. I did the math and it should read 4000 RPMs. Who is right, me or the tach? Is this a common problem? Can it be repaired/adjusted or do I just need to live with it?
Thanks

 

[20774]

My /7 also has a 33/11 rear end and the correct speedometer ratio (1.112). I run a 4.00x18 rear tire...it's going to have a slightly bigger circumference than your metric tire. At 70mph, I believe I'm running somewhere between 4K and 4250 RPM. So, 5K RPM does seem a bit high. Do you have the right speedometer ratio? Unless you changed the speedometer, I would think it's correct.

Snowbum has an equation on this page - http://bmwmotorcycletech.info/ringgears.htm - about the middle of the page to compute either mph or tach readings. It's important to get the circumference of the tire.

Anton Largiader has a gearing comparison chart here - http://www.largiader.com/articles/gearing/ - it doesn't work for me (axis labels don't show), maybe something with Win10. But maybe you can confirm your values with his charts.

Shouldn't be a common problem, provided the bike's configuration is correct. BTW...has this been a problem ever since you owned the bike? It would be possible to fix things...likely you would need to send instrument cluster off. Another thought...the tachometer operates off of the coils collapse rate as the spark is generated...the tach reading is all electric. Apparently the speed is fine but the RPMs are off. This would need some kind of electrical circuit change to properly count the coil changes in order to get the right RPM.

 

[cycleman2]

I'm running a 37/11 rear end ( 3.36 ) ratio in my 95 R1100RT with a Kenna Sidecar. At 100 kph/62 mph ( confirmed via GPS) I am turning approx. 4100 rpm. The stock rear end for this bike is a ( 3.0 ) ratio with an rpm of about 3700-3750 at 100 kmph or 62 mph. With my sidecar gearing 68 mph or 110 kph is about 4500 rpm. So something is not right with yours.

The tackometer on my 95 is electronic, and I'm not sure when they started that on the airheads, but I do know if it is getting too much voltage it will read high. Might be worth check out and maybe somebody else with more experience can provide an answer.

 

[20774]

The tackometer on my 95 is electronic, and I'm not sure when they started that on the airheads,

I believe that started with the '78 models...'77 was still cable driven.

 

[toooldtocare]

I believe that started with the '78 models...'77 was still cable driven.

First, read this. http://bmwmotorcycletech.info/tach.htm

It may offer some help with your tach. And Kurt is right, the 78s had electronic tachs, as did my 78 Motorsport.