Tachometer 'Swing' ?

[rozemab]

Hello,

I was wondering how much needle 'wiggle' you experience with a mechanic tachometer on a /7 (<= 1977). I can be cruising at 3500-3700 rpm and the tachometer can swing up to 250rpm with a constant rpm (my hearing) of the engine. That seems excessive but maybe that's just the characteristic of an older mechanical tachometer. The 'swing' is pretty much equal on both sides of a set rpm.

thanks, Brian

 

[20774]

Have you tried lubing the cable (generally not the correct thing to do but worth a shot) and/or replacing the cable? Speedo cables can internally unravel and "hang" resulting in a hang-release-hang kind of response. That could be what's going on in your case.

 

[Bill Burke]

Is the rpm swing unrelated to bumps in the road? Needle jump on rough sections is normal. Or does it spin up periodically even if you're on the straight and narrow, smooth flat section of road at steady throttle? If yes, then I'd suspect the cable.

Is the tach cable rubber grommet at the front engine case cracked and crunchy looking? If yes, then you have a really old tach cable. Replace it and things should calm down.

 

[rozemab]

The swing is independent of the bumps etc. As I can determine, the swing occurs at 3000-5000 rpm range and is not consistent. I'll try lubing the rev counter cable and see it makes any difference. Does one typically pull the speedo/tacho module from the mount and then loosen the cable? Does not seem like alot of room between the knurled nut on the cable and the headlight bucket.

 

[dennisdarrow]

Does the "swing" stay in place or just bounce back and forth and up and down the range a bit.............Either way is NOT normal.........Yes, you tak off the knurled nut behind the tach by just reaching in and loosening it.......This is one place that it is just fine to use a pair of vice grips or a set of water pumps to do the job. Just try not to let them slip and mess up the knurling.........

In the past I have dealt with this a couple of times with the same results......I tried the new cable....NO GOOD.......Tried putting silicon spray up into the back of the tach......NO GOOD.........Took the tach off and tried cleaning it a bit and then lubing with silicone spray....NO GOOD................
Then when the weather turned colder the jumping increased, screaming of the tach head began, and the needle ended up breaking off........One I sent to get fixed and the current one is waiting for me to get around to doing that........I say....Send the whole cluster off to get repaired and overhauled......the speedo and tach, lights, etc...........God bless.......Dennis

 

[rozemab]

Dennis,

The swing is consistent regardless of the rpm, it still swings in a range about a 'center' rpm, if you understand what i mean. (about 250rpm or one tick mark on the gauge).

So, if everything is healthy, there should be no swing, the needle should stay 'put' on the gauge?

I can buy a new BMW tachometer for $160, what does it cost to have one rebuilt?

Brian

Does the "swing" stay in place or just bounce back and forth and up and down the range a bit.............Either way is NOT normal.........Yes, you tak off the knurled nut behind the tach by just reaching in and loosening it.......This is one place that it is just fine to use a pair of vice grips or a set of water pumps to do the job. Just try not to let them slip and mess up the knurling.........

In the past I have dealt with this a couple of times with the same results......I tried the new cable....NO GOOD.......Tried putting silicon spray up into the back of the tach......NO GOOD.........Took the tach off and tried cleaning it a bit and then lubing with silicone spray....NO GOOD................
Then when the weather turned colder the jumping increased, screaming of the tach head began, and the needle ended up breaking off........One I sent to get fixed and the current one is waiting for me to get around to doing that........I say....Send the whole cluster off to get repaired and overhauled......the speedo and tach, lights, etc...........God bless.......Dennis

 

[20774]

So, if everything is healthy, there should be no swing, the needle should stay 'put' on the gauge?

I can buy a new BMW tachometer for $160, what does it cost to have one rebuilt?

Yes, the needle should remain constant...no reason for the RPM to be changing unless you have a problem with the carbs/engine, but you'ld know that by the seat of your pants.

Probably would cost the same to have yours rebuilt. So, you just have to ask...do I want original parts on my bike or not. Important parts anyway. Sure everyone replaces batteries, tires, bearings, voltage regulators, etc. But I prefer to keep what she came with...

 

[Bill Burke]

My experience was different from Dennis Darrow's above. At 45K on the clock, my tach started the big dance. Tried lubing etc, then replaced the tach cable. (My old one was clearly crunchy at the grommet). I'm not gonna say my tach now runs like an Omega seamaster watch, but it's been quite a bit steadier and no further trouble for the past 40K miles. The cable was about $25 iirc. Cheers.

 

[barron_williams]

Couple of thoughts -

If the tach cable is 10 years old (or older), you will probably want to replace it anyway, if you end up spending the money for a rebuild/clean up of the tach. So, maybe try a new cable first and see if it makes a difference?

There are a couple of good threads about instrument repair on this forum and also on the Airheads web site. Though, most deal with the slipping odometer gear, because that seems to be the most common problem.

One thing you might notice from reading the threads is that if the cable isn't the problem, then it gets progressively worse until a major failure occurs (needle snaps off).

The instruments are one of the areas on my bike where I have elected to have a "pro" do the work. I have had good luck with Overseas Speedometer. A quick web search will get you the web site which has estimated prices and other details. A search of this forum will also yield details of folk's experience with a couple other instrument repair shops.

Seems like the instruments need a bit of maintenance every 15 years or so.

Regards,

Barron