Dancing mechanical tach

[RDG]

I've got a dancing tach on my 77. At idle it's fine. Once moving on tho, the needle dances like crazy, all over the face, 2- 3K increments. After about 20 minutes at speed, it seems to steady up but still dances @ 1K or so...

Replaced the cable. Swapped out with a good known tach. Same/same.

Anyone out there have any advice, comments or recommendations?

[Deleted User 62]

Same thing happens on my '72. I'm hoping it's just the cable going south, but at $55 for a new one, it will be awhile before I know. Taxes to pay and all (grumble mumble)

[BoxerTwin]

mine does it on occasion too on my '77. It will only do it at high speeds in and will jump from 2.5k to 5k when I'm really turning about 4k. Most of the time it works fine however. It is moody.

[moosehead]

I've had this problem on a mechanical tach as well and I found there is a very fine dampening/return spring that get cock-eyed...sometimes the spring winds up nicely (while increasing rpm) and then allows zero return normally. The problem I saw in mine is that the spring was binding up as it would up then,,,wham...let go either up or down.

Its a sob to see but look just below the face plate, from the side. right around the base needle shaft and you'll see it. Use a magnifying glass and you can pick it out. I removed the tach needle and face plate to see if I could "unbind" the spring. It was damn near impossible once it gets out of whack so I junked it after playing around with it forever. Bought used off flea bay ...but I found the old mechanical one hard to find. I think if I had found the bound up spring earlier might have been able to fix it,,,dunno

Not much help but you may get lucky and only have couple spring coils bound up. Good luck.

[dwire]

Could anyone who has experience with these bikes in the days they were un-crated comment on how they behaved straight from the factory?

I have in past tried everything and even tested with an NOS unit I have that has about a mile on it (now...) and it acted no differently than the original, even after swapping several new unbent cables out. Mine, while does as well as I'd expect on smooth roads, either needle can get some movement under rough road riding. I suppose from knowing what's inside, I never thought twice about it; it seemed logical for sharp impacts to make the needle(s) deflect some.

Should I expect more from these units or not? It would be nice to hear what one should really expect out of these things...

[teo]

I bought my R75/6 new in Jan 1975. The tach bounce a little and over the next three or four years bounced a little more. I bought a spare/replacement tach and it worked fine for a few years. Finally I sent them both (and one speedo) to Palo Alto for a rebuild. They worked for about 60K miles after that and are just now showing some subtle movement.

Teo

[dwire]

Thanks teo very much Sir or Madam.

That is very helpful I know for me and should be for others. I regret to admit I have a "record" servicing instruments like our BMW speedo/tach units and it would appear that I need perhaps tear into both my original unit and the NOS one as well, for it is not in the cable(s) engine or transmission. Dang, I hate doing that type of work - it is so easy to spend a mountain of time on. Often it is rewarded highly, occasionally, it is not and you end up with a bunch of "pieces-parts" to repair old meter movements and clock/watch movements with and no more unit for your motorcycle or automobile.

Thanks again. It is hard to have to hear, but I doubt you could know how many people I've spoken with that can't really say how they behaved in the early 70's and what we truly should expect. Now we know.

[RDG]

Hmmm, I'm thinking it's in the engine case where the tach cable ends....is this off the camshaft? Does anyone have any experience with the gear drive system down there, or is it some kind of mystical Germanic Hobgoblin only definable by visiting their spirit world?

Anyone got a schematic? Not for the spirit world, but the tach drive?

[Deleted User 62]

Try the MaxBMW fiche? http://www.maxbmwmotorcycles.com/fiche/ ... nd=7311731

[dwire]

I would think the diagram in the ETK is self explanatory.

I still would be asking more specific questions and I think everyone here requires more specific data to help you much.

The TACH that stopped working and you replaced the cable, was it broken or kinked? Before you replaced it, did it still turn with the engine turning over or running? Did you route the (new) cable being certain it is not in a bind at all?

If needle deflection/erratic indication is the issue, barring the cable and its routing and such, look to the instrument itself. Resounding theme with flaky needle deflection and shaking about 9 times out of 10 with a new cable is it's the instrument itself. My 1971's both got a "control test" with an NOS speedo/tach instrument swap - with brand new cables; neither worked any differently than what was in the bike when the 38 or so year old NOS unit was swapped in. This tells me that simply sitting on a shelf, let alone sitting/use in a bike, they can easily get gummed up and become inconsistent.

Best of luck - the things that can go wrong, assuming the bike does not run like garbage (IE, timing chain/gear or tensioner is totally hosed) are illustrated above, 1,6,8,9,10. Not necessarily in that order. Also a new cable could be crap too. Duane covers this well, but you should have been certain it turns without any excess drag at any one point in 360 degrees of rotation - that'll make an instrument pulsate all day long; this comes often from a cable being coiled for a long time.

If I were a betting man and since we don't really have the answers as to what you found when you replaced the cable and such, I'd still point at the instrument and or cable far before ever thinking something was wrong with the drive helical(s) and associated parts...