Tachometer issues ...

contours · Apr 8, 2013

Excuse my inexperience, but I acquired my 1975 850 Mk3 only recently after giving up motorcycles altogether for 25+ years. My bike has not needed any serious attention since I've owned it so naturally I started making changes. The first thing I did was replace the stock handlebars with clubman bars which caused me to experiment with re-routing the throttle cable. To do that I needed to disconnect the tachometer cable temporarily. Unfortunately, the tachometer has NEVER worked right since re-connecting the cable after I was done (I think I know how to properly connect the cable - I've done this on other bikes countless times without any issues). The RPMs now show a narrow range from about 1500 to 3500 no matter how I rev the engine!

After disconnecting and re-connecting it many times I got fed up, bought a new cable and installed it. Still the same problem. So I guess either my tachometer drive or the tachometer itself is bad?

Any suggestions on how to proceed?

Thanks,
Peter

concours · Apr 8, 2013

Welcome. Does the disconected at the clock cable turn at the same speed as the engine runs? (revving it up)

DogT · Apr 8, 2013

Best to check the clocks off the bike with electric drill and appropriate connector. Easy, remember they run backwards on the drill. Most problems are cable lengths, and since you changed from "Export" to clubman bars, that's a good start of a problem, wrong cable lengths.

Dave
69S

contours · Apr 8, 2013

concours said:
Welcome. Does the disconected at the clock cable turn at the same speed as the engine runs? (revving it up)

Ah - I haven't tried that. I will check that out next - THANKS!

contours · Apr 8, 2013

DogT said:
Most problems are cable lengths, and since you changed from "Export" to clubman bars, that's a good start of a problem, wrong cable lengths.

Dave
69S

That would be a good test and I will try that next time I'm at the garage where it's stored. However, I can't believe the cable length matters that much since I've changed the bars back to standard and then one more time to a European set and had the same problem with each change.

The original cable was slightly worn at the clock end, but replacing it with a new identical length cable should have eliminated that possibility.

I'll post back in a week or so after I check out the tachometer using a drill.

Later,
Peter

rpatton · Apr 8, 2013

contours said:
The original cable was slightly worn at the clock end, but replacing it with a new identical length cable should have eliminated that possibility.
Later,
Peter

Peter,
A possible problem can come up when you put in a new cable. The part that extends into the tach can be so long that it bottoms out and put too much force on the part that it is meant to spin, making it bind. It has a thrust bearing of sorts but it isn't up to taking any force at all. It is adjusted with that little brass wart thingy.

contours · Apr 8, 2013

Okay. Bob, that's good to know.

DogT · Apr 8, 2013

With your cables installed, but not into the clocks, the inner cable should not stick out past the nut when the nut is pulled up tight against the shoulder, like when it's screwed on the clock, otherwise the inner cable will go into the clock to far and may mess things up in there. Easy enough to check. Yes some cables have the brass locking thingies, I'm not sure how that works. I had my speedo cable on bassackwards at first and it was erratic, when I reversed it, it was fine.

Dave
69S

contours · Apr 8, 2013

DogT said:
With your cables installed, but not into the clocks, the inner cable should not stick out past the nut when the nut is pulled up tight against the shoulder, like when it's screwed on the clock, otherwise the inner cable will go into the clock to far and may mess things up in there. Easy enough to check.

Okay, I'll check that too - THANKS.

Hey, I should put the drill in reverse to better simulate the engine turning the cable, right?

DogT · Apr 8, 2013

Yeah, with the drill in forward, you'll just bang the needle against the bottom stop and that won't tell you much, but a standard drill probably won't get the needle much past 1/3 scale, but that should tell you something, especially if it's steady. I forget what I put in the clock to turn it, but it was simple, I found something that fit in the drive and the drill pretty good. Maybe the square drive end of a tap or an 'easy-out'?

Dave
69S

rpatton · Apr 9, 2013

Peter,

There's another thing you might want to check on the new cable. I had some work done to my tach a couple years ago and the guy that did it warned me that most of the new cables being produced can bind in the part that engages in the tach from being too big. This is a separate issue from bottoming out, but it can cause the same damage to the tach. Check it's fit without the cable housing. See if it's a sliding fit. I had to dress the edges of the square section a little on my new cable.
Bob

concours · Apr 9, 2013

DogT said:
Yeah, with the drill in forward, you'll just bang the needle against the bottom stop and that won't tell you much, but a standard drill probably won't get the needle much past 1/3 scale, but that should tell you something, especially if it's steady. I forget what I put in the clock to turn it, but it was simple, I found something that fit in the drive and the drill pretty good. Maybe the square drive end of a tap or an 'easy-out'?

Dave
69S

A wooden kitchen match works good for that.

contours · Apr 10, 2013

I appreciate all the advice from you folks! I will let you know once I check things out in a week or so.

contours · Jun 26, 2013

I'm back to report success - sorry it took so long!

The problem may not have been a problem at all once I threw the new cable on. My complaint was the limited RPM range I was seeing while revving the engine as the bike sat still. Now that I've taken it out several times and open the throttle a little I do see a response. The issue may have been with my perception of how high the revs should go. I recall some information about the limited power band one should expect after installing a single 32mm Mikuni. Now after riding It makes sense that I may not see the revs climb past or even up to 5 grand.

On the other end of the spectrum, the idle speed does not get below 1200 without risk of stalling. So the bottom of the range seems reasonable to me now. (Not complaining about the stalling too much because after it's good and warm the idle is very stable).

Thanks for all the info and tips. This website is awesome!

Peter

concours · Jun 26, 2013

contours said:
I'm back to report success - sorry it took so long!

The problem may not have been a problem at all once I threw the new cable on. My complaint was the limited RPM range I was seeing while revving the engine as the bike sat still. Now that I've taken it out several times and open the throttle a little I do see a response. The issue may have been with my perception of how high the revs should go. I recall some information about the limited power band one should expect after installing a single 32mm Mikuni. Now after riding It makes sense that I may not see the revs climb past or even up to 5 grand.

On the other end of the spectrum, the idle speed does not get below 1200 without risk of stalling. So the bottom of the range seems reasonable to me now. (Not complaining about the stalling too much because after it's good and warm the idle is very stable).

Thanks for all the info and tips. This website is awesome!

Peter

This still sounds lame. Like a slipping clutch, you should easily be able to tell if the engine's feel/sound is consistent with gages. Even with half a carburettor, it should free rev in lower gears easily to redline. At 1200 rpm, stalling would be unlikely. Stalling usually happens below 500 rpm.

contours · Jun 26, 2013

concours says ..

"Even with half a carburettor, it should free rev in lower gears easily to redline. At 1200 rpm, stalling would be unlikely."

What should I expect for redline with this 34mm Mikuni then? I sensed I would not get much more speed or power revving beyond 5,000 RPM today. I wish i knew how to gauge it by sound.

DogT · Jun 26, 2013

+1. I think it sounds lame too. It should be linear with the engine speed. Clutch should have nothing to do with it. And again +1 it shouldn't attempt to stall until it's around 600 rpm or less. I think the thing needs to be tested or better yet repaired. It could still be the cable or the drive mechanism.

Why would it not rev beyond 5K? I know nothing about the Mik, but I wouldn't put up with that.

Dave
69S

contours · Jun 26, 2013

Okay then, more testing is needed. I am currently having the Layshaft bearing replaced. But once I get it back I won't hesitate to open it up and see what my highest RPM might be.

I will revisit this in a little while - thanks for your input.

Peter

Nater_Potater · Jun 27, 2013

Find someone with an inductive pickup tachometer and compare the clock's reading to the electronic tach. Don't expect perfection, but it'll at least tell you if your tach is in the ballpark.

donmeek · Jun 27, 2013

concours is correct. something's amiss. I'm surprised more people haven't chimed in here. I'm not an expert, but as a long time mik user I can tell you that a properly tuned Norton with a properly set up mikuni will run up nicely way past 5000

Tachometer issues ...

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