Accuracy of the electronic tachometer on the R65LS

[DanMay]

My 1982 R65LS tachometer is off by about 1000 RPM at 5000 RPM.  The tach reads 6000 when the motor is actually spinning 5000 RPM.  I checked it with a validated inductive tachometer using the spark plug lead.

Is there any adjustment on the factory tachometer ?  

[Bob_Roller]

The boxer engine that these bikehave, have what is called the 'wasted spark ' ignition system .

Both spark plugs fire at the same time regardless if it actually igniting the fuel air mixture .

So the plug fires twice as much as an engine that doesn't have this type ignition system .

Don't know if that will cause an indicaton issue with an inductive tachometer or not .

If this isn't causing the problem, I don't know of any adjustments that can be made to correct an error in indication of the tachometer .

One member here, wirespokes, repairs speedometers, don't know if he has any experience with Motometer tachometers or not .

Send him a personal message and see what he has to say about it .

[Luca]

The wasted spark will double the RPM on a timing light/tach.

I've heard that some timing lights have trouble on smaller plug wires.  Have you tried checking the indicated rpm against your speed?  I think there's a chart in the FAQ section for speeds at a certain rpm through the gears.

[Barry]

What is the error at lower revs i.e. is the magnitude of the error the same in which case it's a zero error that might be corrected by moving the needle or is the error greater at higher revs in which case it's a span error which indicates an internal fault.

I would do a simple calculation with your final drive ratio and compare the tacho reading with the speedo reading maybe using other means to correct the speedo reading first. What is your final drive ratio ?

[nhmaf]

There is a potentiometer in the tachometer circuit that is used for making adjustments, but I cannot recall at the moment if it is for adjusting the total range or for specific value -- I will try to find a schematic/info on it..

Got it!
Sometime ago we had a thread on this, and one of our members, Adamaster, posted some pics of the tachometer with the back cover removed.
There should be ~3 screws securing the back cover on the tachometer housing that must be removed first, then you will see the circuit panel for the tachometer-the adjustment potentiometer is shown circled in the upper left part of the picture:


From the side, it should have a small slot for using a small screwdriver to make adjustments.  It is also very likely that some paint or varnish has been applied to the rotary knob to help hold it in place/adjustment, though these things aren't foolproof and the potentiometer can degrade over time as well.


I would VERY CAREFULLY remove or loosen the varnish with a bit of electronics cleaner and the tip of a small knife, or at least get it to soften a little so you are less likely to break the potentiometer while trying to turn it.   A VERY small amount of turning will have a significant effect on the dial reading., so go slowly and carefully, and check with your inductive test instrument.  Counterclockwise should lower the needle reading, and clockwise should raise it.   I'd recommend checking at idle and at ~ 4K or 5K RPM for accuracy.  If you find that you cannot get it to be reasonably close at both RPM settings with a single setting of the potentiometer, it may be that the dial needle needs to be repositioned or that the potentiomenter or another component way out of otlerance and is in need of replacement.  

Hope this helps-
Mike

[georgesgiralt]

I, for one, would calibrate the tacho outside the bike.
As there is 2 sparks per revolution, if you feed the tacho with 50 Hz (so 50 periods per second) you should get a reading of 50/2 RPS (Revolution per seconds) and 60 time this figure for RPM. So the tacho should read 1500 RPM. Getting 12V at 50 Hz is easy if you've a small transformer giving 12 V on it's secondary and your mains is at 50 Hz. If you're fed with 60 Hz then you'll get 1800 RPM.
Of course adjusting for end of indication so neaar 8000 RPM is not so easy but you can find a suitable generator and a transformer using your PC and sound card and the same transformer as above, this time reversed (fed by the earphones sound card output at the 12V wiring, and the tacho at the previous primary winding) ....
I did this on a car tacho a very long time ago and it was perfect. (I only had to adjust one point, the design giving exactness trough the whole range once one point was set)
If you've to buy a transformer buy the less powerfull you can find and ask for 10 V secondary. Should be sufficient.Be carefull playing with the mains wiring and insulate things perfectly.. Will be sad to have to get your bike from your estate....

[DanMay]

Thanks for all the replies.  I was first suspect of the RPM reading from the installed tachometer when I was riding the bike.  The tach would redline but I could feel it was still in the power band.  I had the bike on the dyno last week, and the dyno inductive tach proved my theory.  I should have taken samples at each 1000 RPM intervals too see if the skew was linear or not.  What I do know is that my "real" redline is when I peg the tachometer all the way and the needle bounces off the bottom.  

I have an extra "tiny tach" I will install and see if I can correlate.

[Luca]

I'm going to adapt one of Monte's sayings here:
If you don't show us a chart you didn't dyno the bike  

But seriously, I for one would love to see how your bike did on the dyno.  I've only ever seen one chart.  It does show power increasing all the way up to redline, though, so I think by the time you're out of the power band you are at or past redline (though the owners manual does allow for a temporary maximum engine speed that is slightly higher than the maximum continuous engine speed)

[DanMay]

I am racing this R65LS in the US.  AHRMA Superbike Lightweight.  I have not done anything trick (yet) to the motor, just jetted the stock Bings to about a 12.0 AFR.  The rest of the prep was tires and suspension.

See the attached chart, with some explantions.
1.  I stopped the run at 7500 RPM, partly because the bike tach was pegged.  It put out 39.68 HP.  Normally, the peak HP is below the max RPM, so I do not know if it would have gone higher if i let the revs go higher.
2. I could go a little leaner on the main jet to get a little more HP, but the bike responds well with this carb setup.

[DanMay]

Here is some video from my onboard camera.  This is my first time on the R65, I normally race a R50/5

http://www.youtube.com/watch?v=tptpQwMYP1Y

http://www.youtube.com/watch?v=WUb1Ffq6d-I

[Barry]

Interesting stuff. At first I thought the torque curve was incredibly flat until I saw it didn't start until 4750. Still it is very steady. Racing looks like fun.

[nhmaf]

Nice work Dan - your smoothness & speed retention in the curves is an asset to catching up on some of those riders - imagine if you just had another 6-8 HP for those straights!

The engine does sound like it is running pretty good

[montmil]

Are you planning on running AHRMA Vintage Days at Barber this October?

[DanMay]

Thanks for the feedback !  I am running at Barber again this year.  I ran last year on my R50/5 in Production Lightweight.  This year I will most likely run the R65LS and my R50/5 (in Sportsman 500).

I am running next at NOLA with the R65LS.  I am going to keep the R65LS motor close to stock for this year.  I can hold my own with the Ascots, although they have a few more HP.  My gearing was way off at the last race, I was running the stock 32/9 final drive and I was topping out 5th gear just as I hit the front straight.  I plan on running a 37/11 at NOLA.

[Luca]

Thanks for sharing the chart and the videos.  The filming was top notch.  That's a great place to mount your camera.

The riding was impressive too, with all of the big gains being made up in the turns.  I dropped a couple bricks in my trousers when you overtook the first guy in race 5 and grabbed some of the track edge in the process!