Topic: Tach calibration?

[Dunk]

Curious if there is a way to calibrate the tach on a 750? Best I can figure is maybe winding or bending the spring but it looks very fragile. My tach reads about 200-300 RPM high. Not a big deal, and it's already restored and sealed up so I doubt I'd pull it apart so long as it otherwise works well... Just annoys me to see a 1200 RPM idle when it's really more like 900-950. I expect to do a few more of these bikes though and it would be nice to be able to calibrate it so it's dead on, or as close as it reasonably can be.

[dave500]

itll never be dead on,they never were a precision instrument.

[lucky]

First I would buy a dwell/tachometer from SEARS and see if your instrument really is off.
Only costs about $30. 

You need the dwell meter anyway to check your points if they are not new.

[Dunk]

Derailed already...

I would assume when these were initially made they were calibrated to be within some margin of error. I doubt there was no QC before buttoning them up and would suspect assembly line procedure would be something like spin it at x and tach should indicate y +/- 10% or whatever.

I am going by the RPM reading on my timing light and have no reason to suspect it is inaccurate though I can verify against other tachs that I have. I am running a Dyna S and don't recall any instruction to adjust it. Timing is the same on either pickup/coil.

[robvangulik]

When it still reads 300rpm high at 10.000revs that is more like 3% off,and that is not bad for a nigh on 40 years old instrument.
Who cares about the idle revs, standing still isn't what these bikes were made for...

[Bootlegger56]

If it twists you up bad enough to bother with it then you could hook up your known standard be it a dwell or other tach and mark the gauge face with a china marker at the appropriate RPM levels.  I made a pattern for a tach face decal on the 550 I just sold.  LOL....I tried to do the math based on engine RPM, sprocket and tire size that would make the tach function like a quasi speedometer ie 5th gear 3500 = 35MPH, 4500 = 45 mph...ect.  This way it would be simple to relate speed to indicated RPM.  The red line was bogus at @ 13K but reflected the true 270 degree sweep of the standard 650 tach.  (650 cam, gear an tach).  I am sure you get the point.  You could have the new tach face decal made to reflect the correct RPM or twice the normal if you want to impress your friends with a 16K red line.  Just a thought.

[BobbyR]

You have been inside the Tach and the Speedo is the same mechanism. I suppose you could tweak the clock spring inside, but is it really worth it? If you need to know you could get a small electronic tach.

One other thing to consider, we sometimes expect that if it is 100 low at idle it will be 100 low at high RPMs. The error may not be constant.

Interesting post!

[70CB750]

I once tweaked a spring on an alarm clock, afterwards it was the fastest alarm clock you could imagine

Most of the mechanical meters - no matter what they measure - are not linear and you are right, neither is the error.

[Oldtech]

Most good multi meters (Fluke, Snap-On, Mac Tools) come with a tachometer function. According to mine my tach reads 500rpm low. You could always drop by a garage and ask if one of the techs could do a reading for you. Then simply adjust your riding to compensate for the error.