tachometer calibration

o0norton0o · Sep 5, 2016

I've determined that my tachometer is out of calibration. I had the day off and couldn't leave well enough alone, so I took it apart to have a look. Wow, what a bizarre mechanism. I have a thousand questions, and a few observations too...

I hooked the tach up to my 1000 rpm drill and it bounced wildly, just like it does on the bike. The adjuster screw had no effect that I could tell, so I disassembled it and cleaned it. I did some reading online, then put it back together and it was still erratic, so I decided to take it back apart to clean and lube it. That straightened it right out. It bounced very little and the adjuster screw now actually had a gradual effect as I turned it causing the needle to rise steadily as I turned it. It seems like it's going to work but,.......

here's my question,... I set the adjustment to read 4,000 rpms on the tach, when the drill is turning at 1,000 rpms because the tach is a 4 to 1 ratio. BUT, IF the tach is driven off the camshaft, which is half the speed of the crankshaft, shouldn't the RPM's read 2,000 RPM's when the drive cable speed is 1000 RPM's ?? I know that very few people service their own tach's and speedo's, but hopefully I'll still get an answer. Thanks in advance...

guest76 · Sep 6, 2016

4:1 is correct

Don't forget that it's not a direct drive off the camshaft.
The tachometer drive gear, is the bit that reduces your ratio to 4:1

o0norton0o · Sep 6, 2016

thanks for responding. I'm still not all that sure that adjusting the calibration screw to make the clock read 4,000 rpms for a drill turning the cable at 1000 rpms correctly calibrates the tach. (probably because I am over thinking it) It's raining today, so I won't get a chance to test drive it, which will probably be the best way to check the tach.

bluto posted this site link http://www.gearingcommander.com/ which makes calculating the speeds, rpms, and gearing very simple. hopefully, it's correct and I won't have to take it all apart again to recalibrate it again....

DogT · Sep 6, 2016

I would calibrate it off the rear wheel and a cheap RMP reader. Don't ask me how to turn the wheel that fast, that's your problem. You could probably come up with some sort of motor drive to the rear wheel. Sounds like a good project for the winter.

acotrel · Sep 6, 2016

I'd take it to somebody who repairs speedos and tachos and get it calibrated properly.

guest76 · Sep 6, 2016

I have a mode on my strobe light that gives you an accurate display of RPMs - perfect for calibrating a mechanical tacho.

If you don't have an option like that, maybe invest in a laser tacho - which you can get from eBay for less than $10 and take a reading off your crank. They are surprisingly very accurate!

ILLF8ED · Sep 6, 2016

o0norton0o said:
I've determined that my tachometer is out of calibration. I had the day off and couldn't leave well enough alone, so I took it apart to have a look. Wow, what a bizarre mechanism. I have a thousand questions, and a few observations too...

I hooked the tach up to my 1000 rpm drill and it bounced wildly, just like it does on the bike. The adjuster screw had no effect that I could tell, so I disassembled it and cleaned it. I did some reading online, then put it back together and it was still erratic, so I decided to take it back apart to clean and lube it. That straightened it right out. It bounced very little and the adjuster screw now actually had a gradual effect as I turned it causing the needle to rise steadily as I turned it. It seems like it's going to work but,.......

here's my question,... I set the adjustment to read 4,000 rpms on the tach, when the drill is turning at 1,000 rpms because the tach is a 4 to 1 ratio. BUT, IF the tach is driven off the camshaft, which is half the speed of the crankshaft, shouldn't the RPM's read 2,000 RPM's when the drive cable speed is 1000 RPM's ?? I know that very few people service their own tach's and speedo's, but hopefully I'll still get an answer. Thanks in advance...

Smiths magnetic tachometer and speedometer are basically the same unit. What makes the needle bounce is a worn bush at the base of the axle that the need attaches. Calibration will have no affect on this. Frankly I've had several instruments rebuilt. With the new electronic instruments from Smiths, they are about the same cost as rebuilding.....so up to you.

texasSlick · Sep 6, 2016

o0norton0o said:
thanks for responding. I'm still not all that sure that adjusting the calibration screw to make the clock read 4,000 rpms for a drill turning the cable at 1000 rpms correctly calibrates the tach. (probably because I am over thinking it) It's raining today, so I won't get a chance to test drive it, which will probably be the best way to check the tach.

bluto posted this site link http://www.gearingcommander.com/ which makes calculating the speeds, rpms, and gearing very simple. hopefully, it's correct and I won't have to take it all apart again to recalibrate it again....

An engine turning at 4000 rpm has a cam turning at 2000. A cam turning at 2000, driving a 4:1 tach gearbox, means the output of the gearbox, also the input to the tach, is turning at 500. Thus there is an 8:1 overall reduction from engine to tach instrument.

Your description of your calibration method, seems to have overlooked the half speed of the cam.

Slick

o0norton0o · Sep 9, 2016

I got my tach calibrated by mostly trial and error. I think I took it apart and put it back together about 10 times. It was a laborious process. I used an electronic tach on the bike's coil to determin the amount of adjustment needed to the norton tach , then made a drive set up jig with an old broken tach cable attached to a screw gun so I could adjust the calibration screw while it was mounted in the jig. It reads pretty close to the predicted speed/ rpms that gearingcommander specifies for my custom size wheels and 21/42 tooth sprockets. Lubing the base of the tach with a squirt of wurth HHS-2000 really cured the bouncing needle too.

I just took a run down the hiway at 70 and it was reading about 4100 rpms. Gearingcommander predicted it would read 3999. I can live with that amount of error. It cost me $44. for a new bezel, glass, and gaskets since I mangled the old bezel in the process of recalibrating it. It looks brand new,..... and more importantly, it works again.

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