Tach Inaccurate

Status
Not open for further replies.
xbacksideslider

xbacksideslider

Joined
Aug 19, 2010
Messages
1,967
Country flag
My '73 750's tachometer has had cracked glass for 20 years. Something fell on it a long time ago.

Anyway, I ordered a bezel and went to a local automotive instrument repair shop where, in the course of telling me to send it to Nissonger, they put it on their calibration machine and at 7000 it indicated 6000.

Made me wonder if another one of the factory's "Combat fixes" was to set up the tachs to exaggerate?
 
Hi- you can send it to Nissonger, or I have used a very nice gentleman in Oregon named Bob Carter- his eBay handle is 'aprivatepeace'. Does absolutely beautiful work and made me an excellent price for both my speedo and tach. Best of all, they work better than new, and look absolutely brand spankin' new to boot. I recommend Bob to anybody who wants restoration-quality work done. Good luck!
 
I had a run in with Nisonger last time so would rather go anywhere than there. They did eventually make good but had to endure much arguing with their head a-hole. If you send to Nisonger take good pictures of your tach beforehand.
 
Rather than delay the project, I used brake clean to clean out the brass/zinc center shaft area and followed with some Tri-Flow lubricant until it all spun nicely by feel.

The local old school hardware store cut me a piece of round 3.5" x 1/16 glass for $2.00. The new bezel and gasket went in with some minimal clear silicone seal between the glass and the bezel.

I was able to fold over the bezel edge by laying the unit on a cloth on my table and using various pieces of hardwood to press fold and then knock down the high spots. Finished with a 6" Channel lock plliers by clamping the bezel between a piece of hardwood on the bezel side and the pliers jaw on the side being folded over. Happy with the result. Sits nice and flush in the cast aluminum nacelle.

I should say, though, that the re-pop bezel does NOT match the original.

But to return to my original question - does anyone know if the tachs were deliberately calibrated to overstate engine speed? to read 7000 at 6000?
 
xbacksideslider said:
But to return to my original question - does anyone know if the tachs were deliberately calibrated to overstate engine speed? to read 7000 at 6000?
Click to expand...

Like most conspiracy theories always assume incompetence rather than overt action.
 
I sincerely doubt any deliberate factory miscalibration. My 74 has always been dead on.

Nissonger is my choice as well. They've always treated me well and their work is superb, albeit pricey.
 
The first post suggests that the tacho reads 6000 rpm at a true 7000 but the later post indicates that it reads 7000 at 6000 rpm ?

My guess is that American air has got in through the cracked glass. The English are more prone to understatement. :)

Generally they lag a little but are pretty good at constant speed. I've seen a couple of mine on the tester over the years. I don't think that it's fair to talk of incompetence on an almost forty year old instrument. I bet even Nippon Denso don't guarantee theirs for that long.
 
79x100 said:
The first post suggests that the tacho reads 6000 rpm at a true 7000
Click to expand...

That had me wondering for a while, however I eventually understood it to mean the tach was reading 7000 RPM when the calibration machine was set for "6000 RPM".
 
79x100 said:
The first post suggests that the tacho reads 6000 rpm at a true 7000 but the later post indicates that it reads 7000 at 6000 rpm ?

My guess is that American air has got in through the cracked glass. The English are more prone to understatement. :)

Generally they lag a little but are pretty good at constant speed. I've seen a couple of mine on the tester over the years. I don't think that it's fair to talk of incompetence on an almost forty year old instrument. I bet even Nippon Denso don't guarantee theirs for that long.
Click to expand...

I was speaking to the accusation about setting at new. I make no claims for 40-something yrs old crap, because I is such crap. :?
 
Yes, my first post misstates the case.

My tachometer indicates 7000 RPM when the engine would actually be running at 6000 RPM.

I imagine that, within the instrument, the adjustment adjusts the magnets' proximity to each other and thereby adjusts the indicated engine speed. Further, I imagine that over time, with wear or contamination or de-magnetization, the unit might indicate a lower engine speed than actual engine speed. That leads to my speculation that it might have been set up, originally, to read high.
 
xbacksideslider said:
I imagine that, within the instrument, the adjustment adjusts the magnets' proximity to each other and thereby adjusts the indicated engine speed. Further, I imagine that over time, with wear or contamination or de-magnetization, the unit might indicate a lower engine speed than actual engine speed.
Click to expand...


One thing I think you may have overlooked is that the needle return spring (which opposes the rotational force applied to the needle) will also weaken over time which would affect the reading.
 
My combat is an early one - 202093 - and the tach has always been dead on. I don't think a trick like setting the tachs to read high would have worked for long with most Norton owners.

It is very easy to check your tach's calibration on your own bike using one of the speed in gears charts.
 
batrider said:
It is very easy to check your tach's calibration on your own bike using one of the speed in gears charts.
Click to expand...

But....first you'd need to calibrate the speedo to ensure it was reading accurately.
 
Oh that was the other trick - making the speedo read high to match.

The local police have these speed-indicating warning signs around and Marshal Norton tells me they are very accurate!

Russ
 
askdoctorday-

I did a search on ebay as described in your post and ebay does not have a name/handle for aprivatepeace.

Just trying to add reliable contacts to my Norton data base.
 
Well I know that on my bike I can be cruising down the road at a steady pace and glance down at the gauges once in a while and see the tach will be setting at 3500 one time and 4500 the next time. All without changing speed. I figured it must be a variable ratio transmission. :shock:
 
batrider said:
The local police have these speed-indicating warning signs around and Marshal Norton tells me they are very accurate!
Click to expand...

They did try those signs in the UK some years ago, but as soon as everybody realised there wasn't any camera in them then it became a game to see how high you could get them to indicate-so they were rather counter productive.

They soon changed them to a sign that flashes "SLOW DOWN" instead of telling you your actual speed - no fun at all.

Tach Inaccurate
 
L.A.B. said:
batrider said:
The local police have these speed-indicating warning signs around and Marshal Norton tells me they are very accurate!
Click to expand...

They did try those signs in the UK some years ago, but as soon as everybody realised there wasn't any camera in them then it became a game to see how high you could get them to indicate-so they were rather counter productive.

They soon changed them to a sign that flashes "SLOW DOWN" instead of telling you your actual speed - no fun at all.

Tach Inaccurate
Click to expand...

They still have a couple of them here in Pueblo on the interstate but they only go to 99 mph. :D
 
Status
Not open for further replies.

Similar threads

M
  • Locked
Speedo and tach bezels '74 Norton Commando
Replies
6
Views
1,796
marshg246
marshg246
joe czech
  • Locked
tach/speedometer cleaning, calibration, repair....
Replies
16
Views
2,599
Lineslinger
Lineslinger
joe czech
  • Locked
dirty film on inside of speedo/tach glass...
Replies
6
Views
1,632
Tornado
Tornado
M
  • Locked
Electronic Tach frozen in polar region
Replies
3
Views
1,020
JimNH
J
Johnnymac
Tach seized up
Replies
4
Views
739
lazyeye6
lazyeye6
Back
Top