Just bought a really nice 73 750 Commando that runs like a champ and looks really good as well with one problem. The speedo is insanely off, appears to be reading 85-90 MPH when I'm riding around 60-65! Were there different calibrated instruments and speedo drives etc., the gauges I'm sure are correct for the bike and the seller says were the bikes originals. Needle bounces quite badly back and forth over a 10 MPH range as well. The tach is smooth and even and appears to be correct. Any ideas on things to check before I replace it or send it off for a rebuild? Thanks.
It sounds as though you may have the wrong speedometer drive. The Norton Commando speedometer drive ratio is 15/12 which equals 1.25:1. You may have a 2:1 ratio speedometer drive on your bike right now. The speedometer drive ratio is stamped on the felt seal retainer and is easily seen once you remove the speedometer drive from the rear wheel. If the speedometer drive is correct I would ride the motorcycle along a road where there are mile markers or use a GPS and see if the odometer is correct. If the odometer is correct then there is a problem with the speedometer itself. I am assuming your bike has the original 19 inch rim with the 4.10 x 19 tire.
The speedometer cable is a likely culprit for a bouncing speedometer. Check the routing to see if there are any kinks and also go ahead and re-lube the cable as well.
When I finished my T100R 20 years ago and went for a spin up the road I was doing about 30MPH and the speedo said 50. Same problem so I went through a couple of indicated speed zones and found that I was getting my speed in kilometers per hour. As the speedo was a 150 mph speedo, it would work well in Canada where we don't have miles anymore. Only trouble was that the speedo light is on the left side and once past 100 kph the needle went into darkness so I got the right drive and went back to MPH. Cheers, Wilf
Thanks to both of you for the suggestions, I feel a rear wheel coming off in the next few days.
If you want to check the drive gearbox ratio, then the easiest way is to disconnect the upper end of the cable from the speedo and count the number of cable turns per revolution of the wheel. If one wheel revolution = 1.25 cable turns, then the drive is the correct ratio.
The correct Smiths speedo for your '73 model would be a 'Green Globe' 150 MPH SSM 3001/09 unit, the instrument number should be printed at the bottom of the dial, however, by '73 they were also fitting Veglia instruments (in pairs) in which case, if Veglia, it would probably be: 09190919900.
It's not easy to resolve such small increments of a single rotation, and the error you're seeing may be due to wind-up of the inner cable. Why not turn the wheel 10 revolutions, and see what you get out of the speedo drive?
For example, if you were supposed to get 1.25 turns resulting from a single rotation, you should now see 12.5 ... much more accurate way to do it.
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I recently picked up a '72 Commando and am in the process of sorting some stuff out. My bike encounters the same issue. 12 turns of the rear wheel equals 15 going to the speedo. Stock wheels and tires as well. Indicated 100mph of the bike feels like 60 gauging only on others in traffic. Speedo needle does not bounce either.. Any ideas?
I fitted a different speedo on to my bike and now it works perfectly. So, now that I have isolated the issue being my speedo unit, how do I go about fixing/taking it apart. I took the three screws out that hold the guts in and it won't budge. Does the chrome bezel unscrew from the housing?
I find it very strange that it reads so consistently off if it is broken. It doesn't jump around either, as I mentioned earlier.
The Shaw website link you attached was very informative. Towards the end of the read, he mentions calibrating the speedo with a power drill and simply turning a little brass screw. I'd say I'll give it a crack.. Thanks a bunch, Chris
I disassembled the speedo and adjusted it by turning the little brass screw behind the housing. Unfortunately, the chrome bezel got compromised in the adventure. Now I'll be on a hunt for that piece and how to re-crimp it.. I'm still scratching my head as to how that adjustment got so far off. The screw had to be backed out at least 10 revolutions to get it in spec with my spare speedo using my drill and square drive tip used for deck screws.