Speedo impending death?

When I completed the restoration on my '74 Commando, the speedo was in working condition, so I only cleaned the face and replaced the glass and bezel. Sometime later, it started to wave back and forth about 2 mph marks either side of the "actual" speed - something I wasn't happy with, but tolerated. Suddenly, It's dancing wildly all over the place. Does this mean impending disaster? Have the gears gone? Will it possibly seize and do something nasty like break a cable? I should probably disconnect it now, but would be glad for advice. Though I'm confident with getting into every bit of the engine and gearbox, I've never messed with the internals of one of these instruments - is it best left to an expert? If so, who might that be? (I'm in the middle of the USA - Omaha, NE).

Thanks,
Paul.

Paul, start by eliminating the easy stuff.

Unscrew the cable going in to the dial gauge. Is the squareness of the cable end still good, not rounding off, not unraveling?

Is the entire cable lubed good right down and into the drive unit, is that greased inside?

If all that is good, then it may be time to send it out for inspection/repair.

Paul.
This happened to me so I will throw my two cents in. From your statement; “When I completed the restoration on my '74 Commando” did you buy new Tacho and Speedo cables? If so you have to be very careful of the amount of “stick out” of the section of inner cable going into the two clocks which will destroy your instruments in short order if it is too long. :shock: Like most parts that are made for Brit bikes you need to make some adjustments by cutting these shorter depending on the manufacturer of the cables. You might have to get the clock rebuilt if this is the case. Maybe some others have the actual length of stick out to guide by, or measure your old cable and compare to the new one. Try doing a search on this forum for the correct length.
Regards,
CNN

Take an 3/16" rod and grind four flats on the end so it is the same size as the square end of of the inner wire of your speedo cable. Stick that end in the back of you speedo. Chuck the other end in your portable drill. Make like you are unscrewing something. Is the reading steady ? If so, the gauge is probably okay. If not, I have had gauges rebuilt by Precision Instrument Repair in Atlanta. The workmanship was good. The red paint faded after 30,000 miles.


Greg

Likely just cable and drive related than clock itself. That's how my speedos do before the drive goes away. Cables should out last a few drives. The clock end of cable should stick up into clock 7/16". If a re-do of cable don't do it, then plan B C above can be tried or plan D, send clock out. Smiths clocks are more accurate than modern digital if everything in spec. Really should work steady and true as Commandos should at speed.

I had a problem with my speedo jumping a little this spring. Then one day it started getting very erratic readings, I thought the clutch was slipping. Then it was jumping 30 MPH and finally quite all together. Ended up being the sending unit, the gears just wore out. Installed a new and back to steady readings.






















a new sending unit

1 kink in the cable will give you needle bounce.
Cable pulls out at bottom near speedo drive.
Wipe off good and clean and look for broke or loose strands.
If so replace problem solved.
Grease cable w/ water proof grease by hand.
I use surgical gloves get a hand full of goop and have at it.
Easier to reinsert at clock side. Gravity is you friend.
Hook cabe up at bottom near speedo drive to a varible speed drill w/o ruining the cable end.
Electrical tape end first. your looking for slow revs not blender speed here.
Slow speed is all you need. While some one holds the drill at a certain rpms look at speedo for bounce.
No problems time for a $100.00 drive.
Still cheaper than a speedo.
Mine is spot on after 38 years.
I know this cause the PA State Police clocked me at 51mph in a 35 zone last fall.
Trooper asked me how fast I was going and I said 51 I thought the speed was 50 here?
He said its 35 and you were driving at 51, nice bike, slow down and on my way I was.
He looked kinda puzzled while looking at the bike as I think my Norton was older than him
See a Norton can get you out of a ticket almost as much as a blonde w/ an unbuttoned blouse
Marshal

As others said very likely is the speedo drive. I would pull cable and lubricate thoroughly than try it. If its still erratic one way to test it is to disconnect the cable at the speedo drive and connect a drill to it and run the drill, I believe it is counter-clockwise. If speedometer is steady than you know that the speedo drive is bad. A new one will cost about $80 with OldBritts.

Replaced mine about 1500 miles ago, speedometer is jumps a bit but is generally very steady.

MarshalNorton said:

The PA State Police clocked me at 51mph in a 35 zone last fall.
Trooper asked me how fast I was going and I said 51 I thought the speed was 50 here?
He said its 35 and you were driving at 51, nice bike, slow down and on my way I was.l

Click to expand...

I find this hard to believe.











You mean to tell me your speedo said 51, and you were actually going 51?

Unbelievable!

Make sure you have only 7/16" protruding beyond the fitting. Some of the commercially produced cables stick out too far and can ruin your speedo if you don't trim them to size.