T3 Speedo Problem

Hi,

I hope you’re all taking the opportunity to get out in this glorious weather.

I have noticed recently on my 1975 T3 that as I am going along, at virtually any speed in any gear, as I roll off the throttle to slow down, the speedo needle immediately drops to read near zero mph - i.e. at the pin, and then comes back up to a truish speed indication only when I then open up the throttle again to get back on the power.

The tacho appears to not be affected, continuing to read as normal.

I think it must be the speedo cable, but thought I’d ask for confirmation, or a suggestion of what else to look for. I get the feeling that the odometer reading is being affected as well - i.e. under reading distance travelled.

Thanks,
Jim

A very strange one. As you suggest, the cable would be the first thing to check.

Vibration. Something worn somewhere.

Hello Jim, the only thing I can think of is the ball bearing that locks the worm gear on the output shafts has been left out, in normal use the locknut would be really tight and the worm gear would work as usual, if that nut loosens the worm gear would start to freewheel when the power is off. The output shaft pushes to the rear when under power
locking the worm gear and speedo starts working. I would definatly have a good look at the output shaft nut as this could be a warning that its coming undone and in the words of Harry Enfield “you don’t want to do that”
I hope there is a simpler explanation
All the best Roy

Hmm,

I took off the speedo cable at the bottom end, and slid out the inner drive cable to check it out - no obvious signs of damage or excessive wear so I greased it to put back in.

I also took out the worm drive assembly, and the worm gear teeth were slightly worn but seemed to seat down properly with little obvious play.

Anyway, I put it all back and tightened it up, and yet when I went back out on the road the speedo needle was still doing the same thing. :frowning:

I can’t find any decent photos showing the parts of the worm drive assembly, but a parts diagram mentions a ball as part of the main gear output shaft next to what looks like the “toothed rim” referred to as part of the speedo drive assembly, so if the fault is with either of those then the whole gearbox would have to come off…!

Any other thoughts?

Jim

Has this just started happening or has it always done it?
Did you check the worm drive for play up and down, I’m thinking maybe it is lifting up when the bike is decelerating and disengaging from the drive below?
Have you got the olive in the bottom of the cable to stop the cable pulling out of the drive assembly?

Hi Jim,
The ball locks the worm drive to the output shafts as would a woodruff key
it is the easiest part to lose when taken apart. To inspect the locknut and rear drive you don’t need to remove the gearbox but I think you will need to drop the rear swinging arm and carden drive

Ah, now that’s interesting… I don’t remember seeing an olive/spacer to ensure that the drive cable stays seated down in the worm gear…

Perhaps that’s the issue?

The olive should fit on the cable outer below the nut so it squeezes the cable outer as you tighten down onto the top of the worm drive assembly. It stops the cable pulling out of the drive mechanism
They are also used on scooters so are easily obtained, here is one supplier.