I know I'm replying to a very old post.
I thought I'd share my experience anyway.
Last year I sent the original Borletti speedometer from my 1970 Moto Guzzi Ambassador to Peter for a complete overhaul. The reason was a very bouncy needle. He advised that the overhaul would correct the damping issue as well as recalibrate the speedometer, at a cost of £270. I could have bought a reproduction speedometer for around £150, but I wanted to preserve the originality of the bike, so I opted for the overhaul.
Unfortunately, from the moment I refitted it, the needle was just as bouncy as before.
Because I assumed a professionally overhauled speedometer couldn't possibly be the cause, I spent months trying to find the fault elsewhere. I first replaced the speedometer cable with a new aftermarket one, then had a second cable custom-made because I suspected the aftermarket cable might be incorrect. I also dismantled and inspected the gearbox drive mechanism, which is a quite tricky job on an Ambassador. Everything checked out.
More recently, after riding in heavy rain, the speedometer also developed water ingress. Peter initially felt that water couldn't get in through the bezel, but after I returned it to him he replaced the bezel and the water ingress was resolved.
At that point, mainly to eliminate every remaining possibility, I bought one of the reproduction Borletti speedometers from Mondejar in Spain. I installed it using exactly the same speedometer cable and gearbox drive.
It worked perfectly immediately.
The needle is completely steady, the reading is smooth, and I haven't changed anything else on the motorcycle. That left me with the unavoidable conclusion that the overhauled original speedometer had been faulty all along.
