Sticking rev counter

Technical Small Block V Twins (not new V7s)
1

The rev counter on my V50 seems to be a bit sticky. It drops to 2500 rpm and just sits there when revs drop below that lever. If I tap the clocks a few times, then it drops back to 500rpm. I’m guessing its just grime in the mechanism. Is it possible to open it up and clean the mechanism?

I’m thinking to clean it with a very light degreaser and artists brush and then apply some dry teflon grease to lubricate before re-assembling.

2

My bet is that you’ve got dirt on the internal worm drive. If it’s an original tacho with the soft alloy bezel then it can be opened up, but it’s not easy and it will take a lot of patience. I’ve done a few, made a mess of the first couple (fortunately they still worked) and got better with each attempt. You’ll need a towel, a small electrical screwdriver (3mm across) sharpened to a chisel edge, a piece of steel rod about 8mm diameter and 100 long for use as a punch and an 8oz hammer.

Right at this point comes the big fat scary warning:

DO NOT ATTEMPT THIS WITHOUT ACCEPTING THAT YOU MAY WRITE OFF YOUR TACHO.

If that worries you (and I admit, that’s partly my aim because I know how easy it is to wreck things) then get a copy of Classic Bike and find an advert for an instrument repair specialist.

  • Put the tacho face down on a folded towel to protect the bezel.
  • You should see a small gap between the edge of the bezel and the body of the tacho; use the screwdriver with a rotating action to start lifting the edge of the bezel. When it lifts sufficiently, slide the screwdriver underneath and push it forward 3-4mm, then rotate again to lift the bezel. Repeat, repeat, repeat, repeat. GENTLY. GENTLY. If you are not very careful you can split the bezel or push the screwdriver through it.
  • Eventually you will get to a point where the edge of the bezel has lifted all the way round; now do it again (and again) with a greater degree of rotation of the screwdriver until the bezel finally comes off.
  • Underneath is a rubber gasket, a plastic light distribution ring and the instrument glass. Make sure that you reinsert the light distribution ring the correct way up.
  • There are two screws in the base of the tacho. Undo those and the mechanism will drop out.
  • Clean as desired. You should be able to do it without removing the needle and face.
  • Reassembly… this is where you wish you had three hands. Seriously.
  • Reassemble in the correct order and lay it face down on the towel again.
  • Push down hard with one hand to compress the rubber gasket and with the other, use the 100mm rod to fold the bezel back down four times at 90 degrees to each other.
  • Still pushing down and with the same hand, hold the rod over each of the four points and tap the rod with the hammer to ensure they are fully seated.
  • Repeat again for the other four points, so you have eight seats. Now chase the rest of the bezel back into place.

If you succeed you will have a series of tiny creases on the underside of the bezel where you’ve pushed the stretched ally back into place. I have to say this again though:

DO NOT ATTEMPT THIS WITHOUT ACCEPTING THAT YOU MAY WRITE OFF YOUR TACHO.

Last warning for those who have instruments with chrome bezels and are thinking about it: don’t, because they have steel bezels which won’t lift.

2 Likes
3

Thanks for that. Plenty of detail there. I’ll give it a go. There’s a couple tachos on ebay so if I mess it up then it’ll cost me £60. I can live with that.