Tachometer issues

Hello all-
Back again. It's been a while. I have had an on-going issue with my MK3- Seems to "eat" tachometers. When I restored the bike a couple of years ago, it got a new Vehill cable and a new drive unit from Old Britts. I have routed the cable as cleanly as I can, based on what others have done, and the way my other Norton is done. After installing a newly rebuilt unit, after about 3-4 good rides the tach just starts spinning wildly and ultimately staying pegged 180 degrees out. This is the third tachometer in 2 years I've purchased. None of them have worked for longer than the time mentioned above. Any ideas, cause Im out of those, and money! Cheers.
JD

Three tach's in two years? Ouch!

A couple things to check, start with the fit of the square part of the cable that fits into the back of the tach, it should be a smooth slip fit. If you have to use any amount of force to seat it in the tach, you're pre-loading the mechanism and it won't last. I dressed mine down with a sanding disk on the dremel until it was just right.

Then, check the cable length, because if the cables too long, it will pre-load the tach as well. My method was to install the cable in the tach (off the bike), seating the drive tach end as far into the tach as it will comfortably go. Then I measured how much the drive end extended from the other end. Remove the cable from the tach; duplicating that same amount of extension at the drive end, measure the extension at the tach end.
What you want to know is how much of the cable should fit into the tach.

Now install the drive end to the bike and measure the extension at the tach end - it wants to be a little less that the amount you determined would fit. If it's too long, you can cut it down with the dremel cutoff or carefully with a bench grinder. Just be sure that you don't fray the cable in the process, because that will screw up that nice smooth fit you worked for when you started.
Hope this helps,
TC