Bouncing tach

[JerseyJoe]

I've got two problems with my tach and I haven't seen any discussion here on these problems yet. If I've missed them, someone please chime in. If not, then let the discussion begin.

First problem, if engine speed changes rapidly, either from a gear change or goosing the throttle, the tach does a pretty large-scale bounce. Probably up to 4-5k then it bounces down to approximately the correct reading.

Secondly, you'll note that for the first problem, I say 'approximately' because my tach seems off by about 1000 RPM. At idle, barely ticking over at about 500RPM it reads 1,500 or so, and on the highway in 4th at 70, it reads 4,500 or so. I'd like the tach to read correctly, not the least because the car feels flat on acceleration and I'm thinking maybe the clutch is slipping, but since the tach isn't accurate, I have no reference, and I'm afraid the engine may be revving even higher than indicated. I "think" its reading high down low, but maybe its also reading low at the higher end.

The car is a 1971, with good points and cap to my eye, stock coil, ballast resistor and a 4-wire tach.

I found some screws on the back of the tach that look like adjustment points - am I being overly optimistic to think the reading is adjustable?

[Mitchka]

Well, as a general rule of thumb, any tach that is 30 years old is tired and should be replaced. I would recommend getting rid of your old tach. It is an original piece, yes, but they are inaccurate and jump around as they age. Even in good working condition, they are inaccurate near redline. I know Autometer makes a good 5" tach that is still black face.

If you need an original tach, let me know, I have 2 extras with under 40k miles on them.