Jumpy Tach

I have a 1972 911T. The Tach jumps from 0 to 1500 like a bouncing ball at idle and displays the same erraticness throughout shifting. What could cause this?

If the engine is running smooth, the tach is at fault and on the older ones there is circuit degradation, resisters going bad and such. On our 1974 914 I had a speedo repair shop do the tach as it bounced a lot. If you have points and the engine is running rough it could be point bounce. We use only Blue Streak points (1 set per year)in our 1974 914 and they are great. You could also check with a test tach such as a Sun, etc

[This message has been edited by john rogers (edited 02-25-99).]