Fluctuating Tac problem

"I had this problem after several years of flawless tach operation."

I'm curious as to why a resistor would need to be added after several years of flawless operation without a resistor.

Given that this is (was) a very common problem with analogue Rotax tachos  - I am curious to know why a simple resister was not part of the tacho, in the first instance.

Good question Ken Ryan, I would like to know the answer myself but have never heard a reasonable explanation. Must have something to do with aging or small amounts of corrosion. 

I have two theories:

I can imagine that if the amplitude of the pickup pulses is close to the maximum that the tacho circuit can tolerate before it saturates and distorts the pulse producing additional pulses, that may give an incorrect reading. Perhaps the pickup coil is somehow closer to the flywheel resulting in larger amplitude pulses?

Fitting the resistor just provides some attenuation to prevent the circuit from saturating.

Alternatively there if there is some high frequency noise coupling into the tacho circuit then the resistor helps to form a low pass filter to remove the high frequency noise.

I have no evidence of either of these, they are just based on my knowledge of electronics.