Sprocket Tacho wheel pulse drifting?   Ripple Pulse or V pulse

1)  Add a thermal resistor circuit to limit the drift.
 
 

 +5v
   /
   \    3k                                                                                ________> to IC 6 pin 8
   /                                    Thermal resistor 10k (15k)         |
   |______________________/\/\/\_____________________/\/\/\__
  |
  \/    .6 v diode (1N4148 or alike)                                Drift comp. pot about 50K
   |
   |      Ground
 
 

Heat and cool tacho as it is running. Set drift comp pot so the ripple pulse is at 50% when warm and cool.

Ic 6 pin 8 is about .2v in normal operation. This gives an output voltage on IC 6 pin 6 of  about  2.8 volts.  This may need to change a little with temp. This mod can do this.
Sorry don't remember if the Thermal Res. goes up or down with heat.  Did this once it worked great.  Give feedback if you do it.