Over the last several months my Tach has been changing over a 1500
rpm range. This was even more pronounced after we changed the plug wires two weeks ago. I found that I could change the reading just by moving the pickup coil around the #5 plug wire. I moved the pickup coil to plug wire #1 and inserted a 1/4 inch ceramic insulator in the coil/core which held the wire core tightly in place. The insulator is about 1 1/2 inches long and sticks out of the sensor core on both sides. I taped the ceramic to the spark plug wire so it would not move. Solid and correct tach readings have resulted.