Check for for a worn distributor shaft that lets the shutter wheel move around enough to touch the sensor. (look for signs of the "teeth" contacting the sensor) Up and down play in the distributor shaft should be from .015 to .030. You can do a better check of the set up with the distributor out of the motor by hooking to meter, applying power and spinning the shaft by hand.) you can also see the teeth in the gap better and align each tooth to the center of the gap. Also make sure gou have a gap at the bottom of the tooth as well.