Why would a 2003 1200 custom Harley Davidson have spark knock under load?

Answer 1

When the machine is under load, the machine is supposed to automatically retard the engine spark (or do I have that backwards? Anyway...) A common problem for your symptoms is either a bad VOES (Vacuum Operated Electric Switch) or a bad Ignition Module. Here's the skinny. The VOES is connected to the carb by a black rubber tube. When you are wailing on the throttle, the vacuum in the carb goes up, which sucks on the VOES. The VOES detects this sucking, and tells the ignition module to change the spark advance to eliminate the knocking. So, to diagnose this you'll need to test the VOES to see if it is switching correctly (covered in the Service Manual). If it is working correctly, you'll need to check the carb to make sure the port for the vac tube is ok, which it probably is, since that vacuum tube also opens the vacuum operated fuel petcock. If both of those pan out, it looks like you may need a new ignition module.