^ this guy here knows the true reason.
The stock wire and spark plug boot are not RF sheilded, in other words. its a chunk of basic copper wire.
Your speedometer works off the same principal pulse rotatation of a magnet passing a pickup
Your magneto (magnet-magneto see the relation here?) Does teh same thing to generate a spark!
its tripping it out
Any motorcycle stealership should carry plug wire that is shielded, look at the end of the wire it should have an outer cover, an inner "sheilding cover" and of course the wire in the middle, thats what you need.
OR you can get a resistor type spark plug, any auto parts store will have one. for example, a NGK BR5HS
Might as well grab the good wire a good NGK boot and a better plug all at once and be done with it!