[MistahT]

My friend rides a 2003 pregen we built for him. We got it cheap, got it running, and did a mild street fighter since the fairings were toast. It's been running well until a couple weeks ago when one cylinder stopped sparking while on the highway and he had to limp home on one cylinder. The bike has a vapor gauge which uses a wire wrapped around the spark plug wire to read rpm, in this case it happens to be the same cylinder (left side when sitting on the bike). At the same time the bike started running poorly the tach stopped reading signal, and to confirm there wasn't a problem with the vapor as well we switched the wire to the other spark wire and fired the bike. Sure enough there was a tach reading which leads us to believe there is an issue somewhere along the ignition system that is only affecting the left cylinder.

We swapped the plug, wire, coil, and even igniter (ecu?) from my wife's 2004 ninja which runs to no avail, and the parts taken off the non running 03 worked fine on the 04. That leaves us stuck as to what the issue could be... Any help would be much appreciated, especially since we can test any ideas from here pretty easily with the running bike.