'80 engine to 74 frame/harness

[jlvdo]

Help!
I'm trying to install a 1980 XS650 engine to a 1974 TX650 frame/wiring harness. There is an extra plug coming out of the new engine, and I can't figure out where it goes. The engine turns over, but no spark to spark plugs. I think the igniter(?) is out of the loop. The pics below show the color of the wires, as well as the conundrum of the extra plug. Anyone have a clue?

 

[XSLeo]

I think your extra plug from the 80 engine is from the TCI pickups on the stator. This plugs into the TCI box.
Your 74 wiring isn't set up for the TCI ignition, it had points.
If you have the TCI box and coil from the 80 then you will need to wire it in.
To wire in the TCI box and coil shouldn't be to hard. Start by pluging the connector from the pickup into the TCI box. This will leave a black, red/white and orange wire. The red/white wire will hook to one of the red/white wires from the 74 wiring that went to the points. The other red/white wire from the points to the red/white wire on the coil.
The orange wire to the orange on the coil, the black to ground.
If you want to use points then you will need the points and advancer and the rod and bushings to install into your engine.
If you don't have either set up I might suggest getting a Pamco ignition from Mike's. He has three kits with everything you need.
#14-0902 is the least expencive, a bit hotter coil than stock.
#140912 has the green monster coil, much hotter than stock. I am running this one, very pleased with it.
#140915 has the hottest coil but it runs cooler.
The kits come with instructions and the best customer support you can find.