No start MK3 Interstate

Kill switch set to off? Have you got sparks at the plugs? Are the plugs wet with fuel? Check the jetting settings with your old carb. Report back when done/checked all this. 

Assuming the easy start Commando had the "worn carb'" fitted to the bike, do you still have it to hand so as to swap it back and see if that brings the engine back to life.

I'm getting fed up with duff plugs, and I'm not sure they are all fake either! 
Dan 

John,

''it's being going on for a long time'' Sounds like you have had more than one faulty spark plug, in which case I doubt it is the spark plug if you keep getting repeated failures. 

Ashley,
For me its across 10 bikes, and various NGK and Champion plugs. I dont think my ignition systems are causing the problem!
Dan

Below is paragraph from a long screed of information from Rick Parkington that I received from him on the subject of spark plugs.
"If it is habitually fouling plugs the next thing is whether the plugs themselves are the problem (maybe try a different make or grade?) or whether the fuelling is on the rich side - with all the changes to fuels there are a lot of question marks over all that too! Another thing you could try is to disconnect the live feed from the ignition switch to (I presume) the coil and replace it with one directly from battery live to rule out any wiring or switch issues".
Another issue that was mentioned to cover fouling is the modern plugs are made to run with leaner mixtures to appease the environment lobbyists so wont tolerate oily over rich engines.
Suspicion of contaminates conducting through the ceramics is something else to think about. 
Who we should buy plugs from?
The people you feel that you can trust the most especially if you need redress at any time.