Motorcycle Repair: smoke at startup / now chuging, 1998 bmw r1200c, worn piston rings

Question
I have a 1998 BMW r1200c - after it sits for awhile at startup it smokes.  This morning, I started the bike, it backfired and a blue/black smoke plum emerged, now the bike is chugging almost like runing on half the cylinders or something.  I have been told to try replacing the plugs.  Does anyone here have an idea as to what is wrong -is it a valve issue?  Your input is appreciated  

Answer
Hi Joe, It does sound like a valve issue. Do try new plugs, make sure they are the proper heat range for your model. If the problem insists take a compression reading and a cylinder leak down test to diagnose the problem further. This could let you now if you have a slightly stuck valve open valve or worn piston rings etc.