Here's one man's opinion, lifted from Gixer.com forum.
"It's from the crank case vent tube that's routed to the air box.
The oil cools off on the back of the intake valve and leaves a carbon deposit. Connect the crank case vent tube to your pair hose that sucks filtered air from the air box and the exhaust will suck it out.
If ya wanna clean it you gotta get the engine up to operating temp, then turn the engine by hand until the intake valves are closed.
If u turn it to the right position you can pour sea foam into 2 throttle bodies just enough to cover the back of the valves an let it sit for like an hour, then you can suck it out or remove the spark plug from the cylinder you are cleaning, get a rag and cover the spark plug holes to prevent poop from getting all over your bike, and make sure the other
spark plugs are removed also and crank the engine with the starter button and it will suck it in and blow it out the spark plug holes .
The pair mod will prevent it from happening again.
Its important when u do it when the engine is hot, because when its hot the carbon is softer and the sea foam or any top engine cleaner will penetrate better. Repeat until you're happy but don't try to start the motor with the plugs in because you can hydraulic the motor and cause possible engine damage"
Sounds feasible.
To test this theory, remove the crankcase breather hose from the airbox, (plug the airbox breather tube the hose was removed from) and put the c/case breather hose into a small catch bottle secured (cable tied) somewhere conveniently nearby.
Also, clean the backs of the valves until there's no more carbon.
After a 100 miles or so, have a look at the catch bottle and see how much oil is in it.
You'll be surprised!
Then have another look at the backs of the intake valves and see if they'r still clean.
Good luck!