Motorcycle Repair: Oil disappearing, visible signs, burning oil

Question
I own a 2008 fat boy,modified to stage 1&2.It has completed18000 miles.When the oil level is checked the level has dropped considerably. There is no visible signs of a leak on the engine, no oil on the garage floor and is not burning oil. Have you any ideas, I would be grateful for advice on how to rectify this problem.

Answer
It sounds like the Oil is Sumpping in the bottom end of the motor. Drain the motor oil out of the bottom end by removing the drain plug at the bottom of the motor. If more the 4 oz of oil come's out. Then you need to check the oil pump O-ring in the engine. Where the pump meets the motor.  Johnny