I have recently bought a 2007 YZ450F which has been going great. Very well looked after and all original. When I started it the other day, it started leaking a large amount of oil out the drive crank seal. I have now changed the o-ring under the collar along with the seal (also changed oil filter and oil while at it) and still the problem is there! I presume there is too much oil pressure which is forcing oil out the sprocket seal? Help please I don't know what else to do!?!?