The clutch cover design does not provide any type of "check" valve; when the motor turns off this allows the oil to drain back down into the case. When the motor is then turned on their is a dry period to the top end and over time can cause excessive wear involving the rockers, cam, etc...
The second reason is the lack of oil pressure regulation. The bigbear style setup has a spring loaded valve that constantly keeps the oil pressure where it should be in the cooler lines.
If you were to take a closer look at the big bear plate that extracts the oil from the case you would see actually how advanced the part is. The bigbear adapter separates the cooled oil from the hot oil when entering the case. The cooled oil is then delivered to the crank (the most vulnerable part of the warrior motor seems to be crank bearings) and the tranny.
The bigbear setup also ONLY removes EXTRA OIL from the case. This means that no matter what you are always going to get oil to all the parts in the motor even if the cooler gets clogged or if the filter were to clog with crap.
We must remember that Yamaha designed this cooler for the big bear but the oil pump and case are IDENTICAL on a warrior as well. There is a reason Yamaha went through the very costly process of designing these parts and producing them over just tapping the clutch cover. The Bigbear method has proven to be a reliable, reversible, and beneficial mod that cools the priority parts of the motor efficiently.
