sirdeuce wrote:Not really the case here. the late 4AGE, small port head, has a secondary oil return to make up for the inadequate oil return of the earlier models. It is a good size return, 5/8" I.D. planted in the head where OST had shown the AN fitting. The block has a tube pressed into the side and they are connected with a rubber hose. The engineers at Toyota/Yamaha recognized an issue and that was their remedy. Not saying increasing the crankcase venting would not help. could be the drain tube was sized to be a wet vent, 5/8" is quite large.
I am fully aware of the external oil drain on the late model engines; however, fitting in the block is threaded, not pressed.
Also, I believe the external oil drain was also a bandaid fix. Oil pooling in early model 4ag's combined with hard left turns could actually cause oil to be drawn into the intake manifold (thru the valve cover breather port) and kill the engine. IIRC this is the issue the engineers were trying to solve.
Ive been thinking about this since reading Jinx's comment and am not sold that high crankcase pressure is the only issue. I remember reading that MotoIQ series and want to say the guy building it also built FA engines (which ran a dry sump oil system). Dry sump systems are capable of achieving a negative crankcase pressure. If they added the restrictor in addition to this, that suggests oil flow to the head is excessive.