IF you go to mixing oil,just mix 40-1 and stock jetting is fine on the old 250. PLUS the small oil injector tube at the cylinder base has to be blocked off!! Usually oil pumps are dependable. You can check it out if you replace the old SMALL line from the oil pump to the cylinder and while engine is at at idle,hold the oil pump lever all the way UP and if pump is good then you can see oil pump into cylinder. Also the oil pump bleed screw to remove any air is located at top of pump (10 mm) just open and make sure all air bubbles are dispelled and just oil runs out(do this with engine off). OPT