Am rebuilding a 1975 Dt250B . . . . It ran strong on mixed gas with a aftermarket Mikuni with no oil injection port. I put the original mikuni back on and hooked up the oil injection pump. It starts and runs great up to 4000 Rpms which is when it really starts kicking out the smoke . . . . I believe it is getting way too much oil as it bogs past 4000 rpm as if it is being flooded with oil . . .tons of smoke. I have adjusted the oil cam so it should be too light on oil delivery and this made no difference. Any thoughts or help would be greatly appreciated.

My guess is it's getting flooded with fuel. My DT250b has the main jet holder in the side of the float bowl... which means there's a critical o-ring that seals the bowl to the emulsion tube aka needle jet... if that o-ring is bad the engine will run very rich.

Great call on the o-ring! I just replaced it and problem solved. Thank you for helping a newbie out...... am learning more everyday!