Hi

My DT400 every now and then gets air in the two stroke oil line pipe just where it enters the carb, normally after a couple of minutes it clears and oil appears in the full length of pipe again.

Any suggestions on why this may be happening?

The air doesn't travel up the pipe from the pump.

Please see picture. 

1. I believe it's caused when using tubing with larger ID than original. You lose capillary effect.

2. There's a reason Yamaha lines aren't clear.

Sometimes it's best to not know these things if it's running ok. Whoever invented clear fuel & oil lines has a lot to answer for. In saying that, i don't know. The oil pump is churning away all the time so 'maybe'. The motor is sucking & blowing sort of at the point on entry to the cylinder, or carb in this case although reed valve should stop most blow back.Don't know, bur a good case for using premix. I guess it fills & stays that way when running after a minute?. Bleed the pump at the screw on the side & try again. It has a oneway very light spring & ball at the pump union up to the cylinder but if that wasn't seating it should only fill the line completely & slowly drip into the motor depending on what angle the bike is parked considering it enters the carb in this case.