I have run out of the main supply twice and both times I didnt mean to do it. Both times there was no warning, no sputter, it just quit. Today I was running in heavy traffic at 70 MPH  and I mean the cars were close enough to touch and it quit. I almost wet my pants. I am sure that I responded pretty quick but, it seemed like a lifetime before it started running again. My advice is to not let it run out of the main tank because it might cause you serious problems.