Actually i have seen people run there vehicles in races and in normal bashing without the back preesure line from the exaust to the tank. And they say you gain a ton of power and the one guy said he had several gallons through his. Now im not going to do what i just said but yes with the way that 2 strokes work there is some suction and it will run without the line from the exaust to the tank. It can and i have seen it done. And yes it will run lean a little bit lean but not the way that slodsm is describing it. It will run lean but with some little adjustments you can make it work and it wont destroy your engine.