An airbrush works because the air drawing the paint out of the cup is on the atmospheric (low pressure) side of the valve. This moving and expanding air creates lower then atmospheric pressure over a venturi (small hole) and paint is drawn into the air stream. In the case of these fuel lines the transfer pump (FASS in this case) is moving more volume of fluid than the injection pump can use, thereby creating higher than atmospheric pressure, which could become a leak. If this FASS wasn't hooked to anything (a hose open to atmosphere if you will) then, yes a small hole in the line could become a venturi due to the velocity of the fuel. Hopefully that explanation covers it, if not I can dig up some links to explain it a bit better.

 

Batphreak, that was AWESOME. It makes perfect sense. Thanks for clearing that up for me[sm=smiley20.gif]

 

Batphreak, not to start anything butI am afraidI am going to have to disagree with you, even if the fass outlet pipe wasn't hooked to anything it still would have enough pressure on it that a small hole would still be a leak out an air brush has a small venturi where the tube from the paint cup comes into the body, it is this necking down that creates the suction. any leak after a pump will show up as a leak out not in.

 

I didn't say that a hole definitely would let air leak into the fluid in that case, only that it could. You are correct that several conditions have to be met for that to happen. Basically for the hole to let air in the velocity of the fluid has to be high enough that the pressure in the hose becomes less than atmospheric. If the friction between the hose and fluid is enough to keep any pressure in the hose above atmospheric then a hole will be a leak. For reference, here is what we're talking about:

http://en.wikipedia.org/wiki/Venturi_effect

Necking down the tube is the easiest way to bring the fluid velocity higher, that is why connection points are the most likely spot most couplers have an ID smaller than the tube/pipe they're connecting. Another way to create this neck point is if the hose attached to the pump is too small. All bets are off when the outlet of this hose has a restriction, though.

Mayfair, sorry about the major hi-jacking. Did you get that hard start solved?

 

No problems with the jack, I think we've all learned a little somethig from you phreak. I've been familiar with venturi's for quite sometime as they are occasionally used in aquariums on skimmers.