Originally Posted by Myfuture_yourdebt
I just can't see how there wouldn't be a siphon effect once fuel is flowing.
There will be a siphon as long as the output (carb) is lower than the fuel level AND there is fuel in the entire tube. It all works with gravity and the weight of fuel on the "down" side pulling a bit less weight if fuel into the "up" side.
If the carb is above the fuel level, there isn't a siphon -- the carb bowl is vented so there's no "suction" to pull gas up.
If there is a SMALL air bubble in the fuel line, the weight of the gas on the carb side MAY pull the air bubble through the line and start a good siphon.
If there's a slightly larger air bubble in the fuel line, some gas may be pulled under the air and you'll have a half-assed siphon.
If the air bubble is large, the weight of the gas on the carb side may fail to pull any gas up from the tank.
One easy way to remove an air bubble from your reserve tube is to use the reserve line when the tank is full. The weight of the gas above the intake will push fuel through the tube. I'd use a smaller-diameter tube instead of a larger-diameter tube. Removing an air bubble relies on having the fuel flow faster than the air can migrate upstream. Fuel flows faster in a smaller tube and the air will migrate slower in a smaller tube.
Strap on an extra gallon of gas and test your mods.
I have worked with many siphons, but not inside a gas tank.
EDIT: I have one petcock that uses the "main" intake for normal operation and opens BOTH intakes for reserve. This would be an air leak that might kill a siphon.