laymen's terms, it transitions the fuel delivery from low speed circuit to the high?
technical definition...Pop-off is the measurement of the point where atmosphereic pressure overcomes the fuel draw vaccum of the carb. The entire regulator chamber is nothing more than a big check valve. It is the step between the fuel pump, which is providing more pressure than the carb needs to deliver, and the jets, which meter a fixed amount, within a certain pressure range. The regulator chamber is in place to maintain that pressure range.
The whole regulator chamber gets filled with fuel. The fuel is coming from the pump, sits under the diaphram, and is flowing through the jets. If it was just an open flow, with no needle and seat, the jets would be supplied with too much pressure from the pump. As the fuel is sucked out the jets, the fuel supply in thereis drawn out, and the diaphram goes down with it. As it gets to it's low point, it contacts the lever arm, and lifts the needle from the seat. That allows more fuel into the chamber, pushing the diaphram back up, re-seating the needle and shutting off the flow from the pumps, so the jets aren't over-pressured. This happens many, many times every minute as fuel is consumed.
If the pop-off point is set too low, there isn't enough resistance on the needle to stop the fuel pump flow, and the jets are over-pressurized, giving you a rich condition you will never tune out. The same goes for leaking needles.
If the pop-off is too high, there isn't enough fuel pressure to overcome the vaccuum and spring pressure, and you will have hard starts, as the engine is not spinngin fast enought o produce enough pulse, to make the pump supply enough pressure to flow the fuel into the regulator chamber.
It is adjusted by changing the size of the N&S, and the spring tension. There are 4 general strengths of springs - the less the spring is rated the lower the pop-off pressure will need to be to unseat the needle. For example, a 65 gram spring may give you a pressure of 15 psi, where a 115 gram spring may give you 50 (not actual numbers).
You test the pressure with a pop-off pressure gauge. With the regulator chamber open, wet the N&S with WD-40 or gas. With the pump body removed, attatch the gauge to the fuel inlet fitting on the body. Pump the gauge until it 'pops' noting the pressure gauge. Repeat this 3 times to get a nominal reading. Installing a heavier spring will raise the pressure.
video testing pop-off
http://www.shopsbt.com/forum/t29016/
