RE: Pump pressure setting?
GN:
There is too much confusion concerning what a pumped fuel system does, and what a regulated pressure system does, and how they both work.
First, the pumped systems.
The only aftermarket pumps currently available that I know of are the “Perry” VP-20 and VP-30, made by Conley Precision Products. These are both regulated pumps, the VP-20 being operated by the vibration of the engine, and the VP-30 by pressure pulses either from the engine crankcase or a tap in the exhaust. Other than this one difference, their operation is the same.
Both are diaphragm pumps with check valves to control flow. The regulator sets the delivered pressure, but otherwise has no effect on the fuel quantity delivered.
Since these pumps use check valves there is effectively no restriction to gravity flow, a tank mounted too high will tend to drain through, flooding a stopped engine and spilling the fuel on the ground.
The Perry pumps are best used for remote tanks, such as one mounted on the airplane CG.
Another advantage to the positive fuel feed given by the Perry pumps is that by not relying on the carb suction for fuel fees, a much larger carb can be fitted to a given engine and still have reliable operation with a corresponding increase in power.
It is difficult to the point of impossible to adjust an air bleed type carb to work with a pump.
And now the regulated pressure systems.
The Cline and Iron Bay regulated pressure systems are entirely different, allowing no fuel flow except when the carb “Sucks” the fuel through. They are what is known as “Demand” regulators, allowing flow only when the draw from the carb, acting on a diaphragm, pulls the fuel delivery valve open. The biggest difference in the two is the Cline using a plastic body, while the Iron Bay uses an all metal body.
These two require a pressurized fuel tank, they get this pressure by using a check valve in either a crankcase port or the exhaust. The check valve causes the pressure to build in the tank, forcing the fuel to the regulator.
Since these two systems require the carb to draw the fuel into the engine, some engines that will not run well without muffler pressure wont work well with them, but these almost always work well using one of the Perry pumps.
If I’ve left any questions, please ask.
Bill.