Fuel pressure

[azazzera]

Started the engine for the first time after a long winters sleep. I am always amazed on how easy this engine is to start for me. Mostly due to the fact that I do all my own maintenance on it. My question is that my fuel pressure as measured on, I believe, a mr gasket fuel gauge is 5psi. Is that too high. It’s installed just before carburetor and after all filters. Do I just have a strong mechanical pump or is there something to be concerned about.

Thanks Adam z.

[Dave Neptune]

Adam, there were a couple of mechanical pumps for the A-4. Depending on how far the tank was from the motor. The pump for the long runs is the same pump only with a stronger spring for a bit more power IE a bit more pressure.

The Zenith carb should be just fine a 5 psi, although 2~3 is ample.

Dave Neptune

[joe_db]

I did an experiment one day and the engine sounded the exact same at 1 PSI and 8 PSI. Not suggesting anyone go for 8, but 5 from a mechanical pump seems OK. Is that at all RPMs?

[azazzera]

Just at idle. I haven’t looked at higher rpms or under load. What would the risk to the carb be? Fuel spray problems or leaks due to excessive pressure.

[Dave Neptune]

Adam, the carb will not notice anything. The spring regulates the pressure so pressure stays the same until the pump is maxed out on "volume" then the pressure will drop off.

I have seen these beasties run with large Holley 10 psi pumps and be just fine. I do suggest a regulator over 8 psi though.

Dave Neptune

[azazzera]

Dave,Thanks for the reply. why would you recommend a regulator over 8psi? You wrote that the carb would not notice. I understand the point of diminishing return with the pump at higher rpm's. Im assuming much the same as a heart beating faster to increase blood pressure at some point filling becomes compromised and pressure falls. with the mechanical pump what would be the difficulty either at the pump or the carb. with higher than normal pressures. The limited knowledge I have I can assume at some high pressure it could overwhelm the bowl float and allow unlimited fuel past the stopper.

[joe_db]

Eventually you get to more pressure than the needle valve can hold.

[Dave Neptune]

The reason for the regulator is why give the carb more than it needs. The carb can "flood faster" if the needle "sticks" at the higher pressure is a down side for our enclosed engines. The best is to match the fuel pump (mechanical or electric) to the pressure required and a GPH rating as to the applications fuel needs (HP).

Just a bit of safety and I do prefer the mechanical for that reason.

Dave Neptune

[sastanley]

Woo hoo....go the mech!