Slow fuel boost pump for priming

[mafitch]

When I turn on the boost pump to prime the engine, it takes 6-10 seconds before fuel flow is indicated on the gauge. This seems to be accurate, because letting the pump run about 5 seconds after the first sign of fuel flow leads to an easy start. It seems odd to me that the pump takes that long to move any fuel. Is this normal?
This is in a M7-235C with IO-540.

[VA Maule]

Mine Maule although it’s an O360 it takes a few seconds for the pressure to come up. I ass-u-me it’s due to the time it takes to build the pressure back in the line between the pump and the carb. I’ve experienced the same with the fuel servo in a fuel injection setup, the pressure is going to bleed off as soon as ether the mechanical or electric boost pump stops.

Other planes that have an electric boost pump I’ve flown take a little bit of time to get pressure built up before starting. Indeed positive pressure at the carb or fuel servo makes for easier starting ( or it certainly seems to)

[Rezrider]

Just curious. Are you using the electric boost pump to prime carbureted engines prior to starting? I use the hand primer on mine, have I been doing it wrong?
I always thought that the electric boost pump supplied fuel to the carb but not the cylinders.

[VA Maule]

Electric boost pump use before start only ensures that the carburetor float bowl is filled. Has no effect on the panel mounted primer

[Rezrider]

Ok, that's what I thought.
Cheers.

[andy]

On carbureted 180 hp engines the panel mounted primer draws fuel from the gascolator and pumps a fuel mist directly into cylinders 3 and 4. The electric fuel pump is a backup in case the mechanical fuel pump fails. On carbureted engines It pumps fuel into the carburetor float bowl. That's only helpful during starting if there's no fuel in the float bowl. If your fuel pressure gauge shows less than 3 psi when running, then it might be an indication that the engine driven fuel pump is starting to go and it might be time to switch on the electric fuel pump. Those pumps aren't meant to run continuously so it only serves to get you on the ground safely.

On fuel injected engines, the electric fuel pump has a bigger role in starting since it provides fuel pressure to the engine driven fuel pump and clears out vapor in the fuel lines.