Silvaire said it first, all it does is fill the fuel bowl of the carb during start up.
Untill last week I had an aeroplane that was near on impossible to start. “Easy Start” was taken to it a few times in anger. We went through the whole system and found about a dozen things not helping it. Now it would take the key out of your hand! Starts on first turn. Of course a well known old name in UK had signed it off a few flight hours ago, and should really take more pride in his work.
Genuinely interested, what things were wrong that made starting diffcult? What type airplane?
Airborne_Again wrote:
I have never tried to start a carburetted PA28 with the auxiliary pump off, but I don’t think it will work.It does, but it takes some revs to build up enough pressure, so it’s not nice on the battery and starter. Also, if the plane has just been up and some fuel pressure is left you may not even notice.
in that manual, they never advise to switch it off after starting, and then back on before takeoff (as they do in later POHs, say for the Archer II)It does in our Warrior II as well, fuel pump off is the very first item after engine start, then control fuel pressure is still there, then on with the rest.
Aftet you fill the carb’s float bowl with fuel, the starting behavior of a carburated Lycoming with Marvel carb (meaning almost all of them) is completely unaffected by whether the fuel supply to the carb is pressurized.
Silvaire said it first, all it does is fill the fuel bowl of the carb during start up.
I think it may do more than that, particularly with fuel tanks in high wings (Maule etc.) or fuselage (Jodel etc.) which can feed the carb by gravity.
Turning the electric pump on before engine start and then off (either before or after start) verifies that both pumps are providing and holding fuel pressure at the gauge.
Talking of Jodels, do any of them have primer circuits? None of the D140s I’ve flown has a primer, and they all seem to start as easily as any other O-360 Lycoming.
When the float bowl is full, it is valved off from the supply by the float valve until the level in the float bowl drops again. Just like a functional toilet tank/valve two hours after the toilet was last flushed. The fuel in the float bowl is not ever pressurized by the fuel line.
With no gravity flow to the carb, e.g. on a Cherokee, assuming you shut off the electric pump before startup, If you were to crank the engine on the starter for an
extended period it is imaginable that engine could suck enough fuel out of the float bowl that the float valve would open again. If the line is holding pressure from either previous operation of the electric pump or operation of the mechanical pump during cranking on the starter, that will refill the bowl, which will isolate it from the fuel line again. If there were no mechanical pump you might see that as a loss in line pressure on the gauge when cranking for a long period, but Lycomings generally have both mechanical and electrical pumps.
If the fuel line has for some reason depressurized anyway during cranking on the starter, it is regardless unlikely that the float bowl fuel level will drop enough to make any difference in starting behavior. The jet extends well below the surface of the fuel with a full float bowl.
If you have gravity flow to the carb, e.g. on the simplest Cessnas, the fuel supply line is continuously pressurized. On other planes where there may be a pump in addition to gravity flow, I imagine it is intended to prevent vapor lock, but it seems to me unlikely that this would be a factor during startup of a carbureted engine with some head pressure to flush vapor out of the line, through the float bowl vent… but that is conceivable however unlikely.
I have never seen a Jodel with a primer. On the ones I have flown we normally give 1,2,3 pushes of the throttle. That’s not to say there are not Jodels out there with primers as there are many different builders of Jodels and several varieties of engine. The Potez was popular in this area as we had the renowned Charles Olivier (Dr Potez) based nearby.
Just as another data point, the POH of the Grumman Traveller also calls for fuel pump to be off for engine start:
On my O-360-A3A (Rallye 893 E), the fuel pressure indicator (gage) indicates fuel pressure after engine shut down (and of course with the electric fuel pump off) – I must admit I haven´t noticed for how long after the shut down. Am I correct in assuming that this is due to pressure remaining in the “float bowl”? or something else? I see that the fuel pressure sensor is located between the engine Fuel pump and the Carburetor.
893e_fuel_sys_schem_pdf
Yeager wrote:
On my O-360-A3A (Rallye 893 E), the fuel pressure indicator (gage) indicates fuel pressure after engine shut down (and of course with the electric fuel pump off) – I must admit I haven´t noticed for how long after the shut down. Am I correct in assuming that this is due to pressure remaining in the “float bowl”? or something else? I see that the fuel pressure sensor is located between the engine Fuel pump and the Carburetor.
All of that is normal.
There is never any pressure in the float bowl, it is vented to atmosphere. When the pump is on there is pressure in the fuel line, and that’s what makes fuel flow into the float bowl when the float drops slightly and cracks open the float valve. Just like a toilet.
A closed float bowl valve isolates the fuel line between the pump and carburetor. As a result when the engine is shut down with the float bowl full the gauge will show pressure until very, very slowly the valve or fuel pump allows line pressure to leak down – the valves aren’t totally perfect.
If the line pressure drops quickly when the engine and pump are stopped, its an indication of a substantially leaky float valve – which isn’t ideal.
