Carburetted Piper Cherokees: aux fuel pump for cold starts?

Yep not sure if I was misunderstood, but to clarify – our Archer III POH says Electric Fuel Pump ON for start.

My current plane has the same carb’d Lycoming engine and almost identical fuel supply system. In preparation for startup, I run the electric pump until the gauge shows fuel pressure, then shut it off before starting the engine. This as you say then verifies mechanical pump function during taxi and run up, prior to turning the electric pump on again as a backup for takeoff.

Also, if the fuel pressure holds after shutting off the electric pump prior to startup, you know the carb float valve is sealing properly, much like checking a toilet float valve by listening for water flow after the toilet has refilled This issue is highlighted in my head by years of motorcycle experience, in which a leaky float valve can result in the engine being filled with fuel, and also by the very typically leaky float valves of Stromberg carbs fitted to early Continentals, which cause fuel to drip continuously onto the ground if you forget to turn off the manual fuel valve after shutdown.

boscomantico wrote:

In fact I have seen youtube videos where people flip them on before start for a couple of seconds but switch them off again before cranking.

Are you sure you are not mistaking the primer pump on the Archer III or the starting procedure for an injected Lycoming (eg. C172)?

Cycling the electric pump on then off after building fuel pressure is very common practice before starting a carbureted engine.

Silvaire wrote:

Cycling the electric pump on then off after building fuel pressure is very common practice before starting a carbureted engine.

I need to do this now on my Triumph Spitfire (2x Dell’Orto DHLA carburettors) as I’ve removed the mechanical fuel pump since they tend to leak fuel into the crankcase.

If it stands for anything more than a few hours there is significant evaporation from the float chambers and when you turn the pump on you will hear it run for a few seconds as it fills the carburettors and builds pressure. Then it goes quiet (but is still ‘on’) so you know you have fuel for starting. The pump is one of these.

In theory you can of course start the engine on what is in the chambers, so long as it hasn’t evaporated completely, and then turn the pump on afterwards. In this application it is not helped by the fact that the accepted cold-start technique with these carburettors is to pump the throttle 6-8 times to squirt fuel into the intake tract from the accelerator pumps. This is going to use up a fair bit of what is left in the chambers after it has stood, so you put the fuel pump on first.

When I used to run it on only the mechanical pump, not only did the fuel in the carburettors evaporate but the fuel in the line, the pump and the filter would tend to syphon itself back to the tank, resulting in an awful lot of cranking to get fuel into the cylinders.

I think the primer was only optional in the Cherokee II, Archer II, Warrior II series

Hence, it would be tough time early morning to get a cold start with electric pump OFF (or U/S) and no (optional) primer?
Of course, one can still prime it by turning the prop by hand and suck-in fuel into cylinders

At least in Archer2 I used to fly, it will be hard to cold start in winter without electric pump ON even after priming (I would say impossible without primer), then, Pump is switched OFF for taxi and ON before takeoff for the obvious reasons…

Silvaire is explaining this correctly. Assuming that the carburetted engine has run recently enough that the carb float bowl is full, the engine should start and run on that for as much as a minute without the carb being refilled. When the engine runs for a minute, the engine driven fuel pump will fill the carb float bowl. Therefore, and engine which is ready to run when started, will do so without any fuel pump running. If the engine driven pump is not working, and the electric pump is off, the float bowl will empty after a minute, and the engine will stop. This is a check which should be done occasionally by a pilot, and certainly at maintenance to assure that the fuel shut off valve actually shuts the fuel off (I’ve seen them not). The engine driven fuel pump for a Lycoming is a diaphragm pump, which doesn’t produce much pressure, just constant flow, it will pump volume per engine rotation, not much pressure. You can’t have anything near injection system pressures for a carburetted fuel system, the fuel pressure will lift the needle off the seat, and flood the engine. Carburettors have maximum fuel inlet pressure specifications.

If somehow (and it should not be easy or common) the carb float bowl is empty, then yes, you’ll need the electric fuel pump to fill it before a state will be possible.

The airplane has an electric fuel pump in the case of engine driven pump failure, no other reason. It’s a certification requirement. Yes, put it on for takeoff, simply in case the engine driven pump quits during that time, you don’t have time to find and select on the electric pump. Like the engine driven pump, most electric fuel pumps for carbueretted systems are low pressure “click click” volume pumps.

Plunger primers are great for starting, because they will atomize the fuel well, which the carb during a start will not. The plunger primer produces a fuel pressure much higher than a fuel pump could even if it were running, and create an ideally combustible mixture. Yes, you can somewhat run an engine in flight on the plunger primer, I have done it a couple of times. It’s not great, but it’s something when nothing else will work.

Of course, follow the flight manual procedure for starting, but understand that sometimes doing so is as much a systems check, or “back up plan included” as it is primarily necessary for operation.

None of the foregoing applies to injected engines…