fuel pump

G'day.

I'm not sure I'm really following what you're asking... both of the fuel pumps have 1-way valves built into them. And, yes, when the electric pump is on the total pressure arriving at the carby should be higher than when it's just the engine-driven pump alone.  But the effect is limited by flow rate - each pump can only deliver so much fuel and as the flow rate increases the pressure increase from each pump decreases - so it doesn't usually stack up to too much pressure.

Regards,

Doug.

Hey Doug! I am having a problem with my engine slowly loosing rpm to the point of dying when I turn on my standby pump. I replaced my stanby pump with one without a check valve, and still have the problem, so it's not the check valve. The standby pump is putting out 2 psi, so it's not over pressure. It puts out 28 gallons per hour, which is 2 less than the recomended pump from jabiru usa that they use on jabiru aircraft. So I don't see how the standby pump is the issue.

I have the fuel bowl set at 10 1/2 mm with the aircraft level when setting it. I can lower the level to 12mm if that would help, but that seems like such a small difference to have much of an impact. any ideas?

Huh, that's odd!

So, if the engine is idling and you turn the pump on it slows, runs rough and stalls?  That would probably be running too rich, most likely because of the needle valve leaking.

Or, the engine can be at any power, you turn the pump on and the engine gradually stalls out?  That one is a poser, no ready answer for it!

Float level shouldn't be the issue...  Does it die smoothly or does it go rough?  What colour are the plugs after it stops?

Is it a "typical" Jabiru fuel system?  I.e. no fuel return line?  You've got finger filters on the tank outlets, and they're clear?

You occasionally hear about situations where there's a flake of paint or similar in the tank or bowl and, when a certain thing happens, the flake floats up, covers the inlet and kills fuel to the engine... or one of the hoses kinking when there is positive pressure between the standby pump and the mechanical one.  It's a long shot, but it could be something like that.

Regards,

Doug.

Your first paragraph is correct. Idle speed reduces till rough and stalls unless the throttle is advanced.

I have the idle mixture screw at the minimum 3/4 turn out, it was at 1 turn out and the rpm would drop and run rough before I did that. The 3/4 turn out has stopped that so I have a smooth stable idle now, until I turn on the fuel pump. The idle screw is very easy to turn, is that normal?

It is a standard fuel system, finger strainer is clean, new inline filter installed (74micron), vacuum rated hoses to prevent collapsing, it is a sonex aircraft (header fuel tank). I also had the carb taken apart and inspected at a carb shop and they saw nothing wrong with it.

Thanks for all your help and ideas. I want to fly it, but not till this is solved.

How can I check the needle valve for leaking?

If the needle is leaking and the engine is stopping because the mixture is rich one of the easiest ways to check would be to have someone watch the exhaust while it's happening - if the mixture is rich it should be obvious with black smoke chuffing out of the exhaust.  And, as you mentioned, it's not hard to adjust the float level.  You could try moving it to 12mm and see if it's better, worse or the same: you learn something either way.

Is it running standard jetting (needle type, jet sizes etc)?  The idle screw should turn easily, but not so easily that it will move under engine vibration etc.

Regards,

Doug.

Thanks Doug, I will have someone watch and look for the black smoke. I will adjust to 12mm float bowl level also.

It is the standard needle/jet size as supplied by Jabiru for a Sonex from Jabiru usa.

When adjusting the idle screw it turns very easily, what I would call light fingertip pressure. Is there a way to firm it up some? That is my fear that vibration might move it.

My memory is a little hazy on that one... Being an adjustable jet there must be something - either a spring or an O-Ring - on that jet which stops it turning too easily.  The first thing to check would be to see if the spring/O-Ring was still there and replace it if missing.  

Failing that, you could use some fuel-safe thread sealant on the jet which should increase the friction enough to stop it moving.

Regards,

Doug.

Well lowering the float bowl level to 12mm has fixed it when I ran it yesterday. hopefully it is that simple. I will see if it does it again in the future. Thanks for helping.

 

No Problem, wish they were all that easy!

D.