What if I lean the mix with fuel pump on, then turn off the pump?

[turboplanner]

5 hours ago, Thruster88 said:

Brendan, the POH for my 1963 Beechcraft 23 always calls the electric fuel pump the boost pump.

If I we're into nit picking terms, the POH for the Piper Cherokee Warrior II lists several type of pumps in its system; no boost pump but 45 mentions of "electric" pump. Many of these were explanations of how you could diagnose issues, so a POH is well well worth reading.

 

The name itself is not the issue; I was using it to get people's minds off it doing a "BOOST" job in this category where it is used as a backup system for the failures you describe.

 

What the OP and several others were taking about was a boost pump to boost pressure. 

 

Rotax and Jabiru engines have been developed for the upper end of the RA class doing much the same job as the entry level GA.

 

However what to OP is talking about  is boosting pressure and reading a flow gauge and drawing conclusions.

 

Brendan has just given a good example with his XAir or what can be fitted to RA airctaft, where the basic engine had a hand operation redundancy pump, and it's been fitted with an electric pump. These pumps are supposed to fill the supply line then stop when the basic conventional carb float valve closes. In this class of aircraft, which can have all sorts of engines including Harley Davidsons, and with conventional car carburettors with a float bowl and float valve, it can be a a long process to adjust the shut-off leverage to get that valve closed against the pressure of the electric fuel pump. Thats a PRESSURE problem.

 

In terms of saving money by not using too much "boost" FLOW in the cruise on the above system at cruise is controlled by the main jet(s). You don't flirt with reducing jet size because you're only a whisker away from combustion chamber failure of valve heads, broken pistons and seizures, so no savings here. What's more likely is Brendan's experience, the electric pump hasn't cut out and is pumping fuel out of the Carb overflow, hopefully into an open-bottom engine compartment. When you turn the pump off the fuel "burn" per hour get's better.

 

Since, if you know fuel burn at cruise for your engine at a decimal point per hour (not per Nm), you will immediately diagnose a fuel puymp cut off problem. If you have been practising the basics of electric pump control you probably won't know there's a problem unless you have a closed engine compartment and there is wet fuel or stain at the bottom.

 

 

 

 

 

 

 

 

 

 

[Thruster88]

2 hours ago, BrendAn said:

 

 

My latest xair has a 618 and if I leave the electric pump on too long at idle it overcomes the the needles and starts flooding.  Will have to swap for a lower pressure pump or fit a regulator.

Standard xair backup is a squeeze bulb, not much fun operating that if mechanical pump fails.

I have had the carburetors flood out the vents tubes a few times on the thruster 582 while at low idle on a down hill taxi. My electric fuel pump pressure is a little high and am changing the pump to a facet FEP42SV, 1.5 to 4psi. Other reasons for flooding, carbs shaking at low idle and or heavy floats. 

[BrendAn]

Just now, Thruster88 said:

I have had the carburetors flood out the vents tubes a few times on the thruster 582 while at low idle on a down hill taxi. My electric fuel pump pressure is a little high and am changing the pump to a facet FEP42SV, 1.5 to 4psi. Other reasons for flooding, carbs shaking at low idle and or heavy floats. 

Mine only floods on the electric pump if left too long at idle. 2000 rpm in my case.  I don't have a pressure gauge but I think the pump might be a 6 psi .  I will buy the facet model you mentioned above and fit that. 

[skippydiesel]

5 hours ago, BrendAn said:

 

Skippy is implying that you need the pump on to get full power at critical phases of flight. 

 

 

 

Nooooo! Way out of context.

 

The electric Boost/Aux/Back Up/Secondary (whatever name you wish) is ON as a precaution should the Main/Mechanical fail, in whole or part at a critical time in flight ie TO/Climb-Out/Low level opps. Of course it will also be used should the mechanical fail at other times. I have never ever suggested otherwise😈

 

"My latest xair has a 618 and if I leave the electric pump on too long at idle it overcomes the the needles and starts flooding.  Will have to swap for a lower pressure pump or fit a regulator."

 

It may be simpler/less costly to fit a fuel return line (added benefit - will help to remove vaporised fuel). If already fitted - try a larger restrictor jet.😈

[BrendAn]

25 minutes ago, skippydiesel said:

Nooooo! Way out of context.

 

The electric Boost/Aux/Back Up/Secondary (whatever name you wish) is ON as a precaution should the Main/Mechanical fail, in whole or part at a critical time in flight ie TO/Climb-Out/Low level opps. Of course it will also be used should the mechanical fail at other times. I have never ever suggested otherwise😈

 

"My latest xair has a 618 and if I leave the electric pump on too long at idle it overcomes the the needles and starts flooding.  Will have to swap for a lower pressure pump or fit a regulator."

 

It may be simpler/less costly to fit a fuel return line (added benefit - will help to remove vaporised fuel). If already fitted - try a larger restrictor jet.😈

2 strokes don't use the vapour lock return system because they are not affected by vapour. Good suggestion but fitting the correct pump is the way to go. Thruster has supplied the facet model number so it will be an easy job.

[facthunter]

It's probably the motor shaking more at idle causing the flooding.   Nev

[skippydiesel]

4 hours ago, BrendAn said:

2 strokes don't use the vapour lock return system because they are not affected by vapour. Good suggestion but fitting the correct pump is the way to go. Thruster has supplied the facet model number so it will be an easy job.

The vapour lock was only an added bonus (not for you) - Save your $$ and put in a return line with  a Mikuni No 35 or smaller restrictor jet.😈

Edited 12 hours ago by skippydiesel

[BrendAn]

26 minutes ago, skippydiesel said:

The vapour lock was only an added bonus (not for you) - Save your $$ and put in a return line with  a Mikuni No 35 or smaller restrictor jet.😈

No thanks. Don't want an extra fuel line running up to the motor.  Or around the cockpit.

Edited 11 hours ago by BrendAn

[Underwood]

On 25/06/2025 at 7:46 PM, Thruster88 said:

 Below is a 912 fuel pump. A blockage in the vent line might explain your pressure rising over a minute or two.  does this equate to

 

 

 

Could you explain this comment?,

from my understanding a blocked vent would restrict movement of the diaphragm which would reduce flow/pressure to some extent

how would this cause a pressure rise?

sorry If I'm misreading your comment

[skippydiesel]

18 minutes ago, BrendAn said:

No thanks. Don't want an extra fuel line running up to the motor.  Or around the cockpit.

Thats fine BrendAn - FYL; A return line does not have to run from/to the engine. Usually T'd in somewhere between the pump & the carburettor(s). 😈

[BrendAn]

16 minutes ago, skippydiesel said:

Thats fine BrendAn - FYL; A return line does not have to run from/to the engine. Usually T'd in somewhere between the pump & the carburettor(s). 😈

Yes. I thought of that after I posted it. Trouble is in an xair the fuel pump is between the seats , anything I add in front of it would have to be in the cockpit .

[Thruster88]

1 hour ago, Underwood said:

Could you explain this comment?,

from my understanding a blocked vent would restrict movement of the diaphragm which would reduce flow/pressure to some extent

how would this cause a pressure rise?

sorry If I'm misreading your comment

So the fuel pressure in skippys aircraft is dropping from normal 4-5 psi to about 1 psi when the boost pump is turned off and slowly rises over a minute or two back to a normal level of 4-5 psi.

 

Skippy tells me his boost pump pressure is a little higher than the engine pump pressure so the boost pump could force the diaphragm down in the engine pump and It would stop stroking. A blockage of the vent could keep the diaphragm down with minimal stroking which gradually increases as pressure bleds back into the vented section of the engine pump.

[Red]

6 hours ago, Thruster88 said:

So the fuel pressure in skippys aircraft is dropping from normal 4-5 psi to about 1 psi when the boost pump is turned off and slowly rises over a minute or two back to a normal level of 4-5 psi.

 

Skippy tells me his boost pump pressure is a little higher than the engine pump pressure so the boost pump could force the diaphragm down in the engine pump and It would stop stroking. A blockage of the vent could keep the diaphragm down with minimal stroking which gradually increases as pressure bleds back into the vented section of the engine pump.

Lets just remind ourselves how the pump works, the diaphragm could only be completely stuck in any position if the cam broke or part of the the plunger snapped and  fell inside the crankcase or it tore through the diaphragm

Edited 3 hours ago by Red