Boost Pumps

[danny_galaga]

My kit comes with a restrictor jet. Ive forgotten the plumbing now, but if it's inline with the mechanical pump, the return covers both bases.

[skippydiesel]

2 hours ago, kgwilson said:

What is the purpose of a return line to the tank? With a Facet electronic cube pump it continues to run to its maximum pressure when the carb bowl is full and the shutoff valve closes. Pressure does not build up past its maximum. Mine is 1.5psi to 4 psi so when the carb bowl is full it just keeps running at 4 psi with no fuel flow.

The Fuel return Line has nothing to do with the Boost Pump operation.

 

Rotax recommend a Fuel Return Line. It has two related functions:

• When you do your pre start checks and turn on the Boost Pump to check function/pressure vaporised fuel will be drawn off by the return line. Ideally the  fuel return line is from the fuel distributer (on a Rotax) which  is close to the carburettors, thus reducing the amount/chance of fuel vapour entering the carburettor.
• Once the Boost Pump and or engine pump is running, cooler fuel will circulate, reducing the chance of vaporisation.

The Fuel return Line usually has some form of fixed restriction in it - min sues a carburettor slow idle jet. This is so that fuel will preferentially go to the carburettors.

 

 

[RFguy]

Fuel return is to permit circulation of : heated fuel (changing its vaporization behaviour) , purge  / pass  bubbles in the line around the system  .

So, it does provide a very useful job. For a  system without a collector, it does pose some issues to which wing you might put it back into. high wing with collector tank for both wings - fuel return is easy.  Rotax requires that a  fuel return is provided for  in its installations . (past 6 years) .

Skip, so 5 litres / hour now ?  IE something about a bit above  idle  for that engine . or about 4 tablespoons per minute. , amounts to a velocity in the delivery  of (2 x 5 lph) of 2.7mL/second.  for  1/4" ID (VOL = 31.6mL/meter ), the fluid is 'changed out' abotu once every 11 seconds. sounds reasonable. I think you could go half that again  and still have plenty of return . 

Edited July 13, 2023 by RFguy

[kgwilson]

Sound like a good option when there are 2 carburettors sitting on the top of a hot engine.

 

The carb on a 3300A is underneth the engine so not so much of an issue. However if you don't use the electric pump before cranking the engine after the engine has been shut down  for a while on a hot day it will often fire and then stop which I assume is due to vaporisation and the float bowl is not full to keep the engine running. In those situations 10 seconds of electric boost pump and the engine fires instantly and keeps running as it should.

[danny_galaga]

Personally I pre pressure with electric fuel pump before every start.