REFLECTOR: Fuel pressure

[Laurence Coen]

Gilles,

I am running a Franklin with the same setup.  Nine PSI is the maximum
allowable pressure for that carb.  Fuel flow into the carb is regulated by
the float valve.  With pressures greater that 9 PSI, the float can't exert
enough pressure to close the needle valve against the fuel pressure which in
turn floods the engine with fuel.  The reason the engine runs rough when the
boost pump is on is caused by the way the Weldon pump regulates pressure.
It has an internal bypass valve that opens on the pressure differential
between the inlet port and the outlet.  When you turn on the boost pump, you
raise the pressure at the inlet to 6 PSI and the weldon adds it's 9 PSI to
that and now your outlet pressure is 15 PSI and you are flooding the engine.

Larry Coen

----- Original Message ----- 
From: "Gilles Gratton" <gillesgratton45 at sympatico.ca>
To: "Mathieu" <mathieu.gratton at sympatico.ca>; "Velocity Aircraft Owners and
Builders list" <reflector at tvbf.org>
Sent: Monday, August 23, 2004 8:10 AM
Subject: RE: REFLECTOR: Fuel pressure


> Carburetor help,
>                      My Weldon fuel pump on my Franklin engine is
delivering
> 8-9 psi. Is this acceptable or too high? I have already adjusted it down
> from 12 where it was when I installed it. What kind of problem can too
high
> a fuel pressure cause on the carb, if any.  I have a Precision Airmotive
> carb, model MA 4-5.
>            Sometimes when I turn on the electric boost pump while climbing
> at full throttle, my engine runs rough.  This clears up when I turn the
bost
> pump off.  What could be happening here?
>      I use the Facet electric boost pump as recommended by Velocity. My
fuel
> line system is
> 1. sump tank
> 2. shut off valve
> 3. filter
> 4. Facet pump
> 5. engine pump (Weldon)
> 6. carb
>          My fuel pressure gage is on the line from the engine pump to the
> carb.  I also have a parallel line with a one-way valve by passing the
> engine pump and running directly from the Facet pump to the carb.
>    Are everyone using those Facet pumps or is there a better one.
>
> Any theories on the rough running engine.  I have ruled out carb icing by
> applying carb heat to no avail.
> Gilles
>
> --
>
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