REFLECTOR: Fuel pressure

[Gilles Gratton]

Larry,   Thanks for your response. It makes sense.  I got two other
opinions, one from Precision airmotive, who make the carb and one from a
technician at Leavens aviation, the local service center for Precision
Airmotive.  They both told me the same as you did.  They said that my needle
and seat may need replacement, there may be a leak there, causing partial
engine flooding when both pumps are on.
    What kind of fuel pressure are you running your engine at. Do you see
quite a difference in pressure when you turn the electric boost pump on.  I
plan to reduce the pressure on my Weldon pump even more.  Should I aim for 5
or 6 psi?
   Anyway, I removed the carb from my engine tonight and I will send it to
be checked.  I the meantime I will install those primer nozzles on the
intake manifolds.  I have had them for a few years and never got around to
installing them.  Fiddling around with the carb, throttle, etc made me even
more aware that a lot of fuel ends up in the carb intake box and scoop when
we use the trottle to prime the engine.  Have you got the primer lines on
yours?  I see in two other Franklin 165 here that they have the nozzles on
the middle cylinder port,  Would the rear ports (cyl 5-6) be good too.  They
would be easier to install there, the exhaust not being in the way (at least
on the right side).
Gilles.

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org]On
Behalf Of Laurence Coen
Sent: 23 août 2004 12:16
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: Fuel pressure


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

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