REFLECTOR: Fuel pressure

Laurence Coen lwcoen at hotmail.com
Wed Aug 25 11:40:59 CDT 2004


----- Original Message ----- 
From: "Gilles Gratton" <gillesgratton45 at sympatico.ca>
To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
Sent: Tuesday, August 24, 2004 9:30 PM
Subject: RE: REFLECTOR: Fuel pressure


> 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?

The pressure range for the MA 4-5 is 2 PSI to 9 PSI and 6-7 PSI is where
most pumps are set. In theory, you could set the Weldon at 3 PSI and still
never exceed the max pressure of 9 PSI.  I wouldn't try that myself for fear
of the engine going lean during a full power climb. Something that I plan to
look into is a fuel pressure regulator.  I remember these being popular with
the hotrod guys at one time.

>    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).

Yes I have primer lines installed at the center cylinder ports.My nozzles
are home-brewed. I used an AN822-2D elbow.  The primer hole in the manifold
is 28 threads/inch.  A 1/8 pipe tap has the same number of threads, only
tapered.  Thus modified the elbow threads into the manifold and keeps the
primer away from the exhaust.  To form a nozzle, I drilled a #60 hole
through an 8-32 brass screw lengthwise, then tapped the inside of the elbow
and screwed it in.  The primer plumbing comes from the top of my gascolator
(which is pressurized by the electric pump) to an electric solenoid valve
(AS # 05-29823) and tee'd off to the primer nozzles. A 2-3 second push of
the button works great.


> 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
>
> ----- Original Message -----
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