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I am inclined to agree, Dave. Certainly less likely to have fuel from
injectors in the block pushed out the intake compared to those right next to
the inlet like in the TWM throttle body.
Ed
Ed, Thanks for the info. I am quite sure that something like this is
happening, and it does seem to be worse at some RPM but present at almost
all. I will unplug the secondary injectors this weekend and see if that
fixes the problem. I can't immagine getting fuel all the way out from the
stock primary injectors.
Dave Leonard
> Hi Dave,
>
> Here is something to consider. With certain lengths of intake
combined with certain engine RPMs the pulses generated by the rotors in the
intake can actually cause a condition referred to as "Standoff". This is
where the pulses actually cause a "Standing wave" of fuel particles ejected
by the pulses at the entrance to the air intake.
>
> There is some information on the web about "Fuel Standoff" or "Fuel
spitback". Here are a few examples.
>
> We noticed a phenomena, after a race the engine cover was wet with
fuel, the phenomena is called fuel standoff or spitback, during a lecture in
the States, Allen Lockheed(son of the founder of Lockheed aerospace) alluded
to the phenomena saying that this is caused because the exhaust is out of
tune and the pulses were pushing the fuel out of the intake.
>
> Posted a question a while back about my 1986 175 Merc spitting out gas
from thr carbs. I'm in the process of reviving this old boat so I've not
fooled with it much since i posted last (been working on other stuff). Some
suggestions were bad reed valves, gummed up reed valves, stuck floats or
cranks seals. Well this evening I had her running with the carb air box
cover off. It's not spitting gas, its actually blowing the gas out the
throat of the carb. The middle and bottom carbs are the worse. One side of
the top carb is doing it a little. It is blowing so much out, the gas pools
in the throat and runs down the air box. I also noticed once while cranking
it over the middle and bottom carbs puff out white smoke a time two like it
was exhausting thru carb.
>
> Subject: velocity stacks and stand-off
>
> While we were dynoing this 3/4 race engine (280 degree duration cam)
installing the velocity stacks in every case made the fuel "stand-off"
disappear. Without the velocity stacks there was a cloud of fuel in front of
the carb inlets that you could feel the wetness on your hand up to over 1 ft
away. The fuel seemed to be in constant motion into and out of the carb
throat. the fuel did not seem to "blow away" into the dyno room. When the
velocity stacks were installed, the cloud was no longer there, and the
engine made slightly more power from the point where it came on the cam, ie.
about 3200 rpm up to around 4500 rpm. Above that rpm the power was always
less with the stacks than without them.
>
> There is however a mechanical downside to the Weber 4BIDA, and that is
that they are somewhat hard to tune. The carburetor's fidgety nature has
been known to produce a potentially-fiery-phenomenon known as "Fuel
standoff," which can transform a race car into a smoldering pile of ash in
short order.
>
>
> .....is correct on both valves being open at the same time (even if just
a little), the longer duration ,usually the more overlap. This is actually
used to create a slight intake "siphon". The longer the overlap and the more
combustion gas will enter the intake tract. In engines that have short
intake tracts with multiple carbs (like Webers) you will actually get a fuel
"standoff" above (or beside if they mount crossways)the carbs.
>
>
>
>
> So, Don't know if that could be part of your problem or not, but thought
you might want to know. It would probably only happen at certain rpms.
>
> Ed Anderson
>
>
> ----- Original Message -----
> From: DaveLeonard
> To: Rotary motors in aircraft
> Sent: Thursday, October 28, 2004 12:40 AM
> Subject: [FlyRotary] There I was, half way to Vegas...
>
>
> So as you know I have converted to a normally aspirated state. I went
flying to vegas this weekend, and actually won $300 net even after my wife
lost $100.
>
> The engine ran as well as coulld be expected considering my sea level
WOT MAP is only 28". Max n.a. static is 46-4700 RPM. Plenty enough to take
off well over Van's recomended gross and fly at 130 KTAS (using only 8
gal/hr). Better than a typical cessna - but less tan I had with the turbo.
Anyway,
>
> The point of this e-mail is to discuss injector backflow... I have
always had a problem with a faint fuel smell in the cocktpit. As part of
the n.a. conversion I removed the plenum over the intake (see pic.). Every
flight since removing that plenum has included a rather strong smell of fuel
coming in theough the passenger air vent. This smell is only evident after
take off. It turns out that my TWM secondaries, mounted on a downhill part
of the intakes, freely leak fuel down and out of the bellmouths even during
WOT operations. This leaked fuel then seeps out the cracks in my cowl and
goes into the passenger air vent. It also loostly covers everything on that
side of the engine, and the side of the fues around the pax air intake with
2-stroke oil. This was enouth to make me uncomfortable. The wife, who is
still reeling from the emergency landing after the turbo broke, now states
that she will only fly cessnas from now on - due to the very disconcerting
smell of fuel the
>
> While in vegas I tried to program the 'B' computer to use the
secondarys only above 27" MAP but I guess the adjustment did not take
because we still had the same condition on return. I have checked and
re-checked the fuel system far any evidence of a leak. It is coming out the
back end of the intake for sure.
>
> Something to think about if you are installing the TWM injectors near
the air inlet end.
>
> Dave Leonard
>
>> Homepage: http://www.flyrotary.com/
>> Archive: http://lancaironline.net/lists/flyrotary/List.html
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