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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 -----
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 whole trip - and I can't
blamer her.
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
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