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Kevin,
The fact is the engine does run at 2150 rpm for
about 2 minutes before it stumbles. If burning say 12 GPH.
Then that is 0.2 gal/minute (12/60). So you engine apparently burns from
0.2 to 0.4 gallons before it starts to stumble. It runs at
1500 rpm (you do not give a fuel flow) - so assuming the fuel flow is
linear then at 1500 rpm your fuel flow is 1500/2150 *12 = 8.37 GPH, which is
sustainable.
Now since your fuel system (what ever the problem ) can
sustain 1500 rpm or 8.37 GPH (0.139 GPM) that is approx 70% of the fuel flow you
need at 2150 rpm. So lets say your carb's flow bowl volume is 1/2
pint. That is 0.0625 gallon.
So consider the flow bowl simply a part of your fuel
flow. At 12 GPH (0.2 gpm), the bowl would empty (or get too low to sustain
2150 rpm) in .0625/0.2 = 0.3125 minutes or 18.75 seconds. So at
first glance that would not seem to be the problem. However -
that is provided there were no fuel flowing into the bowl at all. But of
course there is - at least at the rate of 0.139 gph (8.37 GPH). So the
draw down would actually be the difference between the demand (12 gph) and the
supply (8.37 gph). That turns out to be 12-8.37 = 3.36 GPH (or .056 gpm).
So taking our 1/2 pint float bowl at 0.0625 gallons and a
draw down rate of .056 gpm we have .0625/.056 = 1.11 minutes in which
it would take a full carb fuel bowl to be drawn down to where it could no longer
sustain the 12 gph flow rate. Now that is not quite your 2 minutes,
but getting close to it . So unless I screwed up the math(or my assumption
about your fuel flow at 1500 rpm is off or your float bowl is not 1/2 pint),
your symptoms sure look like a fuel flow problem to me.
Based on everthing you checked and found OK, I would start
to suspect the carburator myself. I know nothing about aircraft
carburetors, but assume they have a float which regulates how open a valve is
that regulates flow into the carburator and therefore can determine the max rate
at which the flow bowl will fill. Its possible that the there is something wrong
in that area (since your check appears to have eliminated everything else in the
fuel system as a possible cause).
Also, I would disconnect the fuel feed to the carb stick
it into a container and run the pump (electric in this case) for one minute and
the measure and calculate the max fuel flow your system gives you. This
won't prove anything is wrong with the carb but will at least show you are
getting more than 8.7 gph fuel flow to the carb. If you are not getting at
least 18-20 GPH fuel flow I think you have something in your system that
needs fixing.
About all I can think of.
Ed A
----- Original Message -----
Sent: Wednesday, August 03, 2005 7:15
PM
Subject: [FlyRotary] further testing -
-lycoming O-320
my engine continues to baffle me and
everyone else on the airport. I would send it to a mechanic, but I've
spoken with all of them and they have no ideas either. it continues to
look like fuel starvation, but no gauges support this theory. it runs at
2150rpm, burning 11.5-12.3 gals per hr, 3 lbs fuel pressure, for about 2
minutes then stumbles(and will die unless I reduce the throttle to
1500rpm. )the stumbling occurs when cht's are hitting 375 -
410. continued running at 1500 allows the cht's to climb, so I have to
shut things down. the same behavior occurred when I flew the plane the
first two times (until I realized I could duplicate the problem on the
ground) I have a new carb(ma4spa) and 4 rebuilt cyls (on which I am
attempting to set the rings, unfortunately) if I run the engine at less
than full rich the engine won't stumble before temps hit 400+ and I have to
pull power.
I bypassed my fuel filters with no
difference. I installed new filters that have a core of little brass
balls stuck together and seemingly much better flow than the other
filter(no angles either, just straight flow)
I cleaned and backflushed the fuel
pickups and vents, ran with the gas caps off(no diff). my fuel flow is
not changing(until it sputters back to 1500rpm), so my idea that the carb bowl
was slowly not being refilled doesn't show up. the same problem existed
before I had the cyls rebuilt and welded, so I doubt that stuck valves/weak
springs are to blame. also, that would show problems from the
start, not a few minutes later.
everyone wants to blame the electronic
ig. switching between mag and elec. during the first minute shows
no diff. switching after it starts to falter shows no diff.
pulling back to 1500rpm after falter shows no diff. I can't believe
I have a spark or timing problem because there are no times when one works and
the other doesn't. my slick mag is brand new also. I haven't tried
new plugs yet, the current ones are new, although I've heard many stories of
bad new plugs (hey, what do you want for $20?)
my 1" dia. cheapy van's fuel pressure
meter is now showing 3 lbs. I removed the pressure regulator. in
the past it used to show 8lbs pressure (hence I installed the regulator)and my
carb showed signs of leakage which is one reason I replaced it. I have
no idea why it shows only 3 lbs now. I tried plumbing the carb directly
from a gas can. I forgot to switch the fuel line off and shot fuel clear
out on the wing when the engine started. I would guess that the mech.
fuel pump is working correctly. the facet elec pump will pump about 1
gallon in 2 minutes when pumping into a gas can thru the mechanical
pump. it makes no diff. if I run the facet or not. I have bypassed
the mech. pump and run just with the facet, no diff., in fact, when I shut off
the facet the engine still runs until the bowl burns down, maybe 30
sec.?
it is hard to believe that both the
auto plugs and aero plugs are simultaneously failing due to heat. I
could fly the plane at less than full rich to see how long it would fly I
suppose, but that seems unnecessarily risky. I've had two "lucky"
flights so far.
I checked for induction
leaks(pressurized and soap solution) and found nothing.
I am out of
ideas. this problem is just kicking my butt. hopefully one of you
guys will spark an new idea or theory for me.
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