----- Original Message -----
Sent: Sunday, July 03, 2005 5:57 PM
Subject: [FlyRotary] Float Bowl?? -
-lycoming O-320
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.