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You might be able to eliminate the ignition systems by doing a mag check at high power before the problem starts, then do it again after the problem starts & see if you notice anything different during the 2nd check.
Do you have a primer? You can use it as an independent method of fuel delivery. Run the boost pump & when the rpm drops, give it a few slow pumps. RPM might drop slightly while the pump fills but probably won't if you are running the boost pump. If you really have fuel starvation, RPM should surge up (probably will still be rough) when you push in the primer knob.
How rough does it run when the problem starts? Do you have 4cyl cht probes? Do they stay balanced after the problem starts? If no 4probe cht, do you have access to a fast-responding industrial thermometer or laser thermometer? If it gets really rough, you might try to run it with the cowl off & continue to run it for 5 minutes after it gets rough. Then shut down & immediately measure the temps of the exhaust pipes at the heads. If temps don't maintain their normal spread, you might have a valve train path running out of slack on one cyl & holding the valve open.
Charlie
rijakits wrote:
Kevin,
I still would bet on fuel delivery. As it sounds it must be in the carb itself.
As Walter Kerr suggested, try another carb. Do you still have the leaky old one or is that the overhauled?
Anyway 2 min sounds like a the float bowl isn't even full when you start.
I had water in the tank once and a 540 Lyc would run at 70% rpm for nearly 6 min. I thought about fouled spark plugs as it was running rough like that and wanted to warm up to clear them. But it didn't, so I decided to take off, fly a turn around the boat ( to REALLY warm up the engine), land and check mags again ( R-44 on a Tunaboat....). I take her into a hover at around 6 min running time and right there she quit!! 10 seconds later I would have been in the drink!
Turned out to be about 13 gallons of water in the fuel!! My mechanic "checked" the fuel okay - my fault, I trusted him.....
However the 540 ran around 6 min with the fuel in the line and the float bowl, until the water hit. WIWTS is 2 min is NO indication that there is good flow into the bowl.
Your description sounds like something plugs up the fuel line to the bowl or the float/needle is stuck halfway up or some debris in the line (from where the fuel line connects to the carb to the float-bowl)
IF the engine sounds good at 1500 ( no misfires, etc.), ignition should be fine.
Do you have an air filter? Any chance something in the airpath can flip-flop and partially plug the intake?
How long does it take until you can re-start and run it the 2 min at 2100-rpm?
Even if your carb is new, it doesn't mean it is/was clean. In aviation I learned to take nothing (especially if it is new) for granted.
E.g. even with new parts it is sometimes impossible to adjust the gap for the retarted points in a mag correctly AND time it correctly to the engine.
The manufacturing tolerances are too big, ....sometimes.
Check your carb....
Thomas J.
----- Original Message -----
From: kevin lane <mailto:n3773@comcast.net>
To: Rotary motors in aircraft <mailto:flyrotary@lancaironline.net>
Sent: Wednesday, August 03, 2005 6:15 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.
Kevin Lane Portland, OR
e-mail-> n3773@comcast.net <mailto:n3773@comcast.net>
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