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If the problem reduces or vanishes when you reduce throttle, then it is
probably a secondary ignition problem. Over-lean mixture would do that. So if it
improves with richer mixture, there you are. Props on the ground are generally
partially stalled and may not load the engine as well as a flying prop. Higher
airspeed reduces effective angle of attack.
There are several RPM where cylinder filling is slightly better than all
other RPM. This is where you would see ignition problems if it is a marginal
system. Try new plugs or shorten plug gap to .015" all around.
If the problem is gone, the ignition is having trouble lighting the mixture
provided. So richer is easy to light and it becomes more difficult as you go
leaner. The higher the cylinder pressure (best cylinder filling) the less likely
to get an arc.
Lynn E. Hanover
In a message dated 5/14/2011 11:05:25 A.M. Paraguay Standard Time,
bktrub@aol.com writes:
I've had the same thought- that up at higher RPM I might be having
some sort of ignition problem. Adjusting the mixture does seem to
mitigate it somewhat, so that leads me to think that it is not an ignition
problem. Also, it only seems to happen in flight, not on the ground, where the
loads are a little different. So, I still have questions about this, but I'm
going to approach this from the mixture angle. If I am having an ignition
problem, it's only happening at almost full throttle and in flight, so I'm
scratching my head trying to see how to test for this.
Brian Trubee
-----Original
Message----- From: Steven W. Boese <SBoese@uwyo.edu> To: Rotary
motors in aircraft <flyrotary@lancaironline.net> Sent: Fri, May 13,
2011 10:28 pm Subject: [FlyRotary] Re: Engine Tuning
Brian (and anyone else
with more experience than me),
With my engine stand,
primary injectors as small as 21 lb and secondary injectors from 30 to 50 lb
have been used, with MAP up to 30 inches. Mixtures from lean misfire to
rich misfire have been set with RPM up to 5800. Although misfires can be
induced with mixture, at no time was there anything that could be considered a
backfire and certainly nothing like a grenade. In flight, I have induced
misfires with rich as well as lean mixtures again with nothing like the
results you describe. My plane’s muffler is a hollow tube about 6 inches
in diameter 3 feet long with a slightly less than 2” diameter outlet and two
chambers inside separated by a conical wall. This muffler would seem to
be more of a bomb than a grenade if a backfire were to happen. Doesn’t
the fact that there is a viable fuel-air mixture in the exhaust system seem to
indicate that the problem may be ignition rather than mixture? This is
just a question, not intended to be an answer.
Steve
Boese
RV6A 1986 13B NA RD1A
EC2
Since
the weather has been cooperating lately, I've had the opportunity to put a few
more hours on the airplane. On the ground, I can go from an idle up to full
throttle and the engine will be smooth. But when I take off, it seems that as
soon as I'm up a few hundred feet off the runway, I get hellacious backfiring
at full throttle. I can mitigate it a bit by throttling back to about 5100
rpm, and turning the mixture knob to near full rich. It's getting to be a bit
nerve wracking to take off thinking I've got it smoothed out and then get a
series of hand grenades going off under my butt. I'm thinking that I'm still
running too lean up at map address 106 or so. So, I go into the edit page and
richen up the mixture around those addresses. I think I'm creeping up on
smooth full throttle running.
The
good news is that my coolant runs at 175 degrees and oil at less- so the
cooling is more than effective, I just need to close up my air inlet a bit or
restrict the outflow.
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