From:
"Erich Trombley"
Date:
Feb 27, 2009
Subject:
large mag drop on both mags, 912uls
While I can't speak to the 912ULS (I have a 914), I can share this story with
you.
I too had mag excessive drop (600 + rpm) but only on one mag. I tried
everything
I could think of including new spark plugs, plug wires, swapping coils,
purchasing a new coil, and overall conducting a very methodical diagnostic
test program. All to no avail. I could not figure out what was going on with
the engine. To make matters worse the problem seemed to have resolved itself
without me changing anything. I was pulling my hair out. As silly as this
is going to sound, it turned out to be mixture. Yes, mixture.
Interesting it was a function of ambient temperature. All of my testing was
conducted
on the trailer in an unheated garage with the garage door wide open and
it was cold. It was only after the ambient temps increased that the problem
seemed to have gone away, however, I hadn't made that connection yet. Not
realizing
what was going on and with the problem having seemed to have fixed itself
I made arrangements with a flight instructor to conduct a flight with me
the following morning. Well, as you can probably see where this is going, we
got to the airport in the morning and put the plane together. I started up the
plane and conducted a mag check. The problem was back, yet I hadnt changed
a thing. Also, I might add that even with the excessive mag drop the engine ran
very smooth. As such, the instructor suggested we take it up and see how it
performed in the pattern. There were no issues on the take-off roll and we
continued with the flight. After one circuit in the pattern, I performed a mag
drop on downwind... perfect not even 100 rpm drop. The engine was running
perfectly, although I did not know why.
It was during the Rotax class I learned why. The instructor asked if anyone had
experienced significant mag drop. I raised my hand, Yes, yes, I have, I
proclaimed.
The instructor proceeded to walk me down a path asking other questions
such as, was the first time I noted the problem in the winter season or was
it year round, winter, I replied. I also noted that the drop was much worse
on one mag. He then asked if it was a 914, "yes" I stated. The problem
surfaced
on the first flight of the winter season. In the four years prior to this
incident the problem never surfaced. However, the summer preceding this winter
flight I installed an inter-cooler. As ambient temps drop the air becomes
denser, and with this density change the engine gets leaner. It is the lean
condition
that causes the excess mag drop. The inter-cooler only served to exacerbate
the situation, further cooling the inlet air.
The 914 differs from the 912 on the arrangement of the mags. On the 914 the top
plugs of all cylinders are connected one mag and the bottom plugs to the other
mag. On the 912 it is my understanding that one mag is connected to two top
plugs and two bottom plugs (opposing cylinders) with the other mag controlling
the other two top and bottom plugs. The cylinder heads of both the 912 and
914 are such that only one of the two plugs per cylinder is optimized for ideal
flame propagation/combustion (top I believe). Hence with the 914 when you
isolate the top plugs via a mag drop you will note a greater mag drop than with
the bottom circuit offline. With the 912 would see equal mag drops since two
top plugs and two bottom plugs are taken offline with each mag, thus cancelling
out the differences in the head design. With a lean mixture the drop is
more pronounced. The fix, change the needle valve setting to enrich the mixture.
Problem solved. Good luck with diagnosing the problem.
Please keep us posted on your progress.
Erich Trombley N28ET Classic Mono 914
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=448373#448373
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