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 w
ires, swapping coils, purchasing a new coil, and overall conducting a ve
ry methodical diagnostic test program. All to no avail. I could not fi
gure out what was going on with the engine. To make matters worse the p
roblem seemed to have resolved itself without me changing anything. I w
as 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 ambi
ent temperature. <?xml:namespace prefix = o ns = "urn:schemas-micro
soft-com:office:office" />
All of my testing was conducted on the trailer in an unheated garage wit
h the garage door wide open and it was cold. It was only after the ambi
ent temps increased that the problem seemed to have gone away, however,
I hadn't't made that connection yet. Not realizing what was going on an
d with the problem having seemed to have fixed itself I made arrangement
s with a flight instructor to conduct a flight with me the following mor
ning. 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 pla
ne and conducted a mag check. The #$%%^ problem was back, yet I hadn=92
t 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 t
ake 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 circu
it 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 kno
w why.
It was during the Rotax class I learned why. The instructor asked if an
yone had experienced significant mag drop. I raised my hand, =93Yes, ye
s, I have=94 I proclaimed. The instructor proceeded to walk me down a p
ath asking other questions such as, was the first time I noted the probl
em in the winter season or was it year round, =93winter=94 I replied. I
also noted that the drop was much worse on one mag. He then asked if i
t was a 914, "yes" I stated. The problem surfaced on the first flight o
f the winter season. In the four years prior to this incident the probl
em never surfaced. However, the summer preceding this winter flight I in
stalled an inter-cooler. As ambient temps drop the air becomes denser,
and with this density change the engine gets leaner. It is the lean co
ndition that causes the excess mag drop. The inter-cooler only served t
o 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 plu
gs 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) wit
h the other mag controlling the other two top and bottom plugs. The cyl
inder heads of both the 912 and 914 are such that only one of the two pl
ugs per cylinder is optimized for ideal flame propagation/combustion (to
p I believe). Hence with the 914 when you isolate the top plugs via a m
ag drop you will note a greater mag drop than with the bottom circuit of
fline. With the 912 would see equal mag drops since two top plugs and
two bottom plugs are taken offline with each mag, thus canceling out the
differences in the head design. With a lean mixture the drop is more p
ronounced.
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 pr
ogress.
Erich Trombley
N28ET Classic Mono 914
____________________________________________________________
Let great B to B marketing solutions propel your brand to new heights! C
lick now!
http://thirdpartyoffers.juno.com/TGL2131/fc/BLSrjnsKrsn14LnnWTyIZAqJMqZ7
kaIQ0JgZKUfD3v6De7QBZMMk7bA3m0o/
|