In a message dated 12/7/2006 1:06:47 P.M. Central Standard Time,
bakercdb@gmail.com writes:
On the 2nd leg of a long cross country trip, I noticed that CHTs were
running noticeably higher than usual on climb out. After leveling off at
6,500msl, CHTs were uniformly running approximately 40+ degrees higher than
normal. Upon landing, the problem was eventually traced to one of my
PMag ignitions that, was going to full advance at idle (39degrees). This
was a failure mode of electronic ignitions that I hadn't considered
before. Given the engine seemed to be running fine otherwise, I didn't
try switching off one ignition in flight, which likely would have pinpointed
the problem.
Clark,
Low MAP and low RPM are exactly the low power combination that
encourages electronic ignitions to fully advance the timing. Indeed, the
P-Mag may be broken (always delivering max advance), but a
significant advance at idle is not a good failure indication. I
regularly see 34-35 DBTDC at low RPM (idle or close to idle) and my base timing
is 20 DBTDC because of a CR greater than 8.7 (equivalent to your 39 DBTDC unless
your jumpers also are set for a base timing of 20 DBTDC).
Perhaps the P-Mag goes to a large advance because the MAP
sensor is always indicating low pressure. What happens if you set the
jumpers to use RPM only for the timing? You shouldn't get much advance for
operation above 2500 RPM. I have to assume the RPM sensor is working for
the P-Mag to operate at all.
PS: You may have experienced the kind of condition that led me
to the conclusion that I must have a display of the in flight timing for
any ignition that changes the timing.
Scott Krueger
AKA Grayhawk
Lancair N92EX IO320 SB 89/96
Aurora, IL (KARR)
A man
has got to know his limitations.