'Coupe Group,
This one needs the collected wisdom of the Tech List. (The nice
local mechanic helping me sounds stumped; fortunately, he has shown
that he welcomes suggestions from the Tech list.) More than 2 weeks
ago, I tried to depart for the Copperstate fly-in, but the run-up was
uncharacteristically bad. Usually, there's barely any mag drop when
I do run-up. This time, one mag dropped 200 RPM & bounced back up;
the other dropped 200 RPM...and started slowly dropping more. (My
checklist says "max. 75 RPM drop") I repeated the mag check 3 times
with the exact same results, then phoned the local mechanic who is
convenient to my field, and taxied back to my tie down. He came
right over, cleaned the 4 spark plugs he could easily access on-the-
spot, then tried the run-up himself. He said it started rather
slowly and one mag was still dropping too much & running very rough.
So I left it with him.
A couple of days later, he said he cleaned ALL the spark plugs
thoroughly, but the run-up was still bad. A couple more days after
that, he went out to the plane and -- WITHOUT him doing anything else
-- the run-up was then perfect. But it diesel'd slightly on shut down.
I was concerned about possible mag trouble, but called the more-
experienced AI who did the annual in July (he's at a different
field), and he said that it's not likely mag trouble, because mags
don't die intermittently: if they go bad they stay bad.
So I went out myself to try a flight test...but now the engine
wouldn't start at all! It cranked the prop around very sluggishly
once or twice on the first try, making a noise like a car does when
the battery is weak; on the second try it wouldn't even do that
much. Left it with the local mechanic again.
A couple days after that, he went out to try it, and it started up
fine for him, 3 times in a row! Though it still diesel'd maybe 3
seconds on shut down.
On his next opportunity to try it, a few days later, it did start
very slowly, so he took out the battery and gave it a long slow
charge overnight. I guess the battery may have become weak because
of all the times we tried starting &/or run-ups, without flying it or
running the engine very long.
But that still doesn't solve why the run-ups are going suddenly very
bad, and then back to fine, apparently on their own. Nor does it
explain the brief dieseling on shut down.
I've asked him to check that the P-leads & connections look ok.
Another 'Couper suggested I make sure he also checks the spark plug
wires, and the mag timing. Is there anything else that could be
causing run-ups to abruptly go from good to bad and back again, and
also might be causing slight dieseling? Neither problem developed
slowly. The mag drop problem seems to be binary: either it's
perfectly fine (barely drops on either mag), or terrible (200+ drop),
and has been intermittently terrible for over 2 weeks.
I'm getting flying-deprived-withdrawal and getting really antsy to
get this solved! Does anyone know what we're overlooking? Thanks!
Linda
N3437H (Sky Sprite)
1946 415C, with a C-85 engine & Stromberg carb
