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Thanks to all who have so far provided advice. I
truly appreciate this group's willingness to help.
Summary of the problem: Perfect run up at 2000 RPM with small
mag drops, but the engine does not develop max power. (Please trust me when I
say that it is NOT developing full power). This is after pulling out and back in
a cylinder to fix a helicoil and after replacing the magneto harness and the
right magneto's impuse coupling and drive gear. And after installing 4 new
shielded plugs (top).
Another symptom I noticed is that after a little while (engine
is warm?) attempting maximum static RPM (on the ground), the indicated RPM max
out at 2000 and then over a period of a few seconds slowly drop to 1800
indicated, and stay there. All those readings are in the plane's tach which
reads higher than a more trustworthy handheld optical prop tach. So the
true RPMs are lower than the numbers I quoted above.
In response to all your recommendations, we did the following
today:
1. Checked mag timing. It was good, but we fine tuned it to
exactly what the Continental C-75 manual calls for: 28 degrees before TDC on
right and 30 degrees before TDC on left. The problem stayed the
same.
2. Checked compression on all four cylinders. All were good.
3. Checked throttle, carb heat and mixture links and range of
travel. All good.
4. Checked all 8 spark plugs on a Champion tester. All sparked
good.
5. Cleaned the four bottom plugs, which were not new (the 4
top ones are new) and showed some carbon deposits.
6. Replaced one of the bottom plugs with a new one (the thread
was not in nice shape). So now I have 5 new plugs out of 8.
After all of that, the problem is still the
same!!
What
next? Carburetor?
This is becoming a mystery. Any more ideas???
By the way, Ed, the prop is a MC Cauley 1 B 90 CM 7351
Thanks to all
Eliacim
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