Hello Dave
You mention that at lower power settings you are having plug fouling and black
soot in exhaust.
Sounds like you are just running a bit rich at that throttle opening.
I may take that back a little if you are running on avgas as you I think need
the
heat to prevent lead fouling problems.
Full throttle is pretty much main jet, then it is needle jet, jet needle and
throttle
valve cutaway.
Go fly and precision mark where throttle position is in flight at your favorite
power setting. Then next time you can easily look, see what position in
percentage
of opening it is and you can tune that part.
Ask Rotax Guru on suggestions.
Lean at high power settings is not a good thing, best to err on the rich side.
At lower power settings since you are generating no where near as much heat,
erring
on rich will keep temperatures down which is not a great thing and foul
plugs and make soot and hurt efficiency.
If you are at higher altitudes than sea level, especial at lower power settings,
i don't think the constant Pressure carbs fully compensate and lean enough.
I think higher the boost the better the carbs will compensate.
Check with rotax, but a test by moving jet needle down 1 notch or getting
another
needle with a thicker profile where you need it would be my first dart throw.
Keep a keen eye you don't things run lean at high power settings.
One cute trick i used to do on motorcycles is to pick a throttle setting you are
having problems with, and turn off the petcock when it would begin to run out
of gas it would lean things up, if it began to run great, you knew which
direction
to go. It worked remarkably well at a given throttle setting, you could
adjust and run for some by having the petcock cracked just enough.
As a matter of fact the BD-5 with Zenoah 3 cylinder had you lean by starving the
motor by cutting off fuel with a nice adjustable valve.
For a test you could go to your favorite setting and check how thing is running
then starve it and see if things improve. An EGT would be helpful here, look
for a rise in EGT and RPM (No constant speed). If fairly noticeable, probably
too rich.
Ron Parigoris
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