Some thoughts on mid-range RPM vibration…
I have had a flakey left EGT for a while and recently replaced the thermocouple sensor (Dynon EGT, Hose Clamp, 0.75-1.25”, Rotax SKU: #100405-001). I also did a fresh carb sync with a digital CARBMATE and got very good sync at both idle and just-off-idle. Unit works a charm.
My engine now runs smooth at idle and very smooth at 5500 RPM cruise. This is confirmed with the EGT’s. At idle and at 5500 RPM there is less than 10F difference in the EGT’s so both left and right cylinder banks are producing equal power.
The mid-range (3000-4000 RPM) has a vibration, and I believe uneven power L-R, is the cause. I have nearly a 100F difference between the EGT’s in mid-range RPM.
So, my thoughts:
• Idle and just-off-idle are easy to set using CARBMATE. The sync looks very repeatable and precise.
• 5500 cruise RPM is in good sync because both carbs are nearly wide-open throat and running on high speed main jet.
• Mid-range sync is a crapshoot. It’s easy to set cable-pulls to match at low end and high end is pretty much automatic. Mid-range probably has several factors that contribute to miss-match most of which are not controllable. Just musing here… perhaps airflow in the cowling can cause different ambient pressures for vacuum diaphragms that control carb throat opening. The good news is there isn’t much use for the mid-range, just don’t operate there. When on downwind leg, I start a steady throttle reduction to idle. The 912 engine has water-cooled cylinder heads so shock cooling is not a concern. Usually idle, or slightly off-idle, will bring me to the runway threshold, so mid-range is easily avoided.
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