Interesting phenomoma --- can't think of a reason for this happening........
At full throttle, you have "engaged" the full rich carburtetter position ( keeps the engine cool in climb / full load), but this would occur on run up also (with full throttle). If had same RPM on run up, one might conclude the mixture was too rich...... and at altitude, the mixture was closer to correct, resulting in a higher RPM.....at altitude. I would verify tachometer....., tho it appears to be working correctly..... Variable pich prop ??? Interesting challange... Harry
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