My engine is much smoother above 2300 than from 21 to 2300 but I have not noticed much trouble in the 1800 to 2100 range. I recall seeing one reference that says that there are no rpm ranges that are a problem for the 1A90 series of props on the Ercoupe but I remember another reference that said to avoid the range from 1900 to 2150. My solution is to operate only in the range 23 to 2500 rpm because at 94 cents per gallon fuel is not a concern. Bob Condon
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