Yet another cause....
This might be the time to mention this....a friend of mine forwarded an e-mail to me concerning a discovery that the BD-4 boys had discovered on an engine with hot cylinders(#2 and 3) LF and RR...they noticed that the heat problem moved with the cylinders being swapped around the motor(to cool cylinder positions). Upon inspection, they saw that the heat fins between the head and the rocker box (about six of them) were almost totally blocked down inside by flash at the mold separation line.
Careful filing with needle files lowered the cht's on these cylinders about 40 degrees(F)! I checked mine, and sure enough, the hottest cylinders had the most flash, and were blocked about 60%. After an hour of filing on my hottest, I flew it, and discovered no change!!! I think now that the beneficial effect was lost due to the other problem mentioned above....namely, the carb opening mismatch... I sent this e-mail to Steve at Performance Air in partial payment for the immense favor he did for me....send me an e-mail address, and I'll forward it...
Another thought, the flow rate of air through a heat exchanger has an optimum value. Too fast, or too slow, will reduce efficiency. So don't go nuts with the filing, some roughness or restriction may be desirable. Also remember, I'm not a mechanic...ignore everything I say.
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In theory, theory and practice are the same; in practice, they aren't.