I can't say anything about the reason why 5% of the fins was removed. But I disagree with you on 3w engines running high in temperatures. I have a number of 3w engines ranging from 70cc twin all the way up to 212 cc four cylinder. I don't have any issues with temperature at all. But then again, I have never owned one of these specifically. That could be the case with that particular twin.
We fly with several 140cc and 157cc from 3W in our race planes. We've done this modification on all of them. It is easier to get in temperature. With the original heads there is a temperature drop at high speeds.
Very interesting information. I've had my 157cs for 6 years now. Probably well over 26 actual air hours. She's still gaining horse power with no heating issues. But then I dont race the engine. Kind of odd for me, i would think removing some of the cooling fins would increase temperatures instead of reduce them. But as I said, I don't race my dubs. I would still love to own that 140.
You'll have to excuse my English…
What I initialy meant is that it helps increase temperature with removed fins. So yes, you are correct. 
Ah! So you mean to say you're trying g to keep the engine warmer?!? That makes more sense to me now. I've never known a 3w to heat excessively. Not unless the air flow over the cylinders was poor. Now I understand. Thank you for getting me straight.You'll have to excuse my English…
