ORIGINAL: Sport_Pilot
It was powered by a K&B .65 Sportster and was not over powered at all, surprisingly enough. Yes, downthrust is needed with any flat bottomed airfoil equipped model that hasn't had a bit of positive incidence added to the horizontal stabilizer, ala the Goldberg J-3 Cub.
Mine is kit built and flies fine with a .40. I added strip ailerons and took out some of the diehedral. Seems to fly slightly better than the ARF. At leas the ARF I saw with a .40 did not seem to climb as well, or maybe they don't have the technique down, for if you pull up too steep as though it had more power it will bog down. But if you let it build up some airspeed and gently pull up it will climb out very well. Makes a better trainer that way. Lots of modelers seem to think you should jerk the model off the ground, works when overpowered, but teach's bad habits. And yes I increased the down thrust with a .46, the stock downthrust seems sufficient with a .40. With a .46 and the stock downthrust with a 11-4 prop it would do a hans off loop with no down trim. It was probably a bit tail heavy at the time.
It does seem to climb poorly on 95+ degree days, but then only when right over the runway or close to the ground. I think a .40 that is slightly hotter than the K&B or maybe a 50 sized four stroke would be ideal.
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I don't doubt what you are saying one little bit. My last one was powered by an HP .49 VT, which is probably even less powerful than your two-stroke .40. I could have used a bit more power on days when the grass was getting tall, but, once airborne, even the low powered HP .49 VT could be throttled back to under half throttle for training purposes.
I was amazed at how well the .65 seemed to work on this model. The owner used a very large diameter, low pitch prop on his .65, now that I think of it. It made it really easy to spot land it because of the drag of the large prop disk. An unexpected plus, that would have worked just as well with a smaller engine and a similarly large diameter, low pitched prop.