Bought one of these 40 slowpokes at an auction. It had some minor front end damage (had to rebuild the fire wall). I have over 100 engines from an os20 fs to a ys1.20 fs, and from .049 to 1.20 2 cycle. I tried a ys53 first. It was okay but wasn't interesting. It would land like feather, really slow. I bought a crashed and broken os40fx. After repairing all the broken and cracked parts with JB Weld, I tested the engine on my engine stand. It ran very well using a 10x7 apc prop and produced about 14,000rpm. I thought I'd give it a try on the slowpoke. It was SUPER difficult to land. It seemed maybe I had to much power!! After checking the angle of incidence of the wing to the stabilizer, I found that the wing was 2 degrees positive. I shimmed the wing as best I could (without recarving the wing saddle) to about 1 degree positive. That helped, but the plane was way too fast at full throttle....It would fly at idle, and climb slowly with two clicks of throttle on the transmitter stick. I decided that I wanted the horsepower of the engine, but not the speed it produced on the airframe. So, I made the MAGIC fix. What a huge difference!!!!
I put an 11 x 4 apc prop in place of the 10 x 7. It now lands like a feather, will fly all day inverted with virtually no trim adjustment,
is able to hover when I use 15% nitro, still has the speed if I want to crank it up, and will fly about thiry minutes on a tank of gas (Great Planes 10 oz.). It loads in my pick up without disassembly and has become my favorite sport plane to fly.