As mentioned before it may not be getting enough fuel.
But,
I just bought a YS 1.40 for my plane and it WOULD NOT run worth a flip on anything less than 30% by volume not weight % Nitro. I was running Byrons 25% (for 1 flight). The engine was sluggish bigtime in the air and when I landed it the exhaust pipe was blue. A fellow pattern flyer had some Magnum 1.5% (approximately 30%) and I emptied and refilled the tank. The engine fired right off and flew the plane like a scalded banshee. When I landed the head was MUCH cooler and the so was the exhaust pipe. I was promptly educated on YS engines by the pattern guys there, and I saw it for my own eyes. YS 4 strokes must run good high nitro fuel or they overheat. Cool power is great fuel, you may just try higher Nitro.

I would also say if it turns over smoothly and has good compression its probably not hurt. I would try to run it as I mentioned above.
Thats not gospel, just my opinion.