ORIGINAL: frenchie79
Can you check the RPM at full throttle-something is wrong here. Your problem should be just the opposite-not being able to get it down. How much does the plane weigh. This plane should float forever at idle let alone 3/4.
It does sound like the CG or the engine not getting up to power. Maybe a throttle problem?? The Sr, is one of those planes you should be able to hand launch and it will float all day. Wish I could state the best prop for your engine and plane but testing is the only way I know how to do it. As long as the engine is still within it's working RPM range prop testing is a must but that plane should fly with rubber band power. Double check the CG.