I went flying today and a friend had his Yellow F4 and could not get it going fast enough to fly. The plane does not pull to hard on the ground although the engine winds up fine. Is there anyway to check how much trust it is putting out. He has a 91 in it , should be enough.

You will need 8+ pounds. Use a fish scale in the 25 pound range,
Hook it to the main gear and see what you have.
You can't have the exhaust in one outlet.
Darryl Usher www.acsip.com/~dgu