RE: K&B 65 Sportster
Well, the Sportster is an unusual engine in the way it is built, or at least the metal they used. My dad has a .20, .45, and a .65. the .20 and .65 are great engines the .45 on the other hand....well. We think the .45 needs a new O-ring on the head.
Anyway, back to your question. My dads .65 is a good engine, it won't turn a prop as fast as say an OS .61, but I think it would lug a big prop.
I think the problem you may be having, if it is an engine with little or no runtime, is that they take a loooong time to break in. If I remember right one of the reviewers in one of the magazines I read said he put baking soda toothpaste in his and flipped if for a while...I'm not recommending this, although I might try it myself your mileage might vary. Once they are broken in they run good.
On the other hand, if it is a well used engine, the early models tended to wear the crank bushing and suck air and lean out....The .45 my dad has was given to him..actually he was given two, one complete and one in pieces....along with a new crankcase. The new crankcase had a bronze bushing but the original had the crank running in the aluminum. The aluminum had worn and air was getting in and leaning the engine out. The .45 is the oldest of the three engines, and the .20 and .65 he bought new....something else we found out from the .45 is that the 0-Ring can go bad and they loose compression...don't know if the O-ring went bad because of old age or handling while someone had it apart...we quit messing with it because at the time we couldn't find a suitable O-ring.