Perhaps the carburetor has a fuel delivery problem, or the governor is not adjusted properly. Or the throttle linkage and/or governor tensioner spring are not hooked up properly?
You can loosen the screw that connects the governor to the governor arm and try to adjust the arm. I believe you are supposed to leave the governor stationary while you push the arm to open the throttle open 100% and then tighten the screw, but that method of adjustment doesn't always work. I just adjusted a Craftsman (w/ Tecumseh motor) last weekend and I just had to fiddle with it for about two minutes until I found an appropriate adjustment that worked great.