First, I think there is more than one Colt 5 and more than one Colt 8. So far as the ones in the manuals section are concerned, they both have the same single speed gearbox.
You may have messed up the carburetor when you modified it. It sounds as if you may have enlarged the venturi, which is always a bad idea, because it reduces the suction applied to the fuel discharge nozzle and therefore makes the mixture leaner. It does this in a non-linear fashion versus speed. If you want a higher-flow carburetor, use a bigger one, don't bore one out. Also, port work or valve work on a properly developed engine just about always reduces the low and mid speed torque more than it boosts high speed torque. It is useful when you are going to use the engine for performance work at high speeds only, or attach it to a lighter vehicle, but it just doesn't make any sense when you are going to leave it on the same lawnmower.
