k2skier

Did you purchase a B&S starter or an aftermarket? I have seen some really bad (IE weak) aftermarket starters. The battery must be a minimum of 220CCA, the more the better for a twin. Check voltage at the starter while cranking, you should have a minimum of 9.5 volts while cranking, plugs out it should be higher. With a strong battery, jumping straight to the starter, if it doesn't crank well, it may have some drag somewhere, belt, piston scuffing...If it does crank with the starter jumped to directly then you may have a problem with wires, connections, terminals, grounds...

Also check to see if one of the valves opens a little when the piston is coming up on TDC, the cam could have a faulty compression release.

FYI, this mechanic has been putting in 11 (+) hour days, not much time to give away for free for what I charge for...