Stuck rings? pull the plugs, pour some Marvel Mystery Oil in both jugs, let it sit for 24 hours, then crank it over with the plugs out, then reinstall the plugs and fire it up.
If that the problem it will break them lose. Works great in aircraft jugs especailly if it has not been run in a while. Worth a shot before you pull the jugs.
If it is carbon under the valve seats, pull the plug and rig a old drilled out spak plug to a air hose and put about 80#'s of air on the jug, if you hear it leaking back into the crank case, ring problems, if the "hiss" is in the intake/exasust, valve seat problems . I have fixed leaky valves by pulling the plug, feed some small nylon rope into the jug with the piston near TDC, remove the valve spring/retainer/keeper and then chuck the valve stem (carefully) in a drill and spin it. If it is carbon or crud on the valve or seat, it will break it lose. Put it back together and fire it up. Again works, on aircraft jugs, and they are very similar to HD jugs, just about 3 times as expensive.
If it flys,floats or flirts, it is cheaper to rent it....applies to Harleys also......