when you put a wrench on the crankshaft and turn it clockwise, you are cranking both pistons thru the compression and exhaust cycles.
When a piston starts to compress it will require more torque to keep turning. Once it gets past top dead center it will be very easy to turn, because the compressed air is pushing the piston back down.
it should then do the same thing on the next full 360 on the crank shaft, on the other piston, but about 80° off from the other one. If you feel the compression stroke on both pistons (once on each 360 turn of the crankshaft) then your valves and rings are doing what they should. So that is a simple compression test in itself.
If you only feel one piston doing the compression stroke on two full turns of the crankshaft (720°) them something is wrong with one of the combustion chambers.
If you do this with both plugs out it will just spin freely, but you could stick a finger in each spark plug hole and feel if its building compression. If only one piston is building compression, that would be a way to tell which one is bad, take the plugs out one by one, or use your finger in the plug hole.
The reason for only going clockwise BTW, that is the direction the engine runs, opposite from the front wheel. If you turn it backwards the timing chain goes slack and bad things.... dont turn it backwards. One exception: while you are turning the crankshaft over, if you feel it clunk, like metal is hitting metal, you can back the wrench handle off about an inch, then go clockwise again. If you keep feeling a clunk on the wrench in the same spot, that could be a failed connecting rod bearing.
Last edited by KCW; 10-02-2018 at 02:20 PM.