I have a Cub Cadet cc4125 string trimmer not starting

Need a little more information .. but look for the following .. take out the spark plug and look inside with a flashlight ... if the piston is moving with the rotation of the flywheel then you likely have a blown head gasket .. if there is no compression there wont be a load on the pull cord .. a cracked or damaged piston can also do this .. ... if the piston is not moving when you rotate the flywheel then you likely have a bad "shear key" sometimes called a "shear pin" .. thats usually a half moon shaped small metal part that keeps the flywheel locked rotationally to the crankshaft .. it is designed to shear off if there is an excessive sudden load on the engine .. the flywheel will then spin without turning the crankshaft .. the same thing will happen if you have broken the connecting rod .. (a rod failure is often preceded by lots of clatter from the engine) .. the head gasket or shear key are much more likely to be your problem .. if you cant see in the spark plug hole you might try using a straw to feel the piston motion .. its possible that without the spark plug in place the piston might move even with a bad shear pin .. because with no compression the piston will move freely .. if thats the case you can stop the piston motion with a wood dowell thru the spark plug hole .. if it moves in spite of your dowell resistance then consider the head gasket option ..

I have a cub cudet cc4125 not starting. Changed spark plug, oil, put fresh gas in and no luck. I am getting resistance on the pull cord, but its not turning? Any suggestions would be great. Thanks!