I have a 19.5 hp mower that stopped running, no spark. I changed the coil and it ran long enough to mow the lawn then stopped again. Now it tries to start but won't like it has a retarded timing, it won't quite hit. Pulled carb apart and diaphragm is good, valves are not stuck. Pulled mag to make sure keyway was good. I am at a loss. Any ideas? Thanks to anyone who can help.