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Had same problem once. The rectifier (under the flywheel) had vibrated into several pieces and was locking up the flywheel when pieces interfered with turning. Afraid you are going to have to remove...

I have a STX38 about the same age as yours and it is a good mower.
Am guessing someone has put the wrong front linkage on it or has installed them wrong. This also probably means it has the wrong...

I had same problem with same engine. New condenser fixed it. (after cleaning magneto and points)