With the battery ground cable disconnected and the sheet metal lower flywheel cover removed, shine a bright light up into the bellhousing and watch as someone pushes the start pedal down. The teeth should be fully meshed into the flywheel before the linkage touches the switch. It's possible your "professional rebuilder" assembled something wrong. You can test the starter on the bench the same way- - - -the pinion gear should be pushed fully forward, almost touching the housing, before the switch makes contact. If the starter shaft is spinning with the teeth fully engaged, without turning the engine, you've got a bad starter drive- - - -"new" isn't always "good"!
Jerry
