I have a 16.5 briggs & strattion garden tractor when trying to start the engine it is very hard to spin over ,you have to keep turning starter switch to get to spinning fast enough to start . I have tried two new batteries cleaned all connections and terminals replaced starter, wondered if there is a internal compression release that is worn out or is not working. Thanks for any help anyone can give me . Also it runs fine if you can get it started.

More than likely your valves are out of adjustment.