I've seen lots of posts on here where engines will turn over slowly or "jerky", turning part of a turn and then stalling. It seems to be a common failure mode for single and twin cylinder Kohlers and Briggs. I acknowledge that this is sometimes due to inoperable compression releases, but I just proved another failure mode. The starter solenoid on most all of these mowers is a "universal" part and the internal contacts are not particularly robust. Changing the solenoid on my FIL's TroyBilt Pony made a HUGE difference in starter motor strength and RPM.