I have 3 mowers with B&S - 17hp motors, several years old. All have developed a 'hard' spot while trying to crank them over. This is so bad that each comes to a stop while cranking. Things I have tried - crank motor with out belts on, check oil (of course), check started and battery, turn motor by hand (could not feel the spot), with plug removed starter will spin motor over (most times) -- I have found no cause. If I turn the motor back to just before it's compression most time the starter will spin it fast enough to get over the spot and start.-- Any thoughts?