Have a 15.5 HP B&S on an early 2000's craftsman rider. Last year was difficult to start, this spring, impossible. When turning over, it hits a spot where it just can't go further with the starter. Have replaced starter, solenoid, battery, plug, and adjusted valves (which move freely-I have had the head off as well). If I hold the lever that operates the compression release ("easy starting" mechanism), then it turns over really well, but won't start. As soon as I begin to ease the lever into the position it should be in, the motor backfires through the carb and exhaust really badly, and becomes unable to turn over again. Obviously it has something to do with a timing problem or a compression-release problem, but I can't figure it out.