I am not familiar with the 250, but could the harmonic balancer rubber have delaminated and the timing mark be inaccurate? One way to check the accuracy of the TDC mark is to install a stop in number one spark plug hole and rotate the engine til it hits the stop, mark the spot on the balancer, rotate the engine in the opposite direction until it hits the stop and mark the balancer at that spot. Halfway between the two marks on the balancer it TDC.
