A 235 has some flat spots between the camshaft lobes that are there to provide clearance for the sides of certain connecting rods. At certain points in the cam rotation it's possible to get contact between the connecting rods and the cam if it stops or gets out of time. The stroke on a 216 is short enough to keep the cam and rods from getting together.

Pull the fuel pump and use a bright light and an inspection mirror to see if the cam is turning.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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