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Suddenly sluggish and weak - like the timing is way retarded ...
at least that's what mine was like on a Buick 350 many years back.
You should be able to see if the chain is way loose (must be loose to jump).
Wth valve cover off, rotate crank until one of the valves is almost down.
then, rotate the crank back and forth ... determine how much
rotation it takes before the valve (cam) starts moving back up.
I would say a few degrees is OK ... many degrees may suggest a worn sprocket.