Remove the valve covers, disconnect the hot wire from the coil and turn the motor over with the starter, observing for equal movement on all rocker arms. If some of the rockers aren't moving much, the cam has probably gone flat, requiring disassembly of the motor to clean out the shrapnel, followed by a new camshaft and lifters.

If all the rockers are moving equally, remove all the spark plugs and wire the throttle blades wide open. Do a compression test on each cylinder.