Could be anything from a miss fire to a blown piston depending on how server it is..
Get a Vacuum gauge and Compression tester and do the BASICS first..run a vacuum test, manifold source, it should read between 17 and 21 lbs, Steady needle..flucutions in unison with a miss indicate Leakey valves / burned valves / seals..
Do a compression Test and leak-down, low numbers indicate the bad cylinder, norm is within 125 to 150 PSI..
Look at the plugs, are the oil/Gas Fouled? this is another indicator..
Causes could be anything from a fouled plug to a bad / mis adjusted rocker or bent push rod .. to Bad valves..only testing will tell you.
you could have broke a valve spring, how old are they
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