I would suspect sticking rings or valves. I just tried using some of that engine oil treatment stuff in a couple of my tractors, and it just made them worse. Severe knoicking and smoking. So I drained the oil from each and replaced the filters. After restarting, they still were pretty noisy, although not as much. I'm a great believer in PB Blaster, by the way. So I thought, if that oil treatment stuff gummed thinks up, what would be good to ungum them?. So I squirted that PB BBlaster into the oil tube and counted to ten. Started the engines back up and within two minutes they quited right down and QUIT smoking! So, I would suggest doing something similar. Only, pull the plug, squirt some PB Blaster in there, won't need much, and give a couple SLOW cranks with the plug still out. Let it set for a half hour, replug it and start it up. That PB will loosen sticky rings. In the future, run a 20% mix of Rislone in with the oil every oil change.
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