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That's caused by an air leak on the inlet rubbers of thereabouts. I had the same fault on my 550four.
Are you sure of that compression reading? It looks way too low to be running properly.
It doesn't sound logical I agree, in that one cylinder differs in burn from others that are apparently the same.That compression overall reading though I wouldn't by able to resist investigating further. A slightly wide field view perhaps, but if compression is genuinely that low then it's conceivable that the combustion burn time would be slow and as such it would limit the rpm under load to give the 4500rpm ceiling. You could try advancing the timing to compensate and see if it would rev higher at all to give you more of a picture.
I do too. The fact that there's power to be found when the choke is operated suggests that it's running too lean (inlet airleak) and operating the choke enriches the mixture back to what it should be.
It looks like the number 3 problem could be a symptom rather that cause. The plug caps if they are NGK in good condition are reasonably well protected against ingress and run without problems in rain conditions.The problem with lower cylinder pressure is not necessarily one of outright power failing to pull the weight along but more of burn time of the mixture. It ultimately, if too slow, acts as a speed governor above which a loaded engine will not gain revs. It usually gives a very gentle and smooth tickover though.
plugs 2 and 3 share the same ignition system,points,coils,and capacitor,can you swop the plug leads over? are the long enough? and then see if the trouble still stays on 3, that make it a carb issue,and/or exhaust BTW does the 3 exhaust silencer/tailpipe got hot ?or the same as the others ?