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The two N-series tractors I have operated in my life have both had the "irregular popping" syndrome you describe. It isn't a loud "bang", but rather a soft "poof" in the exhaust note. I suspect this is lean misfire caused by the Marvel-Schebler carburetor back suction economizer system working a little too well at high manifold vacuum conditions. Most of these tractors are set up to idle at 600 to 800 rpm. This puts them right in a range where they tend to "poof" at idle. They will pull a load OK at these speeds, the poofing stops as soon as the manifold vacuum drops down. If your carburetor idle system actually works well enough to allow you to set it to the 400 RPM spec, this should cause the "poofing" to stop at the (slooowwww) idle speed. At this speed, the economizer vacuum inlet port is cut off by the throttle blade and it stops trying to lean out the mixture. You should be able to get a very nice, steady, pop-free idle at 400 RPM. The 2N of my youth had the clicking behavior you describe. In that case, it was a loose sleeve that my Dad and I tried a couple of times to secure with various means. We never did manage to get it to stay put. The tractor worked OK throughout my teen years, despite the noise from the sleeve, but we never pushed the tractor as hard as I push my 9N now. (Going above half-throttle back then was unusual.) I would be reluctant to run a rotary mower at 3/4 to full throttle with a loose sleeve -- it is quite possible for the resulting stresses to crack the sleeve, or for the friction to egg out the bore in a way that prevents rebuilding the block, at least not with a simple resleeving. I would not be as concerned about valve noise, if that, indeed, is the source of your clicking.
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