Old Tecumseh V60 Runs Erratically with Occasional Smoke

This engine is on an Ariens Emperor of 60's vintage. The mower was purchased used in the 70's and has given good service. This problem has become worse recently. The engine starts extremely well and rarely if ever dies, but it runs unevenly. When it falters, it emits a puff of black smoke. The plug is black and occasionally sooty. The plug and air filter are brand new. The carburetor was today completely disassembled and found to be very clean. Upon reassembly, the condition seems exactly the same. Needle valve adjustment does not correct the problem, but does affect engine operation as one might expect. It appears the engine is running too rich when it falters. Can someone give me a list of things to look for? Some things I wonder about: Leaky head gasket Badly burned points Bad rings (not likely because smoke is not white and no white caking on plug.) Improperly seating valves A possible clue: When the mower travels uphill and is therefore under extra load, there is generally a series of smoke puffs. Help much appreciated! Frank
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I'm suspecting something to do with the float - especially when you mention the uphill difference. Is there a screen in the inlet ? If so I'd blast it. It's hard to tell how "clean" carb parts are by sight. I'd clean the float needle. It's hard to imagine how the float setting could have changed so I would guess that's OK unless the float has a pinhole and it taking on a bit of fuel.
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The float needle was removed and checked. Very clean, looked like stainless steel. The seat was unscrewed and examined also. (I did notice that the spring clip was missing, so I replaced it with another. Thought that might fix things but no.) It could be that I goose the engine just before I go uphill that could also set it off. So come to think of it the "clue" could be bogus. (Sorry.) I am noticing that the high speed jet adjust does,'t seem to do much until it's almost "closed." Then the engine stops abruptly.
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