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Jonathan, All engines have to breathe and a certain amount of “slight” smoke out of the draft tube is completely normal; the harder that they are pulling, the more the blow-by pressures. All blow-by pressures originate at the combustion chamber and proper clearances control that pressure. Here are some things to think about: When you say, "freshly rebuilt", did you replace the sleeves or re-bore (depending on engine) or did you just re-ring it? Replacing just the rings is a stop-gap/band-aid repair at best. A new ring against a bell-mouthed or wavy bore will never seat. Were the rings correctly installed in regards to stagger and correct top/bottom orientation? Were the expanders and scrapers installed correctly? Were the ring gaps cleaned and were the depths correct? Ring-to-land clearances are critical when re using old pistons. Were the valve guide to valve clearances correct as well as the installation of their respective seals? Lastly, is the ventilation system inlet plugged? Just some ideas, Allan
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