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A lean piston is a air leak [ lean condition] . With the tractor running at half rpm spray some starting fluid around the the intakes to see if the rpms go up. Do this on a cold motor so that the manifold is not hot. There might also be a pin hole in the intake, it is cast iron! If the rpm does go up then there is a air leak. Some times a digital timing light with a tach in it is needed to see a rpm change due to the low rpm of these motors. Another way but you would have to put a vacume port in the intake just above the carb or have a L.P intake on it. Put a vacume gauge on it ! At idel in will produce 12to 16 psi of vacume as rpm goes up the #s will come down to around "0" meaning the carb is jetted correct and meeting the demands of the motor .[ the "0#" you will only see under load] pulling a plow or on a dyno. If that # doesnt come down then there is a air leak. In short go buy some fresh intake gaskets and retorqe them after you have heat cycled the motor a few times.
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