I couldn't say for sure but I don't think that it would affect the air/fuel ratio very much if at all since none of the drain back passages are open to the air. The passages are internal to the motor. Having said that, there are some 1/4" or so hoses that connect the cylinder drain covers to a fitting on the by-pass covers. Make sure they are in good shape and not cracked etc.
If the motor is running too lean, then usually it's a fuel delivery or carburetor issue, or sometimes the gasket on the by-pass covers leaking. If you see any fuel/oil mixture leaking out from around any of the the four by-pass covers, then you may have a bad gasket or loose by-pass cover bolts.