That sounds like the classic "loading-up" syndrome to me. You see lots of smoke when you increase the throttle right?

[ul][*] How many turns out are you on the low-speed mixture? [*] Is the engine upside down by chance?[*] Where is the tank in relation to the engines carb?[*] Does fuel siphon automatically from the tank into the carb when the plane is in a slightly down attitude on landing?
[/ul]

If the engine is upside down and the carb is lower than the tank, then you can get to a point where the siphon is automatic and the low-speed mixture becomes virtually irrelevant.