My bad, the ppruners are right, the simple carbs on the Robbie and many simple helos are not fully density compensated, so they require manual leaning. The venturi does a gross job of density adjustment since the pressure drop across the venturi that draws up the fuel is proportional to the mass and velocity of the air flowing through the venturi (1/2Rho V squared!), but the effect is not perfectly compensated, so we must manually fine tune the fuel flow. Density compensated carbs are more expensive, and also harder to maintain - most WWII piston aircraft had them, BTW.