Relatively warmer air restricts the number of air molecules entering the engine via a given carb throat. For a given needle setting the engine will be richer. The lower amount of O/2 will require less fuel (leaner needle) thus reducing the power output. A leaner needle will help the running, however the overall power output is still reduced.
By the same token, in relatively cooler air the reverse happens. With proper needle valve setting, cold dry air will produce more power output than warm dry air. Add humidity and additional problems do occur. Thankfully in our modeling world they are not really significant. Humidity does NOT act like water/alcohol injection for additional mass. It simply reduces the combustible oxygen.

Sometimes modelers confuse the observed changes in model performance due to the increased True Air Speed ( TAS )/ Ground Speed (GS) relationship in hot or high altitude flights with engine performance. While in either hot or high alt. the engine performance is less (albeit very small in most cases at a given location) the increase in TAS on those hot days can invite the model pilot to misjudge the approach-to-stall condition of an airplane that he has been flying in much cooler weather.
The warmer air with farther spaced air molecules produces a slightly faster TAS, for a given number of air molecule flow (Indicated Air Speed [ IAS ]) which actually controls the stall characteristics of the flying machine.
In other words, one will try to match that previously observed cool-air speed relative to the ground and stall the model.


edit: format