Increase the nitro percent. Hi temps with high humidity give high density altitude, as though you were flying at a higher elevation were the air is thinner and contains less oxygen. The nitro contains its own share of O2.  What engine and what fuel do you use?

   A friend of mine told me that he and his flying buddy could not get their engines running due to the high humidity.  I am guessing that the humidity was close to sixty percent and the temps were near 90 degrees F.

   Humidity (or at least the dew point) seems to have a drastic effect on how they run and the power level, but I have never heard of not being able to get it started, even in conditions far more oppressive. Like the 93 NATs, where the humidity was in the 60's and it was 100-ish degrees - with a dew point in the mid 80s. My problem that week was that it started without the battery time after time, so really, it seemed to make it much easier to start. The only real danger is over-choking it, it takes very little choke in those conditions. 

    The humidity seems to suck out power and smooth out the run at the same time. A lot of the engines that run nicely in the midwest/southeast have problems when it is dry and the converse is also true. One of the most dramatic differences is the "schneurle effect", where the engine want to run faster on outsides, and slower on insides, particularly with the engine breaking into a hard 2 on the second outside loop of the round 8 and square 8. This can be almost a non-issue in humid air, and is a deal-breaker sometimes in dry air. I have come up with or heard no plausible mechanism for this, and have had some epic discussions/arguments with engine reworkers over the topic (not Randy...), particularly back in the 80's with muffler schneurle engines.

    In one case, on advice to "shim the tank", I managed to get to enough shim to get 6 seconds a lap inverted and 5 upright, and it STILL went screeching lean in the outside part of the round 8. Fuel (from FAI to Cox Racing Fuel)/compression/venturi/head shim/tank shim/uniflow/suction/plug all made the expected differences in the way it ran, but didn't make a dent in the problem. And let me tell you an ABC Schnuerle 45 with Cox Racing fuel is one pretty macho engine compared to a wimpy ST46. And it runs out of gas right after the square 8. This engine subsequently ran fine in the midwest summer, no reported issues from the guy who ended up with it.

      Brett