I suspected you would mean the pistonring (because indeed, what other rings are there?), but I don't see a connection between ambient temperature and ring sealing performance, great enough to affect mixture...
Hence my confusion,

I mean, the engine is running a bit cooler (Chris said he did not see the CHT above 120 deg C, while typically, mine reach about 135 in flight on a dutch summers day) but that means EVERYTHING is running a bit cooler in proportion.
Rings don't so much seal on their shape and dimensions per se, but more on the running surface being mated to the liner, and on the combustion pressure getting behind the ring, pushing it against the liner.
Just gutfeeling, but in this scale, and in aircooled engines I don't think that the influence of running temperature has any noticable effect on ring performance, to be honest.

I know that in the large marine engines, temperature changes affect ring performance but more in the sense of increased wear and greater chance of mechanical ring faillure. But the conditions under which those rings perform their task are barely comparable with what our rings encounter, both in absolute temperatures and pressures, as well as in the chemical environment they live in. What is more: Those engines have controlled linertemperatures (liner dimensions basically being constant), active piston cooling keeping the piston dimensions in check, while ring temperatures change with load, combustion temperature and pressure, and thus their dimensions vary, as opposed to our little aircooled engines where everything more or less expands or contracts a bit more in concert.

Mind you, I am just extrapolating here, because I don't think there is much research on this particular subject (plenty of that on the large engines, but very little WRT our toy engines), therefore as said, just a gut feeling, I could be wrong.