All of the Yamaha four-strokes, three and four cylinder up through 2005, have a heavy rubber flap that covers the engine. Many riders have found it is beneficial to flip the flap up away from the engine as riding temperatures increase.
Why is it in there? To help retain engine heat in extreme conditions. This is why it makes perfect sense that it isnâ€™t needed when riding in warmer temps. We tend to leave it up unless weâ€™re getting closer to zero degrees (F), so if you want a number then use 10 degrees as a general up-down toggle. Try it both ways some day and see what (if anything) happens to the running quality on that day. As it gets warmer out, you will notice more of a change (with it up) and then again as it gets colder (with it down).