Hi all. Does humidity affect our engines? The reason I ask is I have a Yamaha YT125 Tri-Moto (DT125 engine just detuned a bit) and I've noticed on humid days that it runs a bit different. The humidity has been between 70-80% for the last month so. When it is humid it doesn't like to idle and will only start if the throttle is slightly cracked open, and above 3/4 throttle it likes to cut out and just doesn't seem to have the power it usually does. On a non humid day it day it will start right up and idle all day, and doesn't cut out above 3/4 throttle. At full throttle on a humid day I can hear it struggling to suck air. It's all stock running the factory air box and filter with a Mikuni VM20SS #25 pilot jet #200 main jet. If anybody has any ideas it would be great. Thanks

Air dencity  usually affects 2 strokes, [ high altitude ]  ,,,dunno about humidity,  as its cold here  " always " 

Of course it does, hot, cold, humid, dry it all affects the mixture. Humid is normally hot. Hot air is not as dense, it also contains more moisture so you have less actual air in the cyl and an amount of water that will degrade during the burn to cause an increase in oxygen further disturbing the mixture.

As far as a fix goes there isn't one. Unless you want to fuel inject your bike or change the jetting daily.

Thanks everyone