What causes mixture to change suddenly?

[tudorv]

Hi everyone!

Any idea what could cause my bike to suddenly behave like it's running lean and too hot? I haven't touched the pilot screw in over a year and it's been running great.
The only thing i've done recently is to change the oil from a 10w40 to a 15w50 to prepare for the hot summer.
Now suddenly when it warms up, the idle goes too high... like it's too lean.

I'm thinking maybe bad gas? Or gunk in carburator?
I will pull the plug to confirm the mixture problem, but the cause of it evades me. Any suggestions?

Thanks!