What causes mixture to change suddenly?
 

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!

If I had to guess I'd say you picked up a little dirt.Some SeaFoam or Berryman's may fit you right up!! ;) :)

Thanks for your reply. Do you think it would be possible that the simple fact that it's gotten a lot warmer outside be causing some sort of change in the way the bike runs? I mean, are these carbs so sensitive?

Later edit: I pulled the plug and this is what it looks like. To me it looks lean, so I turned out the pilot screw half a turn to make it richer. Is that correct? Anyway it seems to run smoother now. I will try to find some sort of equivalent to Seafoam to clean the carb - as it's not available in Romania.

Thanks!

To access the pilot screw you have to remove the cap, don't you? Don't mess with that stuff unless there is a reason to. And for now, we don't really have a reason to assume that it needs to be messed with. Your plug looks pretty decent. A touch lean but that's about what they look like with a factory setting.

Just stick with the carb cleaner in every other tank and it will clear up.

Also, I don't know that we've ever recommended going up to a viscosity that thick just because of the summer. The standard recommended weight will cover everything from winter to a decent Southern Summer. There are some really small channels in the lubrication system and the only way that oil makes it from the sump up to the head is by being pumped through these really small channels. The thicker the starting viscosity, the harder it is for that oil to reach the head, especially at start-up. You don't want to make it harder for oil to get pumped up there and unless you lived in the Sahara, I don't think you're slightly warmer morning temps are going to necessitate the weight change.

Thanks for your input! I'm by no means an expert, but i enjoy working on my own on the bike. I'm learning by doing. I've fiddled with the pilot screw before, after a mechanic (!) messed it up. And I've been happy with the setting for the better part of a year.

As i've said.. the only mystery was why there was a sudden change in the way the bike behaved. But until I figure stuff out, I couldn't let it run lean and crappy. Especially since Seafoam is not available to me.

As for the oil.. I'm 'by the book' on this one. I ride mostly in the city in heavy traffic. And it gets insanely hot during rush hour. I believe it's better for the bike to have a bit more viscosity under these conditions.