How does weather affect jetting?

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Here's the skinny on weather:

  • Cold air is more dense (less activity w/ the molecules, thus they're closer together)
  • Hot air is less dense (More activity between molecules)
  • Moist air is lese dense (more H2O molecules between the O2 molecules)
  • Dry air is more dense (more O2 per volume of air)

So... dry cool air is more dense, moist warm air is less dense.

Your carburetors mix a certain amount of fuel with a certain volume of air. There is an ideal ratio of fuel to air called the stoichiometric ratio, which is about 14.7:1. At this ratio, theoretically, all of the fuel will be burned using all of the oxygen in the air. In reality (your bike/my bike), the fuel to air ratio varies from 14.7:1 quite a bit. More air = less gas (lean); more gas = less air (rich).

Your job when jetting your bike is to get as close to this ideal 14.7:1 ratio as possible, (which also produces the most power). The problem is denser air (Cold/Dry) produces a leaner condition and thinner air (hot/wet) produces a richer condition. The other problem is that the weather is constantly changing. So in the end, yes you can constantly chase the perfect mixture (thus the advantage of fuel injection, which is constantly adjusting for weather) or you compromise and set up the jetting to work well at about 70 degrees... a happy medium.

A lean condition can hurt your engine over time, but you would certainly know it's lean (spitting, sputtering, etc.). Duke ran 92 mains in his carbs for a long time and had no problems until he pulled the airbox top (allowing more air to flow - thus creating a leaner condition). When he did that he could immediately tell it was too lean. 105's (stock) are in it now and it runs very well.