Leaner gets better?

[SanDiegoMike]

I remember reading recently in this forum that a lean condition will get better when riding and a rich condition will get worse. How does this work? While I am still trying to get to the bottom of my bike's poor midrange running condition, I am noticing that it does seem to run a bit better after I have been riding for 10-15 miles. The stumble/sputter is still there, it just seems be in a smaller range than before, if that makes any sense at all.
I think all of this lends to the advice I am getting from all of the pros on this site that I probably need to up my main jets by one size due to the intake/exhaust configuration I have. I don't know why this has only become a problem for me in the last couple of months, but it has.

[XSPastor]

SanDiegoMike,
The simple answer is that a cold engine requires a richer mixture than a warm engine. That's the whole reason for the enricher (which most folks call the choke) circuit.

Tim

[skids]

It may have something to do with hot cylinders turing the mist droplets of fuel into gas that is more redily combustible.