Altitude, Temperature, Humidity

[D36-108A]

How do these rank in order of importance to proper carburetion?

The reason I ask is because I took my 02 CR250 with stock jetting* (& slight airscrew adjustment) to some low hills in dense fog at about 50 F. It abso-freaking-lutely ripped, pulling part-throttle wheelies, just crisp and sweet. OK, so it was colder than normal, so the air is more dense, fine. But with 100% humidity (goggles misted up immediately; you could wipe the water off the number plate and fender) shouldn't that cancel out the not-very-low temperature?

A week later, I took it back to the same place after changing the air filter, about 60 F, clear blue sky, and it was relatively weak, with worse throttle response and soft low-to-mid power. Just ten degrees and some fog.

I'm trying to get my low-altitude (<1000') baseline jetting before I come up with something for Moonrocks, which is at 4,500' and should be 20-40 degrees F.

* = 380, 32.5, -30-74 needle @ #2

 

[dirt bike dave]

There are 'calculators' that will allow you to input various conditions to show their effect on horspower potential. If you play with the numbers, you can see the relative impact of changes in temp, humidity and pressure.

http://wahiduddin.net/calc/calc_hp_dp.htm is the first one I found with a google search, but there may be better ones.

 

[D36-108A]

Very nice, thank you. Plugging in the conditions in which my bike ran near-perfect shows a near 100% relative air density. Reno conditions are about 87% of that.

Now all I need to do is figure out how to change the carb settings to match the 13% lower air density. Somehow I think it's more complicated than simply using 13% smaller jets.