Like from 80s and 90s to 40s and 50s I would notice a difference?
As air cools its density increases. So a simple answer is yes, you will be getting a larger volume of air on every intake stroke of the piston. I would say a 10-15 degree difference is negligable, but a change of 50 degrees might make a noticeable difference.
Yup, it will go a bit leaner, but it depends on your bike if it will be enough to require a jetting change
The fuel also doesn't atomize as easily with the cold air. Ideally, you would jet for the temp range your riding in, and as someone said, from 40's to 80's, I think you'd notice it with most bikes. Again, an ideal situation would be to be running with the same coolant temp in cold weather as warm weather. You don't want your oil on a 4t running too cold. You'd have to have something like a Trailtech that has a coolant readout. Then, you may have to cover up the rad(s) to get the summer temp of coolant.
Yep, richer on the needle worked for me with my JD kit in my Husky WR
If you had your bike running crisp with jetting spot on for 80"s - 90's and now it's in the 60's and you plan on running it hard, like racing, then I would definitely make an adjustment. I found that out the hard way.....
btw, I've noticed that temp changes of 20 degrees or so seem to equal elevation changes of 1500-2000'.... but that's just my seat of the pants dyno...
Here's a generic mikuni main jet chart just to see the correlation between altitude and temp. and what % change in the main size between.
also, even if your bike is jetted correctly you don't want to cold seize it by running it hard before warming up. In order for all the fuel to be burnt making the correct mixture parts have to be hot enough to atomize the big fuel blobs on contact. Otherwise it'll run lean and seize.
I would consult your owners manual or dealer for a jetting chart, every bike should have one.
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