How much does temperature matter?

[07YZ450F]

how important is temperature to the YZ450Fs? i ride from 70 to 90 degrees at 990ft on stock jetting. hard starting (3-5 kicks), a slight bog when i snap the throttle, and decel popping. what steps should i take to fix this?

[KJ790]

There are a couple ways to fix this. If it ran great where you are at 70 degrees with that jetting then I would go leaner with the main and adjust the fuel screw.

[BergArabia]

Not sure on the technical as I have never done the jetting thing.

I can tell you my bike starts much easier in warm than in cold..

As in first kick every time. So I guess mine might also be a bit lean as it is 3 or 4 kicks when really cold (here that is less than 70 degress) warm is like 100.

[Yamahafan]

how important is temperature to the YZ450Fs? i ride from 70 to 90 degrees at 990ft on stock jetting. hard starting (3-5 kicks), a slight bog when i snap the throttle, and decel popping. what steps should i take to fix this?

I am assuming you have a 2007 YZ450F.

Temperature is very important. In general your jetting should get richer as the temperature gets warmer. You should generally go down one main jet size (i.e. 168 to 165) for every 15 degrees the temperature goes up. I am running 165 mj 45 pj on my 2006 450F with perfect starting and slight decel popping from 80 to 90 degrees. I also have the Zip-Ty mod. A little decel popping indicates ideal four stroke jetting. Usually you can adjust the fuel screw to reduce decel popping.

Hard starting every time ot only the first time of the day? If first time - blip the throttle once or twice to prime the carb before starting. If hard to start every time - open the fuel screw a half to full turn, if this doesn't work go up one size on pilot jet. These will both help with decel popping.

Bog fix - do a search on this and the jetting thread. All these issues have been addressed many times.

[Dano426]

I am assuming you have a 2007 YZ450F.

Temperature is very important. In general your jetting should get richer as the temperature gets warmer. You should generally go down one main jet size (i.e. 168 to 165) for every 15 degrees the temperature goes up. I am running 165 mj 45 pj on my 2006 450F with perfect starting and slight decel popping from 80 to 90 degrees. I also have the Zip-Ty mod. A little decel popping indicates ideal four stroke jetting. Usually you can adjust the fuel screw to reduce decel popping.

Hard starting every time ot only the first time of the day? If first time - blip the throttle once or twice to prime the carb before starting. If hard to start every time - open the fuel screw a half to full turn, if this doesn't work go up one size on pilot jet. These will both help with decel popping.

Bog fix - do a search on this and the jetting thread. All these issues have been addressed many times.

What he said:thumbsup:

[07YZ450F]

i thought u went leaner when the temperature got higher becuz the air is less dense? and richer when it was colder because the air is more dense requiring more fuel to combust at the proper ratio?

[grayracer513]

That is true, but you shouldn't need to rejet for a 20 degree swing in temps. Nevertheless, one cause of excessive decel backfire is a lean idle mixture. The other is an air leak in the exhaust system. Some decel popping is normal for a YZF.