Hello, I am new to the forum and I apologize if this exact post is on here somewhere. I know there are jetting discussions but I can't find one to my exact situation. I recently bought an 08 YFZ 450 se. I am purchasing some parts for it and would like to know exactly what I should choose for jetting. I am putting on a full aftermarket exhaust (I think Dasa)and a K&N air filter. I guess first off do I need to re-jet it and if I do, what jets should I get as far as size? I have no idea how to jet or what to do with them. Also a side question as I just bought it; when it is at idle and I punch the throttle (in neutral) it bogs down and dies. What could this be? When I am riding and racing around the throttle is fine but it just seems like when I punch it before I have the engine rev'd up at all it doesn't get air or fuel fast enough.

 

For jetting, check with the exhaust manufacturer, but generally you need to go up one or two sizes on the main, and maybe 1 size on the pilot, coming from stock.

I would only use K&N in the dunes, every where else foam is better, jmho.

Sounds like you are experiencing the dreaded FCR bog. Common on a stock carb. Try adding throttle just a tad slower and it shouldn't bog. In real world riding I have never had an issue. There are a number of fixes, including the Boyeson powershot and its' clones.