400ex carb question?

[chevymanrick]

Ok I have an 07 model it runs fine and is starting fine but at idle it seems to spit or cough occasionally, and sometimes backfire when I turn it off, has full yoshi exhaust, and k&n filter with outerwears, airbox lid is still on, filter came in a kit, with a dynojet, and I had the dealer install it when it was serviced, anyone have any ideas about the cough in the exhaust note?

[JOHNDOE83]

Everything you described means its running lean, DONT EVER have the dealer install or tune anything!

You can try turning your a/f screw out to 3.5 turns and if it still does it you need a bigger pilot jet.

[chevymanrick]

Ok thanks, I bought a 40, 42, 45 pilots assuming the 38 is still in which I'm sure it is which is a more appropriate jet to use?

[Crosshairs]

The 42 seems to be the most popular for the 400..that's what I used and it works very well.

Check the main jet also, I would guess you should be using at least a 155 or 158

[chevymanrick]

Ok sweet ill pick up some mains also, to be honest I'm terrible with carbs, I don't really understand how to rejet for lean or rich situations, its pretty confusing to me so I appreciate the help, the pipe instructions said it didn't need re-jetting, which seemed kinda sketchy cause I thought u always changed jets with any performance mod, and I thought it might be alittle lean, cause after a lot of riding I could see the header pipe glowing slightly when it got dark out. Thanks again

[426kidz]

mine did that and it was because my exhaust stud bolt was loose and just needed tightened just check that maybe not it but never know...

[dxcody]

426 sounds pretty close on this one, before you start jetting + stuff just be sure to check and make sure you dont have any air leaks first. Make sure the muffler fits good and tight on the header and also make sure the carb and intake boots are tight on the carb.

Just to be sure before you tear into the carb.

Johndoe knows what he talking about im not saying he is wrong, just saying that you will NEVER get your jetting right if you have an air leak, i went through this with my old 400ex. My muffler wasnt tight onto the back of the header so it was causing it to run lean and after a ton of jets,gas soaked hands, and frustration later i figured out it was an airleak..

[426kidz]

Most of the time it's something small and thats where we come in and have done all the bs for you so you don't have too:D

[chevymanrick]

Ok thanks guys, I did read your post a few days ago about the loose stud bolts on your head, so ill double check the whole exhaust and make sure its all tite and there aren't any spots for it to suck air before I starring tuning the carb