YZ 400F jetting, cant seem to get it right

I had no help in the jetting form...

I ran stock 175 Main Jet, and backfired coming out of high rpm's

So I stuck a 178 Main Jet, got the same result.

I have the fmf powercore with 450 airbox(dont know if the 450 airbox is bigger then the 400) just cleaned the acc. pump

should I mess with the fuel/air mixture screw?

I just dont know what to do with it anymore

What year is the bike, what elevations and temps are you riding at?

it is a 1998, riding between yadkinville and steel creek...

temp betwwen 75 to 90( 95 is the highest I rode in)

run 93 octane

How bad is the popping, Is it only on decel or constant and excessive. What is the Elevation? A certain amount of popping with these bikes is normal, if it is excessive I would start with your pilot jet and not the main. the fuel screw should be your next step after that. I had the same problem and will post the link below to my thread and also Here is a great technique from Eddie Sisneros that got my bike running like a clock....until I cracked the case (another story).:thumbsup: Go back to the stock main and try this brother.

http://www.thumpertalk.com/forum/showthread.php?t=327405 Eddie's Sticky

http://www.thumpertalk.com/forum/showthread.php?t=793134&highlight= My post

Also for the fuel screw adjustments, if you want to make life a little easier on yourself, spend 12 bucks and get a Tusk Fuel Mixture Screw-$11.99 (It's a dial, with numbers that sticks out so its easier to make the fuel screw adjustments and keep track of # of turns) Best of luck to ya man.:thumbsup:

how to I access the piolt jet?

how to I access the piolt jet?
It's forward of the main jet in the bottom of the carb. On a 400, you should be able to rotate the loosened carb far enough for access. Before you do that, try the pilot adjustment as outlined in the third link I posted. There may be no reason to change it.

Download a manual here:


If you you can't get the link to work, let me know here.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now