YZ 400F jetting, cant seem to get it right

7 replies to this topic
  • rmx250rider

Posted July 31, 2009 - 02:21 PM


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

  • Aaron_YZ450F

Posted July 31, 2009 - 02:30 PM


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

  • rmx250rider

Posted July 31, 2009 - 02:43 PM


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

  • Aaron_YZ450F

Posted July 31, 2009 - 03:00 PM


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.thumperta...ad.php?t=327405 Eddie's Sticky

http://www.thumperta...3134&highlight= My post

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • Aaron_YZ450F

Posted July 31, 2009 - 03:06 PM


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:

  • grayracer513

Posted July 31, 2009 - 03:22 PM


The main jet has nothing to do with exhaust backfire on deceleration:


Confirm that you do not have an air leak in the exhaust system, then visit the jetting DB and find what others with similar setups are using for a starting point, and read this:


...and this:


  • rmx250rider

Posted July 31, 2009 - 03:35 PM


how to I access the piolt jet?

  • grayracer513

Posted July 31, 2009 - 06:00 PM


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.

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