Jetting question


3 replies to this topic
  • Yamiryder

Posted April 10, 2011 - 01:39 PM

#1

I just bought an 07 yz450f that has a two brothers racing full exhaust system on it. :ride:The problem i have is that it bogs a little off the bottom and when you let off on deceleration, it has a terrible sputter/popping. It has an adjustable air/fuel screw, and it seems to run best when the screw is on 4, and it goes from 1-4. I'm not very familiar with carbs, and i need to know if i need a bigger main jet or a bigger leak jet or what. I think i need a bigger jet, but i'm not sure. Thanks.

  • 2grimjim

Posted April 10, 2011 - 03:08 PM

#2

How familiar are you with tuning carbs? The FCR has a lot of adjustments to get in trouble with. Before you get too deep into changing jets you need to make sure the basics are good: Float level set @8mm; Accelerator pump is adjusted correctly; Spark plug ingood condition; valve clearance correct. If you are certain all of these things are good then you can start making jetting changes.

If you can turn the idle mixture screw all the way in and the engine still runs you have a pilot jet that's too big. Ideally you should be able to kill the engine by closing the idle mixture screw and have good idle quality and easy starting with the screw 1.5-3 full turns out. Some popping in the exhause on deceleration is normal as long as it's not really bad. Opening the idle mixture screw slightly will reduce the popping but opening it too much will make the bike hard to start when it's hot.

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

Posted April 10, 2011 - 03:20 PM

#3

How familiar are you with tuning carbs? The FCR has a lot of adjustments to get in trouble with. Before you get too deep into changing jets you need to make sure the basics are good: Float level set @8mm; Accelerator pump is adjusted correctly; Spark plug ingood condition; valve clearance correct. If you are certain all of these things are good then you can start making jetting changes.

If you can turn the idle mixture screw all the way in and the engine still runs you have a pilot jet that's too big. Ideally you should be able to kill the engine by closing the idle mixture screw and have good idle quality and easy starting with the screw 1.5-3 full turns out. Some popping in the exhause on deceleration is normal as long as it's not really bad. Opening the idle mixture screw slightly will reduce the popping but opening it too much will make the bike hard to start when it's hot.


Thanks for the info. I have yet to check all those things, but i will. And when i said the air/fuel mixture screw, i meant it has an aftermarket air/fuel screw on the bottom of the carb. I wasn't messing with the idle screw. This popping is also worse than it should be.

  • grayracer513

Posted April 10, 2011 - 06:39 PM

#4

Read through the threads in the Fuel system section of the Common Threads Sticky, in particular the ones regarding backfire on decel, how to verify the correct pilot jet selection, and jetting by the seat of your pants. Then also read the jetting used by other members with similar bikes/riding conditions, but be sure to understand that because someone else did it doesn't make it a good idea.





Related Content

 
x

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.