HEADS UP!!!

Site upgrade in progress... Core site functions are working, but some non-critical features/functions will be temporarily unavailable while we work to restore them over the next couple of weeks.

Please post any bugs you encounter, but before you do, check to see if it's already listed.

Thanks for your patience while we work to improve the community.

Sign in to follow this  
Followers 0
yz400fdude

1998 yz400f carb problem

12 posts in this topic

Start with the pilot jet, also known as the slow jet. Could also be an air leak, you can test this by spraying some carb cleaner on all of the intake joints. If engine revs when you spray a joint then that is where your leak is. Other options could be idle set too low, try turning it up about 1/2 turn at a time and see if that helps. Try these things and let us know how it comes out. Good luck.

Josh

Share this post


Link to post
Share on other sites

I can turn the idle all the way up and it still wont idle but yet I can turn the choke on and turn the idle all the way down and it idles pretty decent. Where is the pilot jet located? There is a stripped screw on my carb and I cant get it all the way apart. I will try the airleak method you suggested also.

Share this post


Link to post
Share on other sites

Be sure to check that the hot start is not leaking air by pinching off the hose that leads to the head. If that improves it, it's leaking. Also be sure that your pilot fuel screw hasn't fallen out.

Share this post


Link to post
Share on other sites

Is there an exploded view of my carb somewhere online so I can be sure which jet is the pilot jet?

Share this post


Link to post
Share on other sites

OK I Looked at the manual so now I know what I did. I already cleaned the pilot jet and all the other jets in the area. So on to looking for air leaks and the diaphragm. Also the pilot screw is there but I do have a question, how tight should it be?

Share this post


Link to post
Share on other sites

The pilot fuel screw is he fuel trim adjustment for the idle circuit. As a starting point, it needs to be turned in until it gently seats, then backed out 1 3/4 turns. Then follow the procedure below (it's for a 250F, but it works the same way):

http://www.thumperfaq.com/jetting.htm#PJ

Share this post


Link to post
Share on other sites

I have one more, take a look at the business end of your idle screw and make sure that it is contacting the throttle cam and actually adjusting the slide position to control the idle. I have seen the cam come out of the TPS slot and this holds it just far enough out of position that the idle screw does not line up with the throttle cam. This will be visible under the plastic cable cover on the right side of the bike. Hope this helps, if you are not sure try to take some pictures and post them up.

Josh

Share this post


Link to post
Share on other sites

I'm thinking pilot jet is clogged. Also i have seen FCR carburetors where the slide is put in upside down. On the front of your carb slide, there is a plate with an o-ring gasket. Make sure thats installed proper. Not upside down. Another thing is to make sure your pilot jet and leak jet are the proper size. Just because you can see daylight through them doesn't mean they are clean. You can use numbered jet drills to check. I will post the according jet sizes and the proper number drill to use tomorrow if i can remember my chart.

Share this post


Link to post
Share on other sites

I had a crack in my carb on the out side of the hot start plunger area. mine would also run with the choke out.we even sprayed carb cleaner on the out side of the carb and the idle didnt pick up. so you need to realy look hard if there is a crack.

Share this post


Link to post
Share on other sites

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
Sign in to follow this  
Followers 0