back where i started - stalls in idle...

WR400 that I picked up for real cheap, wouldn't idle. Turned the idle screw all the way up and backed it out 1 full turn, and it idled/ran fine. Starts up on the first 3 kicks. Monday I did a whole bunch of stuff at once: Changed oil + filter, bled brakes, replaced air filter with HiFlow Air Filter, and the grey wire mod. Now, still starts super easy but once again the bike won't idle - once it's good and warm it will idle for anywhere between 15 seconds - 1 minute, and then it'll stall with a little pop. I tried to turning the idle screw fully in and it still dies. It has some sort of Supertrapp can on it (can't find any info about it on the internet and the PO told me everything is stock...) The header pipe is definitely still stock (blocks the oil filter cover, pretty cool yamaha) but the can makes me think that I may need to rejet...

I'm wondering where to go next - definitely going to pull the plug and replace it. I ran a bunch of sea foam through the last tank, but should I pull the carb and clean it out? Could it be the jetting? Dunno what else to look at...

You made changes to the bike that undobutedly made it run leaner.

Jetting would be strongly advised.

JD jet kit.

Hey check the piolt jet make sure its not pluged check the fule mixture screw on the bottom of the carb make sure you take it out all the way squreart carb clean into it but make sure that the small washer spring and o-ring is there then reinsert it into the carb 1 1/2 to 2 turns out then start the bike tun the idile up until it stays running then mess with the fule mixture screw untill you get the highest idle the turn your idle down and repeat untill you have the bike idling good then turn the fule mixture srcrew out 1/4 turn and it sould idile

If it was running fine before, and it sounds like it was, there are 2 things you might want to try: First, turn the fuel screw out a half turn or so. If it idles there, good to go. (Note that if your fuel screw is out more than about 3 full turns, it's time to go 1 size fatter on the pilot jet); the other thing that may be going on is that you have too much oil on the oil filter and it's choking the flow. Look to what you changed to find the problem...(and the gray wire shouldn't have any effect on idle assuming you unplugged the right wire...)

Two things:

replaced air filter with HiFlow Air Filter
then it'll stall with a little pop

Those two things tell me "Lean" - probably will want to go one or two bigger on the pilot jet, main jet - you might get away with what's in there if it was rejetted for the exhaust that is on it. If your "Supertrapp" looks anything like mine (see below) then it's a White Bros. E-Series (no longer made) and depending on the number of plates used it'll run lean.

6 plates is approximately stock, 12 is going to make it pretty lean.

I put the quiet insert into mine so it's open now, had to rejet mine with both bigger main and pilot.

Rule of thumb is - If your pilot screw is not between 1.5 and 2.5 turns out for it to idle right when warmed up, you should rejet the pilot for sure.


Since your problem is with idle you should check your pilot jet. Make sure you can see through the small opening. The small opening tends to clog up. Use compress air or carb cleaner to clear the passage. Most of the time idling issues are caused by clogging of the pilot jet. The other two screws that affects your idle are the fuel mixture screw on the bottom of the carb and the idle knob. Initially set the fuel mixture screw to 1 5/8 turn and adjust as needed from there. You can also use the idle knob to adjust the idle. Don't turn the idle knob out too much or else it'll be too lean and you'll not be able to kick start your wr400. Don't ask me how I figure that out.....

Thanks for all the help! I adjust the pilot screw, turned it all the way in and then back it out turns, set the idle screw at 1 1/4. The bike ran much better, but still a little hesitant on the idle, so I backed out the pilot screw to 2 1/4 and turned the idle in a 1/4 turn to 1 turn and she idle just fine! Thanks!

matt4x4 that can looks very similar to mine, that pic is with the silencer on though right?

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