'98 YZ400 won't idle

20 replies to this topic
  • Tim_Whisnant

Posted February 19, 2005 - 08:21 AM


:) Help!! I bought a '98YZ400 and it starts fine but won't idle after it's warm and the choke is off. The bike was sitting since May so I replaced the gas and cleaned the pilot & main jet, and still no luck. It also pops alot when I have to keep it running with the throttle.

I took it to a local shop and they cleaned out the carb and a new plug and it still won't idle !
They suggested a whole new top-end job !!! The guy I bought it from says it was running perfect when he last rode it and that there's no way it needs a new top-end. (He seems pretty honest, an old Dist. 37 guy)

Anybody have any suggestions out there ?? I'm getting desperate !! :)

  • Chris_from_Oz

Posted February 19, 2005 - 09:32 AM


Have you adjusted the fuel screw?

  • sanpedro426

Posted February 19, 2005 - 09:34 AM


sounds lean to me , ide turn the mixture screw out a half turn and see what happens then. Is the altatude much different where you are as apposed to where you bought it? :)

  • timoyz426

Posted February 19, 2005 - 09:50 AM


just bought a yz426f 2000 for around $3250 its in really good condition is this a good price? Also do they have many faults? what do i need to do to maintain it and keep it running well, any advice would be greatly appreciated regards tim


Posted February 19, 2005 - 10:52 AM


just bought a yz426f 2000 for around $3250 its in really good condition is this a good price? Also do they have many faults? what do i need to do to maintain it and keep it running well, any advice would be greatly appreciated regards tim

Please dont hijack this dude's thread. Start a new one.

Tim, try adjusting your fuel screw and see if it does anything.


  • sanpedro426

Posted February 19, 2005 - 12:28 PM


When you say wont idle , do you mean it dies or reves to high instead of idle?


Posted February 19, 2005 - 12:55 PM


I think he means it dies on him.


  • sanpedro426

Posted February 19, 2005 - 01:30 PM


Then thats REALY lean huh.

  • thumper245

Posted February 20, 2005 - 12:04 AM


I just bought a 98 yz400. It had been sitting for just about all of its life. When I got it the carb had to be rebuilt. I dont think that is your problem but I did play with the fuel screw and I had the same problem. Would die everytime that I let off the gas. I turned the fuel screw out some and took care of the problem. I hope this helps and it wont hurt to change the plug. It seems like it is a simple problem not a rebuild, start there and see what you get.

  • Tim_Whisnant

Posted February 20, 2005 - 06:55 AM


Thanks for the info ! I'll give it a try. Is the fuel screw the one in front of where you get to the main jet ?? And what's the easiest way to get to it? :)

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  • sanpedro426

Posted February 20, 2005 - 08:19 AM


Shoot i was going to tell you where MY fuel screw is but i dont know if our carbs are the same . I have a 2k2 426 but any way its on the bottom of the carb in front of the bowl and it is difficult to get to. I use a shorty screwdriver VERY short and it has to be thin too.It has to be very short because the bottom of the carb is not far from the top of the tranny , AND the screw is way up in a hole almost a half an inch up in there. HOPE this helps.Turn it OUT to RICHEN and IN to LEAN a 1/4 turn at a time. :)


Posted February 20, 2005 - 03:03 PM



Completely clean the carb......... (there are 5 jets in the carb, clean each jet, need to see thru each jet), change oil, replace gas, clean air filter, new plug again.

Bike sounds lean, but if a D37 own and maintained it, it should be something simple.............


Posted February 20, 2005 - 03:05 PM



fuel screw underneath the carb, requires a small screw driver or special screw driver........

  • henrysdad

Posted February 20, 2005 - 03:15 PM


what happens if you turn the idle speed up?

  • Tim_Whisnant

Posted February 21, 2005 - 08:41 AM


Thanks for all that info everyone !! The idle screw didn't help at all by the way.

I have another question though.
Do I need a special plug wrench or will a deep socket work to get the plug out? And what size is it?

I didn't realise the plug is so deep down and hard to get to!! :)

  • sanpedro426

Posted February 21, 2005 - 11:29 AM


I use a plug socket with a 3 inch extension and a swivel on top of that , to get around the frame, then a 6 inch extension and a ratchet. :thumbsup:There are two sizes of spark plug sockets , its the small one. I recomend the spark plug socket because it has rubber in it to grab the plug so you can get it out of the deep hole its in. :)

  • mtndjd

Posted February 22, 2005 - 08:04 PM


New to this forum. I read through this thread trying to find my own no-idle problem with my 99' YZ400F.

My experience may help someone else, maybe not; enjoy!

Situation: Perfect running YZ400F. Then - my bike sat for 4 months in a very hot shed with no use and the gas turned off. What I suspect happened was that the gas in the bowl evaporated and left minute carbon deposits. When I turned on the gas and tried to start the bike, these particles became dislodged. In those first few seconds, they made their way to my pilot jet. Upon taking my carb apart for the 3rd time I changed out the jet and compared it to a new one. The hole which I thought was clear and open, was much smaller on the old 45 then on the new 45 pilot. I then realized that my cleaning job of spraying carb cleaner did no good the first and second cleanings. It took reeming out the pilot with a small diameter wire to dislodge the deposits. What a difference a tiny minute upsize in pilot diameter makes! These things are touchy.

Back together, and wa lah! I guess I wont sell my YZ after-all.

  • Tim_Whisnant

Posted February 23, 2005 - 08:38 PM


:) Thanks, that sounds like good advice. I'm going to try the new jet along with a new plug. Could that be why it pops when I have to keep it revved to keep it running ?? :)

  • henrysdad

Posted February 24, 2005 - 03:48 AM


Sounds very lean to me on the pilot circut OR you have a air leak between the carb and the head.spray some liquid around the rubber carb boot and take notice of any engine speed changes.if there is any change fix the air leak.

  • Tim_Whisnant

Posted February 25, 2005 - 01:30 PM


Thanks ! I'll give it a try. :)

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