Idle Issue

5 replies to this topic
  • AngryCandy

Posted April 18, 2000 - 04:47 AM


First Post....great forum BTW.

I just got a 98 yz4 and have ridden it only 3 times. Love the bike, but the last time I rode it, after about 1 hour in the saddle, the bike refused to idle. Started fine w/o choking, but would not idle. The idle screw is all the way out. Any ideas of how I can increase it? I am just above sea-level and the pipe is a FMF Power Core 4. Thanks

  • Chris_in_the_Mojave

Posted April 18, 2000 - 07:36 AM



Okay, I'll take a stab, if I can't get you going we'll hand off to Clark "The jetting God" Mason. :)

I'm running a PC IV and the stock head pipe. I've got the stock needle and a 180 main, 2.5 turns out(I think, I'll recheck tonight at home) at 2500 ft. When I put the PC IV on I didn't require any idle adjustments.

For starters set the air screw to 2.5 turns out. Look in the manual for a setting for the idle adjuster. You shouldn't be "all the way out". Next CHECK FOR AIR LEAKS( my gut says this is where your problem is). Make sure the hot start hose is hooked up to the hot start valve...BE SURE you haven't left the hot start button "out" :D (You wouldn't be the first!)

What is your jetting setup? Are you running the stock head pipe? What altitude are you at? Will the bike idle when it's cold? The bike idled on the other two rides?

Chris in the Mojave
'98 YZ400F

[This message has been edited by Chris in the Mojave (edited 04-19-2000).]

  • AngryCandy

Posted April 18, 2000 - 07:47 AM


Chris, Yes, I am running the stock head pipe. I don't know anything about carbs, so I couldn't tell you the jetting setup. The bike starts fine cold & hot, runs great - holds consistant speeds well, and accelerates really smoothly (makes me not want to dick with the carb.) I do get some backfire when I close the throttle, but I think that is the nature of the beast.

The bike idles fine when cold, but stalls now when hot. If I close the idles adjust screw at all - forget it - no idle ever.

I will check for air leaks - I am positive the hot start button was not out. Also, I will try setting the air screw 2.5 turns out. Thanks for the input.

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  • yzracer49

Posted April 18, 2000 - 07:04 PM


I just picked my 98 up last month. Mine would only idle with the idle screwed all the way in, I then set the air screw and it made a world of difference.
Check the air screw on the bottom, set it, as Chris said, to 2.5turns out.
Try screwing the idle screw in, this will push on the throtle plate and actually open the slide. By turning it out you closr the carb opening completely.
Also check the idle jet, if the bike has been in storage before you got it it may be plugged.

98 Yz400f
93 Xr100
90 Kdx200
00 Pw50
71 Ke100

  • Matt_Porritt

Posted April 18, 2000 - 10:22 PM


I just got a 98 yz4 and have ridden it only 3 times, but the last time I rode it, after about 1 hour in the saddle, the bike refused to idle. Started fine w/o choking, but would not idle. The idle screw is all the way out. Any ideas of how I can increase it?

Is the hotstart stuck out a little?
I think the idle screws in to increase it.

**Ride it like you Stole it!**
Matt Porritt
99 YZ400F
Rubber Chicken Racing

  • Chris_in_the_Mojave

Posted April 19, 2000 - 06:48 AM


Okay, I checked my notes that FMF gave me last night. With the PC IV SA and the Stock header pipe they recomend a 180 main jet and the Air screw ( the one on the bottom of the carb) 2.75 out. If you have the stock main jet I'd go 2.5 and work with that. The change in main jet isn't part of your idle problem.

To get at the air screw you'll need a small 90 deg blade screw driver. I bought a small ratchting one at Sears that Brian clued me in on. Turn the screw in CAREFULLY until it stops...not "tight", just seated. Now turn it back out, keeping track of the turns, to 2.5. On the idle screw ( the one with the black knob ) you can fine tune the idle speed. You shouldn't have to go all the way in or out. Keep us posted....

OBTW - I TOTALLY understand not wanting to screw with the Carb...6 months ago I felt the same way. That said, changing the main jet is easy, and we'll talk you through it. If you can remove a screw, you can change a main jet. Remember the engine is just an air pump and you've improved it's ablity to "breath" by changing the pipe, to take advantage of the additional air you need additional fuel. Anyway if you feel brave, let us know when you have the jet(180)and we'll talk you through it :)

Chris in the Mojave
'98 YZ400F

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