Idle Problem

What are things I should try and check to figure out why my 98 YZ400F won't idle? I have ordered new gaskets for the exhaust, valves are not too tight, cleaned carb, stock jetting, stock exhaust, 93 octane, hot start valve works properly. I've just plain run out of ideas. Tried tuning with the pilot screw with no luck. Is there something that I'm missing in the carb? Any ideas anyone? :D:applause:

You did not mention the idle screw, black knob on the left side under the choke and hotstart pull knobs. I have seen where these will not engage the throttle linkage on the right side also. This is usually caused by someone pulling the TPS and not re-installing it properly. Anyway, check to be sure your idle screw is working by pulling the black cover off of the right side and watching to see if the end of the idle screw is contacting the linkage. Hope this helps.


Ditto to what Josh suggested. Are you saying it won't idle at any RPM, or are you saying you can't adjust the idle, and that turning the idle screw neither lowers or raises the idle. Can you manually get the bike to idle using the throttle? If it idles using the throttle, then it will be as Josh informed you.

The bike will not idle at all. I have to use the throttle to keep it running. The idle screw is working properly but it is in as far as it will go to help keep it running. When it is adjusted out there is no change in idle. It will idle with the choke out. The more you turn in the idle with the choke out the faster the idle. As soon as you push the choke in it will die after a second. How do you adjust the TPS? I haven't touched it since I've owned the bike. Thanks. :applause:

You might have an electrical problem. I have a WR450 that wouldn't idle proberly. It would idle for 5-10 seconds and just die. Thought it was a fuel problem for the longest time. Even had a good mechanic troubleshoot it for 7 hours and still couldn't find what was wrong.

Finally started swapping electrical parts and it end up being a bad ignition coil even though the coil gave a good reading on an OHMs meter.

How do you adjust the TPS? I haven't touched it since I've owned the bike. Thanks. :applause:

If you haven't messed with it then don't. It is probably not the problem anyway. From what you said about the idle increasing with the idle screw with the choke out it sounds to me like you may have a jetting problem. Have you done any jetting recently? Is it possible that you replaced the pilot with one that is much smaller than the one you were using? How does it run other than the idle? Have you ridden it to see if there are any other symptoms that you don't notice on the stand? I had a bike one time that would idle for a few seconds and die, but it would die when you tried to give it a good ammount of gas. Turned out to be that the needle clip had fallen off and the needle was just sitting in the main not moving. Just throwing some thoughts out there, you have a pretty strange problem here. If you have a buddy with the same bike then you could try swapping parts such as the coil and CDI to check a few of the electrical components. Good luck and let us know what else you may find.


I know this is way simple but make sure your carb is perfectly seated and sealed

I have ridin it and the only other problem I'm having is a little bit of popping on decel. No bog anywhere. The top end does seem like it loses power at high revs. That is about as precise as I can get. I'll try some of the suggestions. Thanks for your time and thoughts. I'll keep you posted. :applause:

I am having the identical problem with the identical bike ('98 YZ400F). Your symptoms are exactly what I am experiencing and I have also tried everything I can think of to correct the problem. I have even gone so far as to tear the carb down (twice) and adjust the float level, blow out every passage, etc. I also thought it might be sucking air between the carb and intake...but again, no dice. I've actually given up and booked it into the local dealship but they can't get me in for 2 weeks. As long as the bike is running, it has great response and power as soon as you're off idle.

If you figure out the problem, please post it!


Could possibly be too rich off the bottom, requiring you to open the throttle (thus more air) in order to idle. I'm no jetting expert by any means...just a thought.

check the cold start plunger. If it isn't seating properly is will cause idle problems.

If I figure the problem out before your bike is looked at i'll be sure to let you know. If I don't would you please let me know what the shop finds or doesn't find? Thanks. :applause:

If the bike has been sitting for a while, or you have dirt in your gas, clean or replace the pilot jet. It is the circuit that effects your idle. Also, if it backfires on decel, you are lean, again suggesting it is a clogged pilot jet. It doesn't take much to clog it. I had the problem on my 99 yz400. You only noticed the piece of debri by holding the jet up to a bright light. I really would check this again if I were you.

I'm still waiting to get it into the shop, but I took Tom's advice and checked the ignition coil (even though it seemed unlikely). The readings are definitely off from the parameters in the manual. 0.5 ohms on the primary coil and 21,000 ohms on the secondary. I've ordered a new coil but it's back ordered from Yamaha. I'll report my findings when it comes in.

Sounds like a start. I'm going to dig into the timing and make sure everything is correct there. My cam timing is right, but maybe the (I'm not sure what to call it) electrical or initial timing is off. I'll check my coil as well. Keep me posted. Thanks. :applause:

The bike's at the shop today...I'll keep you posted. The ignition coil is b/o until the middle of July! But we'll see if the shop even diagnoses that as being the problem.

i had the same problem with my yz400f and here it was the air screw on the bottom of the carb bcz it was screwed out way to far when i bought it and it never idled unless i gave it gas..just a suggestion...keep us posted tho :excuseme:

Got the bike back from the shop...they said it was the hot start valve (even though I checked it before). I was skeptical but I took it home. I got home and fired it up with the choke on, and it fired up 2nd kick. I let run for about 30 seconds and then as I was watching, the choke valve snapped in by itself. I quickly pulled it back out and it snapped in again. I couldn't get it to stay out. So I decided to leave it in to see if the bike would idle, and it did exactly what it did before I took it in to the shop...idle for about 5 seconds and slowly die. So I tried to start it back up thinking maybe it just didn't have time to get warm enough, but now I can't get it to start at all! No matter what I do! So I've now paid for someone else to fix my problem (something I hate to do) and now I have worse problems (even though it wouldn't idle before, it still started on the 1st or 2nd kick every time). I called the shop to bitch and they asked me to bring it back tomorrow. I've got a trip planned for this weekend and no bike to ride at this point.

On another note, it turns out my boss has the identical bike and is pulling his ignition coil off to lend to me to find out if that's the problem. It'll save me waiting til mid July for the one from Yamaha.

Anyway, no further fact a big step backwards!

Well, back from the shop again, with much happier results. They tell me it wouldn't start last night due to a loose electrical connection. But the original idle problem is still being blamed on the hot start valve. I had seen this problem posted on here before and had checked it...but obviously not well enough. It's idling fine now...we'll see if it lasts.

I am still having having problems with the choke valve popping in on it's own...worn out? out of adjustment? I'll have to dig in to it.

Thanks for the help here. ran well all weekend. The idle seems to be a little eratic still...but at least it did idle. Still having overheating problems but that's a whole other thread.

Voodoo, did you get yours figured out?

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