YZ400 WR Carb Problem,Too much gas.

My 98 400 wr is getting too much gas. It is a stock carb. All out of the blue it is using too much gas. I dropprd the needle down a notch. It seemed to help. But it got wosre again. I also even put a new needle and seat in it. Took apart and cleaned it about 6 times. I look at the plug and it is Black.Replaced it a few times. Thinking about taking it to the dealer and let them fix it. And also have them time it ahead too.

Just woundering if anybody had this problem and knows how to fix it.

Thanks for any help.


:) I may be being obvious but have you checked the airfilter???

A choked airfilter will cause the same symptoms, or a rag in the inlet boot?

or is the choke circuit stuck on

right thats me out of ideas! :D

[ January 26, 2002: Message edited by: Guy ]

Yes,I have cleaned the air filter,took all of the hoses off on the left side,sprayed and blew them out with carb cleaner and air compressor. Also the hot start button took off and cleaned.

And also the bike kind of bogges a little at first if you give it some good revs at idle. And full open it kind of cuts off or something like that. And also I for got to mention was that some times you get a loud back fire when you try to start it.

thanks for your ideas


wow, I am having the exact same problem with my bike. It just started happening the last five weeks. I've experienced all you wrote even the backfiring when just started. Never had any problems before, always started on 1st or 2nd kick, but now I have all these problems. It'd be great if we could get this figured out together. My bike is slow to rev, which really sucks on the track. What to do? Any insight from TT's out there? Thanks :)

My bike was the same way. But mine started about 1 yr ago. I tried to start it in the winter. Tried for like 2 weeks. I coulnd't believe the plug was or would be fouled. But it was. The bike used to start on 1st or 2nd kick too. Sometimes still does. I ride and do a little bit of racing. Hare Scrambles and Enduros. Do local Hare Scrambles. And Enduros with the ECEA (east coast enduro association). You can check them out at www.ecea.org. Yeah I hope I can find something out. Just might take to dealer soon.

Check the timing on your accel pump squirt. Check to make sure the squirt just misses the slide as it raises when the throttle is cracked open.

Also I have heard of some problems with the pump actuating devise being dirty and not allowing the pump arm to return as quickly as it should.


I hope this isn't going to be a trend.

If the carb checks out, check the TPS with an ohmmeter. Make sure it's in spec. just because it's easy to do.

Have you checked valve clearances lately?

Have you done anything to change the ignition timing?

I have not not checked the TPS or even the valves. I was planning on doing that soon. When I bought the bike I asked the dealer when I should check to make sure the valves are still gapped right and they old me you don't have to worry about them. And I didn't do anything to the timing or any thing like that,yet. Thinking about getting timmed ahead like others are doing.


If you've had severe cold weather this will richen the bike considerably during start-up until it reaches operating temp. One way to compensate is by starting with hot start button on instead of choke.

APJ squirt timing ?

Thanks for the input. I will check the TPS and the pump squirt. My valves have not been checked but I will try that after the other two. I need to service the valves any ways. Never been done.

Thanks guys. I'll let ya know the results. Good luck Jim with your bike.


it sounds like your floats could sticking or way out of adjustment.also inspect float make sure it doesnt have any holes and needle valve isnt worn.a blocked vent line will also cause bike to run real rich.last but not least check vacuum hoses and hot start hose for leaks.try starting bike them tap floatbowl with wrench sometimes thats all it takes to get them unstuck.

Thanks to every body that had put good segestions. They are all good. Not one bad one. I'm tring all of what I did not try. I have checked the floats a few times. I had the carb tore apart about 7-8 times. What a pain in the butt.


Remember cold or wet weather will make bike run rich.

In my experience its been smartest to start simple, overcomplicating carb problems is common. Here are some "simple" things to try;

1. Turn your fuel screw in and count the number of turns. Turn the screw 1/2 turn further out than it was. Continue to try 1/2 turns until you've reached 3 turns. If the problem isnt solved turn it back to the original # of turns.

2. Check you float level again. Most YZ/WR's run better with a slightly lower float height than stock and you get better mileage!

3. Make sure your air filter is clean and NOT wet.

4. Lastly clean you accelerator pump.

Good luck!

Mr T

cold weather richens the mixture. the air pressure is lower in the winter. fuel finds it harder to vapourize.


How about checking the spark arrestor screen in the muffler? If the screen has carbon build up it could give you these symptoms too.

My `98 has been spared these problems so far. But it hasn't been ridden as much since the 02 arrived.

Air density is affected by the air pressure, temperature and humidity. The density of the air is reduced by decreased air pressure, increased temperatures and increased moisture. Depending on where you live the temp changes might be offset by increased moisture or decreased pressure, just depends. Some guys have a device called a Relative Air Density guage that does the math for them. Right now it's dry, cold, and high pressure where I live so you would definately have to richen up a bit and get some ice tires.

I think your on the right track with the valves. Mine got tight pretty fast.


Here we go again.... :)

Originally posted by Mr Toyz:

Remember cold or wet weather will make bike run rich.

Cold weather LEANS the mixture.

Mr. Toyz' suggestion of adjusting the fuel screw OUT will RICHEN the mixture. I don't think you want to do that if you already are backfiring and giving symptoms of richness.

I tend to discount the fuel screw and mixture as the source of your problem. Particularly if it is colder now than when it was running well. It won't hurt anything to try it though!

Others on the board have had the valve clearance cause this symptom. Check it. It's easy to do..


Steve T

so the weather chart is just a load of bollocks, right!


:) Didn't we just have this conversation last week....? I thought that it was mathematically proven that Taffy can't be wrong?! :D:D:D

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