Help! 2000" WR400 running problem

Hey guys,

I wonder if anyone has ideas regarding the problem I am having with my 2000" WR400:

It often (no always though) happens that when I downshift, rev the engive and then upshift back (like when I climb a hill) the engine starts "coughing", looses power and for a while doesn't run smoothly. After a while the cough goes away. It doesn't sound like your typical backfiring (loud pops) but then my previous bike was a two stroke and it never backfired so I may not know the backfiring sound too well.

I had the bike serviced, the valves, timing, rings, plugs, etc and the garage gurus say that there seems to be nothing wrong with the bike.

Actually I once had a problem which felt very similar with my 360 KTM - it turned out to be clogged carb drain hoses that caused it. Needless to say that I checked & cleaned the carb hoses on WR - didn't help. Also it seems that when I clean the airfilter for a while it "coughs" a bit less but then it comes back.

My bike is totally stock (exaust, air filter lid, all that).

Any ideas will be greatly appreciated! Thanks in advance.

P.S. It scares me that it's gonna start with that "coughing" when I am on the jumping ramp - and then I am gonna dive head first :-)

I sounds like its running lean at lower rpms and on the transition from the straight section to taper on the needle.

Try the following

1)Turn open the fuel screw in 1/2 turn increments and see if its gets better.

2)If you get past 2.5 turns out then go to a larger pilot jet, the next size up from stock, a reset the fuel screw to 1.0 turns out and repeat step 1 above.

3)If still having trouble then raise the needle by lowering the clip one position on the needle.

4) If still having trouble then go up one more size on the pilot jet and repeat step 2) above.

A totally srock bike is jetted too lean and needs to be richened up a little across the board. I would also invest in a couple of main jets the next three sizes and try them one at a time after you resolve the problem described above.

Keep the forum informed of your progress and the specific changes you make and we will help you out as best we can.

Good Luck


[This message has been edited by Clark Mason (edited 09-30-2000).]

It stutters after he has wound it out a bit and upshifted - and ceases (almost) to cough when he cleans the air filter, and you suspect it is running lean in the low speed circuit? Could you please explain the logic?

Where does it state that it ceases almost??? I read "it seems" and not "it ceases". It seems is pretty weak compared to "it ceases". At lower altitudes all WR owners I have helped out had to richen there bikes up usually through jet changes. On a totally stock bike maybe the fuel screw adjustment will be enough but I doubt it.

The combined TESTING experience that I and others have had and the inference from reading every topic related to jetting on this forum is that the WR has trouble coming off and going back onto the straight section of the needle unless jetted perfectly, having the accelerator pump set perfectly, etc. And every thing changes as the owners modify and upgrade their bikes.... This is why we here "stumble on upshift", "Stumble on rapid throttle opening", and "poping or a misleading backfiring on decelleration"

Usually when you upshift there is a drop in rpm and the engine is trying to spool back up in the next taller gear (ie more load). If it experiences a momemntary studder this is a classic indication of a momentaly lean condition. Many bikes, especially four strokes, which are jetted to lean in this area of throttle opening will run great until presented with a variable load at which time the momentary lean condition causes the engine to stumble because it cannot satisify the power output required due to lack of energy--fuel.


I appreciate the explanation Clark. I was following a rather generic line of reasoning that told me if the stuttering seemed to smooth out after cleaning the air filter, he may have a rich condition. I don't know the quirks of these bikes - I don't even have one now, but am waiting for one, so I'm trying to absorb all the info I can in the meantime, hoping it will be a help later on. That is why I asked for an explanation. Hope your info is of help for Phillip, and reading your posts has certainly been a help to me. Thanks again. -Chris

Clark - thanks for the reply!

I am going to try tweaking with the fuel screw and the needle this weekend. I will let you all know how it went and what the results were.

BTW, the "cough" does almost disappear for a while right after I clean the air filter.

One additional piece of info - I just borrowed a used WR400 aftermarket exaust from someone - the French CRD (or SRD) is the brand name, to try it out on my WR400 - I put it on and the "cough" while accelerating is gone! At the same time, the bike now "stutters" or "coughs" ONCE in a while while going at steady throttle at about half-2/3 open. This exaust is a lot more open (and loud) then the stock. Should I keep it?



I am not confident to give you an answer, Clark has more experience with WR's and YZ's. Do you know your needle code and clip position? There is one thing of importance. The stock needle and clip position is DRQ#4 for the non-USA models. This is close to DVP#6. This is the most rich low-midrange (1/4-1/2 throttle) of any model or year WR/YZ400 US or EURO spec. It will be unlikely that you will want to move the needle clip to a richer position than #4. (DRQ#5=DVQ#7) Your best jetting will probably be richer on the pilot jet and leaner on the clip.


This is an unusual case, would you agree?


Take into consideration that the WR's imported to Israel are European bikes. They come with the YZ tank and seat which is good, but they come with a very restricted silencer that does not have a silencer cap which can be removed.

Phillip, replace that silencer.


Husaberg FE501 1996

Need to know the needle and clip position in this bike before further comment. If it is a DRQ then I concur with James Deans comments. How about posting the exact jetting in the carb.


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