Running Rough

Hey, I hope someone can help me. I bought a stock 99 WR400, with the exception of a ground off throttle stop, opened up exhaust and removed airbox lid, last month. I'v been out riding two times with no problems, until my third time out. It just started running real rough all of a sudden at about 1/4 to 1/2 throttle. I checked the air filter which was clean and plug which was really sooty, so I replaced it with NGKCR8E. That didn't change a thing. So I tore the carb off and cleaned it very thoroughly, put it back on and still have the same problem. While the carb was off I checked the jets and they are all stock and the clip was in the #3 position. It was running great before so I don't dare mess with the jetting until I trace the problem. What should I check now? Any input is greatly appreciated.

I have a bud who experienced the same problem.

Does it go away when the bike cools down?

If so theres a good chance its the CDI unit :)

He has killed 2 in 3 years on a YZ426 :D

Might not be, but we chased our tails for ages around the carby path to no avail. :D

If you have a mate with the same bike do a swap and check it out.


I have the same bike and have been going through some problems myself. Check out post (Need help after Mod's). It explains where I started and where I'm at now with my bike. Lots-O-Information in there!

I know before anyone can help they will need more information.

What elevation do you ride at?

What are the exact jetting #'s? I know you say it is stock, but I don't think the clip in the 3rd position is stock on all 99 WR400's. (Is it a US, Europe, Aus, CDN, or ???)

How thoroughly did you clean the carb? Lost any O Rings?

Probably the biggest question is did you clean the accelerator pump real good (were you riding in any water)? Check for cracks on the accelerator pump diaphram. Also check for a cracked slide.

Check into the "Taffy Mod" let me know if you want me to email you pictures on that mod. I haven't taken the time to figure out how to post the pictures (been working on my bike the past few weeks), but I'd be glad to email them to you. I wouldn't do any Mod's like the Taffy Mod until you figure out why it started running bad all a sudden,(mistake I made) but I know the mod is worth looking into.

If you find nothing wrong with the carb, you should probably start running through the electrical tests in the Service Manual. If you don't have a Service Manual I would absolutely get one. It has helped me out a ton.

Keep us posted on the progress!

I forgot to add that when I had the carb apart there was water and a little bit of dirt (no rust) in the accelerator pump housing. I checked the slide for cracks and found none. I cleaned the hot start circuit. All the o-rings are in place and the diaphragms in the accel. pump and air cut valve are okay (no cracks,etc.) and the float height is slightly off spec: it specs at 9mm, I measured it at 9.5mm. Here's what I know on the jets. They are all stock:

Main-168; Jet Needle-OBDTM; Pilot Jet-45; Pilot Screw- 1 1/2 turns out; Clip position is #4 stock-was in the #3 position when I opened it up.

I didn't mention that it only runs rough when I try to hold the throttle steady between 1/4 and 1/2 throttle. If I accelerate hard through the gears it seems fine until I let off the throttle and engine brake, then it backfires excessively.

I hope this helps. Thanks again.

Okay, so today I pulled my carb of for the third time and checked everything over for missing or broken pieces. It's all good. I reinstalled the carb and made sure that everything was tight (no air leaks). Before I ran it, I drained the tank and gave it some fresh fuel to rule out that possibility. I also pulled all the electrical connections and hit them with some cleaner. Then I ran it up and down the street. When I crack the throttle open fast in neutral and when I decelerate, the bike backfires, pops, and spits at me. What could it be? :) This bike still has the original tires on it! Could the valves be out of adjustment? I've got to get this thing figured out. I can't go another week without riding. :D

Thanks for helping.

its strange that it was ok a couple of times and then went bad.. did you ride it really carefully those first 2 times and perhaps the change in your riding style on the 3rd occasion made the problem show up ? I just got a 98400f and it came with a comp exhaust, it was running badly too, but I fortunatley got the original exhaust with it and when i stuck that on all was perfect. I knew the bike hadnt been jetted for the comp exhaust, so I wasnt suprised. (it took me a while to make that conclusion tho, because at first, till I got used to the bike, It seemed to be ok 1 minute and bad the next) If your bike has standard jetting with the lid off and un-corked, perhaps you need to re-jet the bike.

I decided to check the valve clearances today. Here's what I came up with. Intake valves spec at 0.15~0.20mm and Exhaust spec 0.25~0.30mm. I measured the Intake valve closest to the cam gear at 0.15mm and the other two at 0.10mm. I measured both Exhaust valves at 0.20mm. Could this be causing my problem? :)

Any help/suggestions appreciated.

Bingo!!!!! Get those valves back on line and you'll see a whole different bike. As I wrote in an earlier post, before valve job had a slight miss at 1/2 throttle under load and after valve job smooth as it gets. I lot of riders will monkey with the carb to "get the perfect burn" when all their doing is covering up a problem somewhere else. I'm still running the stock needle OBEKN on #3 and backed off on the main to 160, thats it. Lid, grey wire and rear sprocket, runs 100% and very easy to start. 1 year on same plug. 4000ft. Eats Hondas!!!!!!!! :) 2000 WR400 converted to a 426.

Your fuel screw is what controls the fuel flow at 0-1/4 thottle. Check out the March addition of Motorcross Action Magazine. In it you'll find an incredible article on fuel screw adjustments. I'd give that a try before doing anything else. I've found my bike runs differently when the temp changes. I bought a zip-ty fuel screw adjuster and I'm now able to do minor adjustments and it seems to be fine now.

It's been awhile since I've reported my progress. Sorry, my computer's modem fried. Well, I think the bike is fixed. I ordered the proper adjusting pads and installed them and it seems to have fixed the problem. I think I just need to get my fuel screw adjusted now, because I'm experiencing a bad bog when I snap the throttle open fast from 0 - 1/4 throttle. Everything else seems fine. Thanks for all the help.


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