WR400 carb issue?

I just can't figure this one out, maybe someone has some good guidance; My '99 WR400 is "missing" (coughing and spitting) when running down the road at a steady speed, between about 30 and 45 MPH (about 1/4 to 1/2 throttle) most noticably. At lower speeds (and RPMs) and when accelerating up to and over the cruising speeds, it runs like a top. Even when it's missing, if I crack the throttle, it's off like a rocket, but settle back into a steady speed and it starts back up with the missing. It's not real a real steady or cyclic "miss", and I just can't seem to narrow down what it might be. I'm thinking so far it might be a carb issue, but nothing I've done with the carb seems to change the miss. I've gone through the carb (full clean and rebuild) and I'm running good fresh gas. Any thoughts?

..And I did try disconnecting the TPS with no noticable change to the issue...

WR's have a known CDI issue that is just like what you describe....but seeing as you have an older motor, I would be your carb's emulsion tube and needle are worn out.

I think I got it figured out, finally!

Thanks a bunch for the insight Krannie, I'm sure I'll be doing a jet and needle kit in the near future, but I couldn't just let it sit for another night. I went back out and took the carb off the bike and apart ...again. (I'm etting pretty darn good at this particular project!) Looking it all over, I decided to try bringing the float up a bit and that sure seems to have helped. A little frustrating when I was so sure that was not it to begin with!! (So much for "stock measurements")

Again, thanks for the help Krannie.

The "stuttering" that you felt while cruising at a steady throttle, is very common. Some have gone so far as to replace the stock ignition system with an aftermarket one like a Dynatek. While this does fix the issue so I've been told, it is very, very costly. And some of us get our jetting dialed in, which really helps to mask the stuttering issue, and virtually makes the "stuttering" un-noticeable. I almost have to "look" for the stuttering while cruising to notice it, it lessens it so much.

Yup, the float adjustment did it. Put a few black-top miles on it last night, and although the "studder" is still there, it is WAY LESS noticable. I figure I'll pull the carb off and apart one more time before the true riding season gets here. I plan to change up the jets and the needle, and one last tweek on the float. -Thanks for the thoughts Maniac998.

I just bought this bike this past fall to replace a much more tired old XR600. Funny thing; the kid I bought this one from said the carb had "just been rebuilt". The bike is pretty clean, but I would argue the float bowl and jets had not seen the light of day since the factory! It ran "OK" on a couple rides I got in during the fall, but I'm looking forward to this coming season and some better fuel delivery!

I'm just starting to research the jetting, so I'm certainly open to suggestions. I live in the metro Denver area and do most of my riding up in the hills west of here, usually around 8,000-12,000 ft, with a few rides each rear down at Rampart Range (around 6000 ft I think).

dad 7.jpg

Thousands of us have had incredible results with the JD jetting kit. Others have purchased needles, pilots, leak jets, etc. individually from Yamaha and had equal success. So the choice is yours!

I've never heard anything bad about anything from JD Jetting, likely to go that way.... Thanks! :thumbsup:

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