Midrange sputter - Again!!

Hi all I've searched this board for a while now to try to find an answer to this midrange sputter/missing whatever you want to call it problem. Has anyone come up with a resolution and if so could you steer me to the right string?

My problem is at freeway speeds (55-65 mph), trailing/neutral throttle probably 1/8-1/2 range. Doesn't do it over 70 or below 50. Has done it since bone stock and every other jetting combo I have tried so far. The bike runs strong everywhere else and bangs the rev limiter in top gear. I don't notice it offroad because I'm never at a steady throttle.

Current setup:

01 WR426, YZ Timed, no lid, Cut gray wire, Dr. D pipe, 168 MJ, 200 MAJ, EJP #1 thru #5 #4, 45 PJ, 75 PAJ, lot's of pulled hair and pissed off family cause I'm in the garage so much.

I'm even ready for a lashing from Taffy!

By the way I'm, at sea level and the pilot mixture screw is set at 1 turn out.


[ April 17, 2002: Message edited by: Edrow ]


I'm way out on a limb here. There are two jetting related cutting out syndroms or misses, a rich one without popping and a lean one with popping. I can't explain why it would be rich on highway and not in constantly moving throttle on dirt. But I can explain why it would be lean on the highway and not with constantly changing throttle, the Accelerator Pump hides this lean spot by supplying extra fuel as the throttle is twisted.

This is where it gets convoluted and unlikely. You have a Dr.D pipe and you feel the missing and cutting out but you don't hear the popping. The other is you haven't done the Taffy mod, the BK mod, put on a P38, or removed the leak jet (if you have one on a 426). This means that you have a long duration, high volume squirt that could hide the lean condition low on the needle when on dirt.

Although this is a long shot, it would be easy to test. Simply pull the current needle up one or two clip positions (lower clip to larger clip position number) and see if it goes away. Your jetting may be screwy, i.e., you may need a different needle to clean up just a narrow band of throttle position (whatever the TP is from 50 to 70mph on the freeway). If so, go to school (Patrick Burns and Jetting Q's), ask gurus (Taffy and JD), or buy JD's spreadsheet and find one by trial and error there.

OK, here is a new longshot. Especially if it backfires occasionally when it cuts out. You have a weak coil (or bad CDI). If jetting isn't it (that was a longshot too), get a multimeter at Radio Shack (or better if you are familiar with them), one with a tach builtin if not too expensive, and go through the ignition troubleshooting section of the manual. You know, I like this even less because I can't explain why it wouldn't happen on dirt.

Whew, This is thin, real thin. (from Lethal Weapon).


p.s., if it is the needle, call me. I would like to see if I can drive JDs spreadsheet on something like this.


my 426 is bone stock and I have the same "problem". While driving on freeway, steady on the gas at 1/8 to 1/3 (like), there is a sudden loss of power, just a 1/4 of a second. First time it happened I thought I was running out of fuel and just gave more throttle to check, no problemo. At all other times it runs fine, it is just on transports. You sit there, bored numb, listening to the drone of the engine (euro zorst, cant hear the exhaust :) ) and suddenly a dip in the revs like a missfire, no pop that I can hear, and then it runs fine again.

It would be nice to get rid of.

I am still trying to get a feel for how a carb works and thanks to TT, especially the thread "Jetting Q´s" with Taffy and JD, I have learned a lot - but am still very much a beginner, so I am still struggling.

Airbox lid is still on, jetting is (I suppose, havent looked) as per the manual, i.e:





65Starter Jet

DQR clip4

Fuelscrew as from factory!

Help on how to troubleshoot this would be appreciated, even if sarcasm from Taffy is included :D .

i supopose you'll take this as a knock but just lately we've had a lot of 426 riders come aboard saying "i've checked everywhere...". there is a thread here, now, at the top explaining the midrange stutter.

i suggest you make an effort to raise both eyebrows so you can see the bloody screen for a start. :):D

anyway just to prove i'm all heart underneath this basil fawlty exterior... drop the MJ to a 160/162 that MAY cure it. you can also put a 160MAJ in and go to 155/158 MJ this will take you further in the right direction. these two will cure it and the bike will be faster than at present.

mark you were badly wrong, confusing everyone. that disinformation is now going to be read by 20 i*~!s and we've got to break all their pre-concieved ideas down.

all he really needs to do directly to cure his problem is drop the needle. but he can't because he's on clip 1. (clip 1 is at the top, right edrow?).

the above two mods will cure it.


OK guys thanks for the info. and I will now tell you I have done much more than I mentioned. Ihave set the main jet for max speed using my trusty little Panoram and repeated top end runs. I have lowered and raised the stock needle (LOT's of popping in clip #1 and then with the EJP and had little change in this particular area. I also have messed with the ACV by first blocking the vaccum hole and then removing the spring both had little effect other than adjusting the PS open or closed. I have a gut feel (and only that) that it is the relationship of the needle taper and straight vs the ET at that particular throttle position. Changing the clip position just changes the RPM/speed slightly at which this PHENOMENON occurs. Offroad I'm never at a constant throttle position so the problem is hidden (Mark I see what your getting at with the fuel squirt issue here). I used to use the tuning theory "ask for fuel" (open throttle) problem = rich. Closing throttle problem = lean but this dam thing acts rich under closing throttle. Anoter thing is that the plug is bone white (as it should be) but the mixture ring is very close to the tip of the insulator, which indicates a rich condition. Weird. My bike runs really strong in all other areas but this problem has really got to me. I thought since this has come up here many times before someone has come up with the answer. SOOoooo I guess my family will remain PO'd until I get this thing resolved. In summary I will seach "midrange stutter" and will also try the MJ/MAJ reduction theory Taffy suggested. Mark I'll test the coil too just for the hell of it. I'll also go throught the Patrick Burns article. I'll post my findings whenever I get the answer.

Thanks for the input James (I was hoping you would chime in). Both are excellent suggestions. I will mark the throttle and also put in a new plug and do a plug chop when the condition is occurring. I'll then know the exact throttle position and subsequent plug reading. I'll look at the manual/carb tonight to see how I can block the ACP. It may take me a while but I'm determined. Stay tuned.

PS Mark, Although I don't really want to, I do have a meter with a tach and can trouble shoot the ignition system. I may wait a little on this to see what comes out of the other testing.

PSS Taffy, I had to order the 160 MAJ. I'm kind of afraid to drop all the way down to a 160 MJ without a smaller MAJ.


I understand the frustration on this issue. You aren't the first to be baffled by it. This has come up before and gone around and around. I had my '00WR400 dual sported and observed the condition only on a few rare occasions while riding on asphalt. I mostly rode dirt and it wasn't worth fixing when it didn't happen there. There was no satisfactory answer that came from it IMHO.

The things that weren't tried (from my foggy recollection) were main air jet and TPS/igition timing. If you were to mark the throttle and give us a specific throttle position, would it be 1/4, 3/8, or 1/2? I though it happened very close to 1/4 throttle in a slightly trailing situation.

Here is another far fetched idea. What if the high vacuum from the engine at that particular speed were to draw from the accel pump circuit, without the pump diaphram being pushed. Purely a vacuum driven condition in a narrow throttle range. The only way to test it would be to block the flow to or from the pump. :)

I will try to run my KTM400EXC (EJN needle) on the same roads this weekend to see if the condition is there with it too.


[ April 18, 2002: Message edited by: James Dean ]

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