Beating a dead horse?

10 replies to this topic
  • BlueThunder

Posted July 30, 2001 - 10:04 AM


Ok T.Talkers, need some input here. Y2K WR 400FM
with these mods:WR timing,grey wire disconnected and insulated, airbox lid on but
have drilled 1/2" holes(3)down center for more airflow(water crossing paranoia warrants the keeping of the lid :)),stock exhaust w/baffle on (til my Vortip isn't back-ordered from Fineline),throttle limiter cut to YZF spec.,13 tooth counter,170MJ, DVP@#4 clip setting,45PJ and pilot screw @1.25-1.5
turns. I've tried every fix in the forum to get rid of the idle-1/8 throttle stumble(pilot screw settings,DRS needle replacement
with a DVP, clip position settings, accelerator pump linkage adjustment, checked for air leaks at manifold and hot start,etc)
but nothing seems to solve the problem :D? This problem is more noticeable on the workstand while blipping the throttle quickly and will even kill the bike if I wick the throttle to WFO, than it is under load while riding but it definitely hesitates under load as well, usually when I need to bump the front over a big log or rock! Do any of you TTer's have any other remedies that I have overlooked? Any help would be appreciated, since I have a lot invested in this Blue bike and am considering the tallest cliff I can find or a CR450F to solve the problem!

  • Bill

Posted July 30, 2001 - 10:48 AM


Try moving the fuel screw out 1/4 turn at a time. If you get out to 2.75-3.0 turns, consider a larger pilot. You may also want to try moving the clip to #5.

One change at a time and take notes.


99 WR400f, YZ timed, MX-Tech suspension, Scotts steering damper, White Bros E-Series (12 discs), tapered header and a/f. Kouba T-handle for the fuel screw. Works Connection billet throttle tube and frame guards. Cycra Pro-Bend, triple clamp mount handguards. Thumper Racing rad guards, Renthal Jimmy Button highs, YZ Tank and IMS seat, YZ number plate, odo removed, EKP, 48PJ, 175MJ at 500-1000' Thanks James Dean!


Posted July 30, 2001 - 10:54 AM


Get rid of the stock exhaust
and cancel the backorder of the vortip
try a yz stock exhaust, if you can stand the noise. You can buy an insert for the stock yz pipe.
here is the link

I myself use a steel washer from the hardware store stuffed from the inside of the end cap.

the yz pipes can readily be found on ebay for less than 100 bucks (cheap considering the alternitive)

they are also much lighter shaving pounds not ounces

yz timing is also a step in the right direction.

I think all these things will help your bike give you what you are looking for.

p.s. the cr450f may not be all its cracked up to be lets wait and see. does any one remember the cannondale (bike of the year :) ).

I will keep an open mind but was fooled by hype before.

  • James_Dean

Posted July 30, 2001 - 10:55 AM


Change to needle clip #5(DVP) and 2 turns out on the pilot screw. Make sure your accelerator pump is squirting and set your idle a little faster.

There will always be a limit to how hard you can wick the throttle open down low. This is a big carb and the 450F's is going to be even bigger (41mm!). Pull out the exhaust baffle to see how much difference it makes. In real riding situations it is less of a factor and takes almost an extreme situation to occur. Keep the revs up and it won't bog at all.


  • Jason_Williams

Posted July 30, 2001 - 11:43 AM


I tried alot of things when I first got my bike to get rid of the wick the throttle problem and finally gave up. I was wicking it like a two smoke. and I guess that doesn't work with a pumper carb. I adjusted my riding style a little and haven't had a problem since. It will do it really bad when its on a stand in my shop. some times if you wick it fast enough it will kill it.

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  • BlueThunder

Posted July 30, 2001 - 11:46 AM


Wow, you guys are fast, replies by the time I get back from lunch? This forum is truly a godsend to WR owners everywhere! Q&A's in order of your replies are as follows:
Bill-tried all your suggestions already with no results....turning pilot screw out with 45 or stock 42 pilot jets did not cure the hiccup...seems to run best at 1.25 turns with 45PJ...will try #5 clip setting on needle since this is a common solution from both you and JD. Next step would be a 48PJ ?
vincegatcore-YZF's exhaust is just TOO loud for is no object if only someone would produce a QUIET aftermarket exhaust. Washington State has become a real battleground with the "Greenies" and I'm willing to compromise a little performance to keep them off my back and keep our riding areas open(my only other alternative would be shallow graves for all the "huggers" who scowl at us when we pass them on designated ORV trails that are funded with money from ORV tabs!). Sounds tiring and messy to me!
My bike is quiet and dual-sported and I still get the inquisition from the rangers on National Forest marked trails because the "Eco's" got elected into office in this state! They did not get there with my vote, I can assure you!
JD- Will try the clip setting and pilot screw change that you suggest but I am confused on one point? Couldn't this stumble be caused by the accelerator pump squirting too much fuel when the throttle is initially cracked, causing a severe rich condition between idle and 1/8 throttle. Read an older post by fisher titled "2000 WR-Finally runs great" where he commented on fixing a low throttle stumble by adjusting the accelerator pump linkage screw IN so the pump was not squirting fuel immediately after wicking the throttle open. You replied to him that you were looking into this adjustment with Hick. Any revelations with this adjustment? I tried it, but got very minimal results so I adjusted the screw back where it was.
BTW all, was really not buying into the hype on the CR450F, never believe it till you ride it.

  • James_Dean

Posted July 30, 2001 - 12:18 PM


Stay close to stock on the pump delay screw. Not much to gain there.

Too much pump is not likely to be the source from experience. The stock baffle is EXTREMELY small and designed with all the stops you took off. The carb was selected for an open exhaust, straight pipe. Don't expect to wick it open hard with that much restriction. Try clip #5 and you may have to adjust your wrist as needed.

BTW, I am in east Federal Way, neighbor! :)


  • BlueThunder

Posted July 30, 2001 - 12:51 PM


Will try your suggestions JD. Good to know you're a neighbor of mine. Where do you usually ride? Done Naches yet this year? May be getting together a group to do it this weekend if you're interested. All intermediate riders who are fun to ride with, 30+ and no SQUIDS. Let me know. Thanks again for your invaluable advice. You are a "Yoda" on this forum. :)


  • Hick

Posted July 30, 2001 - 03:17 PM


Further pummeling of the deceased equine:

In my experience with both my YZs the correct needle and clip position is where to start when trying to rid yourself of the Yamabog. After that I am thoroughly convinced the accelerator pump delivers too much fuel and shortened the stroke on both bikes. Throttle response is excellent, and worlds better than it was as delivered (esp. on my ’00, but that thing had the wrong needle in there).

But if you have a very extreme or noticeable lag, bog, hiccup or whatever, you probably need to change your needle and/or clip position.

Unless you are at elevation I would think the first order of business is a test ride on # 5 (as Mr. Dean suggested). Lots of times a bad stagger, cough or whatever is an overly lean clip pos. If you have already exhausted your options there I would try a richer tapered “E“ (or YZ spec.) needle next.

Hope this helps.

  • Rich_in_Orlando

Posted July 30, 2001 - 06:14 PM


All above suggestions indicate enrichening the slow circuit, but I don't see it that way. And I don't know how adjusting the needle clip setting would affect 0-1/8 throttle.

The few times I've had similar experiences was with an overly-oiled air filter. I just went through it last week. Too much oil on the filter made it a little harder to start and it stumbled on the bottom (and it lost accleration on top--still made the same terminal velocity, but it took longer to get there.) I find that a very light oiling is still effective and doesn't choke off the breathing.

If that doesn't do it...

Since you haven't made extreme modifications to your engine's breathing, e.g., airbox lid still on and exhaust baffle still in I'm thinking that a 45 PJ may be too rich. Another indicator is that your fuel screw is pretty closed off with a 45 at 1-1/4 turns. Try a 42 and adjust your fuel screw accordingly. It's worth a try and won't cost much to test.

  • lewichris

Posted July 31, 2001 - 09:57 PM


I have a stock yz exhaust if you are interested. it is from a 98 but is in good shape.


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