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stonews

2004 wr450f bucking in top gear @ low RPM's

22 posts in this topic

I have an '04 wr450 done in SM gear. 17" rims with I believe a 40 tooth rear sprocket. Motor is stock w/gytr exhuast baffle. gray wire cut and the air box oppend up a bit. I believe the carb is still stock with just a quick adjust FS. Plug looks good... pipe gets a little carbon at tip over time, so I would say I'm on the rich side of things but not overly.

While on the road holding a steady speed say in 5th gear at low RPM's (4000-5000) I get, a for the lack of the correct term... bucking.... In lower gears at the same rpm's it does not seam to be as pronounced. This correlatets to the 45-50mph range which is where I seam to spend alot of time. Is this in the realm of jetting or is it more of a mapping issue? Even if it is a mapping issue are there settings to minimize the issue?

Thanks for any help offered!!!!

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It wouldn't be a mapping issue as it's not fuel injected, you are correct on the the jetting issue, i'd say drop the needle one clip and it should help, wr are notorious for that surging, bumping, missing when you hold it at a constant rpm, there are just not designed for road use at a constant rpm, not a whole lot you can do about it.

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Search "TPS" (Throttle Position Sensor) in this forum... you'll find your answer, or at least an explanation of what is happening. Your ignition timing is being adjusted by design. A lot of people DS'ing these bikes just unplug the TPS.

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By disconnecting the TPS any idea of what I will be loosing there???? I have been away from tuning for awhile but I always recall there are usually trade offs...... Day in and day out I'm more concerned about the 4 to 6K RPM range not what is happening at 10K. Thanks for the help!!!

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Like Don said, unplugging the TPS is the easiest solution. It takes the throttle position out of the 3d mapping. Look for the plug on the left side of the carb. You can also adjust the TPS per manual. The real fix is proper jetting w/ a JD jetting kit or other. I have an 04 that I dualsport and jetting is the key.

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By disconnecting the TPS any idea of what I will be loosing there???? I have been away from tuning for awhile but I always recall there are usually trade offs...... Day in and day out I'm more concerned about the 4 to 6K RPM range not what is happening at 10K. Thanks for the help!!!

I disconnected it for a while on my 04', I did not experience any trade off other than the desired result which was the "stuttering" going away while at low RPM cruising speeds. I later plugged it back in because I only rode the bike off road and hence never experienced or noticed the stuttering.

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What was your final set up for jetting???? Are you running w/ or without the TPS???

Any thoughts on the Vortex CDI??? After reading another thread it appears to have solved the same problem that I'm having..... but at almost 5 bills thats hard to swallow.

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its a common problem. it happens to my wr. From what i found it comes down to a ignition problem. yes jetting does help, in some cases completely takes away the surging problem but from what i have followed on the many threads that go through this forum it comes down to a ignition problem. I believe by getting a differnt cdi will solve it.

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Have you heard of any other aftermarket CDI's that work besides the Vortex set to program 7??? The cost just hurts. I guess I'll try dropping the needle one, do the O-ring mod and first try adjusting the TPS voltage to .75v at closed throttle.... if no change just plan disconnecting it. Nice to see another WR SM in NH!!! :thumbsup:

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My '98 has an external ACV that is supposed to keep it from popping if you cut the throttle quickly. This also caused them to surge at steady throttle opening. On the trail I never notice it, but if I have to ride the fire roads for some distance at a steady speed for some distance I do.

On my '03 it appears to be built into the carb. You might look for other posts. Also, on both I needed to go up a size on the main jet after usual mods.

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Has it always done that, or is it a change in the way it runs? I might tend to think it is the nature of the beast due to resonance in the intake tract at certain steady state rpm. Or that at that rpm you are not developing much vacuum so the carb is getting weak signals. In high gear at fairly low rpm the engine has a high load on it so it would tend to exagerate the condition. You're pulling pretty tall gearing....not sure what your normal riding is, but maybe gear up a little to bump the rpm up at cruising speed.

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Good thought Dave, I got the bike geared this way but have always considered going up on the rear from the 40 to either a 42 or 44. Most of my riding is back road or dirt roads. So that fith gear is a place i end up quite often.

I just pulled the carb off last night just to make sure I knew what I had for jetting... and it is all stock. So before I button her back up...... any strong suggestions on jetting???? As stated before engine and carb are stock, air box has been opened up, GYTR exhaust insert, gray wire cut and o-ring mod done. I received one suggestion to drop the stock needle one..... I would think, with it breathing a little easier and currently all stock jetting I may want to fatten it up a bit not lean her out more??? but this is why I'm asking, because I don't know!! I'm also looking more for the smooth and reliable side of the equation. I would certainly give up a couple hp at the top for a stronger mid range. I appreciate all recommendations!!!!

Thanks!!!

Bill

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My vote would be it's geared to tall, and to go ahead and change the sprocket. What countershaft size do you have? My 08 came with 13/50 stock so if your stock size was anything like that you are geared way up from that. If it runs/starts fine otherwise, it aint jetting.

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stock was 14/ 50 and I'm at 14/ 40. it is running good it's just that darn surge that is most present in 5th gear but present in lower gears as well in that 4 to 5k range. I was hoping jetting may minimize it although I'm hearing the vortex in program 7 will make it go away but the $$$ is making me stay away for now.

Thanks and I will probably pick up a bigger rear sprocket as well

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Ya, I think you're over geared. I have a hard time seeing how ignition curve change could be involved...just my opinion. One other thing that might help is that thing that goes in the inlet side of the carb with the plate across it...can't think of the name...power up or something like that. I'm betting its an intake velocity/pressure thing, and that device is supposed to help 4 strokes carburate better.

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I have seen them in Dennis Kirk etc... Don't know of anyone trying one. may be worth a try.... Any one out there have any experience with them????

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Confused. an 04 with a TPS?

They put a TPS on bikes with Fuel Inj.I thought

My 06 has a card, so how can an 04 have FI ??

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Bucking? In top gear at low RPM? Downshift.

Jetting/ignition changes will not solve a 'problem' that really does not exist. You have a race bike, not a cruiser. RPMs should be above 6K most of the time.

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