Throttle position sensor

Hi guys, sorry if this has been covered before but Im just trying to work out whether disconnecting the TPS on my WR 426 will reduce the stutter at steady throttle.

Ive looked at a couple of threads in this forum explaining how the TPS works but Im just not sure about one thing. From what I can understand, the TPS flicks between different advance maps for light and heavy throttle inputs, so what is happening when you hold steady throttle and why does it miss and splutter?


I'm going to attempt a simple explanation for this. With the TPS hooked up the CDI map is a 3D map ie: RPM vs Timing vs Throttle position. With the TPS unhooked it is a 2D map ie: Timing vs RPM. With the TPS hooked up and the throttle at a steady state the engine is at a constant high RPM and the throttle will be at a fairly small open position ( cruising) so with this info the CDI will try to turn the timing back to a more retarded position, much like it would if you were at high RPM and chopped the throttle, the map goes to full retarded timing. This is where the TPS info influences the CDI ( high RPM - little throttle means retard the timing) With the TPS unhooked that part of the equation is missing and the CDI only adjusts timing depending on RPM. IF you are holding it at wide open throttle and it's sputtering then it's due to it being on the rev limiter.

So what is happening at a steady throttle position, the CDI is trying to back the timing off as much as possible, causing the sputtering and occasional pop in the exhaust. A very laymans explanation I realize , but hopefully you get the idea. WR Dave

Great explanation Dave.

I'll tell you, this stuttering is the ONLY thing wrong with my WR and I get really frustrated at it. I have almost taken it out back and put it out of my misery.

I have some questions that maybe you, or someone else can answer. I am sure it will help out the original poster too.

I still have the stutter after unplugging the TPS, is it my Jetting that is causing the problem?

I have changed pilot jets, changed the turns out on my fuel screw and still can't get rid of the stutter. I figured by the throttle position when this stutter occurs it must be my pilot circuit but nothing I do seems to help. So should I try the needle next?

How bout I drop my bike off in Calgary and let you tune it?

I had a bit of the stuttering issue when I first started on the '07 , but after jetting and I also changed to the '06 YZ450 stock needle it went away. I'm no guru, I just tried a few things till I hit a combination that worked. WR Dave

Thanks mate that was a pretty good explanation. The bike doesn't stutter at WOT, just at steady throttle. It hasn't bothered me up until now because I usually ride flat out but Im going to register so it might be a pain in the arse on the road thats all.


Once you get the bike working well, try a different clip position or a different needle, see if it helps or just unplug the TPS for road riding??? WR Dave

If you do a search for the stutter problem there was another thread where a guy replaced the cdi with a vortex cdi unit and it fixed the stutter. If i remember right it was a TT member and James Dean from JD jetting that were trying to fix the stutter.

had the same problem with my 01 426, i dropped the needle one clip (ie raise the circlip one notch to lower needle), this didn't solve the sputter compleatly but did make a huge differance.

had the same problem with my 01 426, i dropped the needle one clip (ie raise the circlip one notch to lower needle), this didn't solve the sputter compleatly but did make a huge differance.

Thanks DoctorChopper, I will try that this spring to see if it helps. To be honest any improvement will be appreciated.

My stuttering gets worse the warmer it is, so I think that means the bike is a bit on the rich side. So I'll lower the needle (by raising the clip) one notch.

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