2006 YZ450 Light Throttle Stutter

I've been tweaking away with the YZ while I've been away. Fitted a new standard NFLR needle. Yes I have learnt I have a 2007 bike thanks to Greyracer. The bike was sold to me as a 2008 but I quickly realised it wasn't and thought it was a 2006 but Greyracer told me how to identify the difference, thanks mate. I've fiddled with the idle mixture screw too (I have installed an RFX thumb screw which allows easy adjustment), its 2.25 turns out now and the bike in my opinion is 95% there. I've done a few miles on the bike this weekend due to fitting a set of supermoto wheels and to be honest, its the most fun I've had in years. Its just ballistic in the corners. Under 100 miles and no more chicken strips on those new tyres!!


Away way, I'm still having a bit of a popping misfire at between 5-7krpm on light throttle/cruise (45-65mph) but I think I have realised what the problem is but I'll go into that later. I just wanted a bit of reassurance that the TPS is set correctly as I've just check it again tonight to put my own mind at rest. readings are as follows:


Coil resistance (Blue and Black wire) is 5.04 ohms

Coil variable resistance (Yellow and Black wire is 0.81 ohms closed and 4.06 ohms fully open

TPS voltage output (Yellow and Black wire) is 0.61vdc @ 1850rpm


The ohms reading were checked at the TPS plug and CDI plug and the voltage was checked at the CDI plug but set previously at the TPS plug so I'm guess no problems with the wiring loom.


Does that all seem about right??


OK, back to why I think I may have realised the reason why it is still popping misfiring. Well, when I installed the new NFLR needle, for some reason I thought clip in position 3 was standard. After just checking the manual tonight, I read the clip in position 4 is standard. What's peoples opinions on this error making the difference I need to get the bike running right? I'll move the needle down in the morning and hopefully I'll get out on it tomorrow night to try it.


On a different note, I have to add, I went to start the bike tonight to check the voltage the TPS was putting out, rolled it out of the shed, cold engine, not run today, pulled the kick start out and gently pushed it down on it to feel for compression and the engine just burst into life. Ticked over beautiful too. I must have something right eh..?? lol. I'm putting it down to a couple of twists of the throttle while checking the variable resistance. Really pleased with the way it started. Would make all KTM 660 SMC owners angry that would lol.


Anyway, hopefully somebody can contribute to my findings of tonight.



So I did have the clip in the correct position for stock so I lifted the needle 2 positions to richer it up and it dropped the misfire down the rev range. I'm confused at this point so I've just pulled the spark plug to find this


Does this tell you it's running lean? If so, do I need a thinner needle installed or a larger main jet? It pulls pretty well high up to around 10krpm but I've nothing g to compare it to.

Help please.

Well, just switched the NFLP for an NFPP set at position 4 and I'll try it tomorrow night. It's not recommended to do a needle swap while smoking a fag pist but 'Hey Ho' as we say in the UK. She does sound fit blipping the throttle at 10 o'clock at night in a quiet country village though :D

Does this tell you it's running lean? If so, do I need a thinner needle installed or a larger main jet? It pulls pretty well high up to around 10krpm but I've nothing g to compare it to.



Unleaded gas?

Yes. 95ron here.

Unleaded gas will generally leave the plug looking rather white regardless of the jetting.  Can't go by the plug.

Ok thanks for clarifying that.


So tonight, with the NFPP needle installed the bike feels way better, sharper, faster and smoother. It's still not 100% but I'd say 99% there. I can't get my head round how the needle diameter is so far from standard yet the bike is running better. That's 20% more fuel in this range. This makes me think that maybe the main jet is too small too although I can't really say I can tell. I was thinking of just sticking in a size bigger main anyway. If I increase the main jet size will this only effect 3/4 to full throttle fuelling? This is what I understand from the manual.


So next, initially I was thinking go for the thinnest diameter needle, the NFPN but a little further thinking lead me to think about the Standard Pilot jet, should I increase this size from a #45 to a #50 (#50 because it's in stock at my supplier) as the issue now is at very little throttle, and I do get pops when gently opening the throttle on deceleration.


I've played with the idle screw again and I now know it's set correctly as adjusting it makes things worse.


Thanks for any advice again :)

Just ordered a #162 & #165 main and #48 & #50 slow. Should give me something to do when they arrive.

I know this is for the WR250/450 of the same era, but it sounds very similar and I went through this on my 2006 WR250F, especially noticeable after I converted the bike to street use. I found this information on JD Jetting's web site. Might be useful... interesting none-the-less.



Yamaha WR250/WR450F Stutter Condition-

This is an ignition problem that Yamaha has designed into the bike to make the motor run cooler at light throttle and medium speeds. The stutter is VERY COMMON, and can sometimes be reduced by disconnecting the TPS electrical lead from your carburetor. The condition is most common on flat roads at light throttle positions, when producing under 10hp at the rear wheel. The stutter is harmless and will not affect the power under acceleration, if it is not too annoying, leave the bike as-is and just ride it. The problem is mostly noticed when riders want to do road riding with their bikes.

Try disconnecting the TPS lead and see if it helps your condition. The other alternative is to try running either clip position #2 or #4 on the Red marked needle. This is a work-around that forces you to use more throttle than normal and lightens the stutter, but the bike will make less power when accelerating.

See- http://www.pbase.com/jdjetting1/image/96130587 and http://www.pbase.com/jdjetting1/image/99387782 and with replacement CDI and no jetting changes-


Coil resistance (Blue and Black wire) is 5.04 ohms

Coil variable resistance (Yellow and Black wire is 0.81 ohms closed and 4.06 ohms fully open

TPS voltage output (Yellow and Black wire) is 0.61vdc @ 1850rpm


There's a problem here: TPS continuity between the blue and black leads with the throttle stop fully closed is supposed to be 4-6 THOUSAND ohms, not 5.04. 


Resistance across the yellow and black leads is supposed to be 0-2 K (thousand) ohms at fully closed, and 4-6K at full open.  The only way your readings are right is if they were taken on the ohms x 1000 scale, or on an "auto-ranging" meter, and you read and transcribed them wrong.


I suspect it was a mistake in your reading/transcription, because the voltage at idle is in range, which could not be true if the resistance were that far off.


Your main jet has little or nothing to do with fueling at the low throttle opening where the upper needle diameter is effective, and it isn't 20% more fuel, either.  It's two "steps" richer, each step being worth about a 5% change or less.  Your main jet should be about 162-168 for sea level to 3500 feet. 


The pilot jet is another matter.  Should be a #45 or close to that.  Read this linked post: http://www.thumpertalk.com/topic/271735-how-to-confirm-your-pilot-circuit-setting/


Because the pilot circuit handles the transition from idle onto the main circuit, there is some overlap, and if the pilot is wrong, it can adversely affect light throttle cruising.

BGoyins, I think I have that stitched up with the new TPS. I pretty certain it's not ignition related now.

Hello Grey, my meter was set to 20k ohms yes, it wouldn't read at anything less. It's not the best meter.

According the the manual, one step thinner on the needle equates to nearly 10%. Is this not +10% fuel?

I am never above 2000ft on this bike and rarely see +25deg, so I will put in the 165 when it arrives and regarding the slow/pilot jet, I'll read the link later but that's in advance for your help.


I've read through the link thanks Grey.


It would suggest that my pilot jet is probably OK as it is set to 1.75 turns out, but like I think you suggest, I think there is an overlap where the pilot drops off and the diameter takes over and it's at this point I think the pilot can't supply enough fuel before the diameter comes into full play, even though the idle is good. We will see as I have had a dispatch email and my jets should arrive today :D I really hope I dont need to order another needle!! lol.

Well what appeared was a #50 pilot so not a lot of use. Nothing done really bit I did strip the card again to check the float height. It is 8mm as manual says. Removed all jets and cleaned, again, along with all passage ways, again. No rush at the moment as the swing arm is out for repair now.

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