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Arjan van Druten

Sudden drop in acceleration / power at full throttle and high RPM

17 posts in this topic

Hi All,

 

I have just been working on my WR400F camshaft - valve timings and

 

  • Adjusted valve play according spec. Set to max play.
  • I have set the Exhaust cam opening earlier, according spec (WR enduro setting).  

Results: Noticed a much smoother running of the engine at lower and medium RPM. Only had to tune the idle screw a bit (counter clock). The only problem I now have is that the engine suddenly extremely drops in power when I accelerate with full trottle and get into the higher RPM region (6000 RPM or so). I did not have this issue before.

 

My questions are:

 

  1. Do I need to go down (or Up) on the main jet setting or
  2. maybe (?) also to change something with the needle (did read about new needles with different taper shapes or adjusting clip position). See graph below.

 

I guess that High RPM results in a better mixture in cylinder and therefore the setting of the carb main jet is to Rich (at full trottle opening and High RPM).  

 

Any ideas?

Thanks
Arjan

 

calchart.gif

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Can you be more specific about the power drop

 

- does it only happen when you 'whack' the throttle, or does it happen also when you roll it on

- does if feel like a kill switch, or does it feel like it's 'drowning'

- can you get past it by babying the throttle

- have you inpsected the plug/plugwire/stator pickup/kill button for possible arcing

- does you carb have an accelerator pump, and have you verified it is working

Edited by Kah Ran Nee
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Can you be more specific about the power drop

 

- does it only happen when you 'whack' the throttle, or does it happen also when you roll it on

*** Have to check it, but I believe also when rolling it on, will try tomorrow.

 

- does if feel like a kill switch, or does it feel like it's 'drowning'

***Feels more like a kill, Full power loss and than picks up power (when RPM drops a bit) en loosing power again....(shocking feeling, also very unpleasant to ride)

 

- can you get past it by babying the throttle

*** You mean like very slow increasing it...I will try....(same remark as with point 1, I think>)

 

 

- have you inpsected the plug/plugwire/stator pickup/kill button for possible arcing

*** Guess not the issue, because it did not occur before I changed cam shaft position (outlet earlier open). BTW the TOP Piston liner was not correct also before i worked on the bike. Therefore the inlet valve timing has also changed.....

By the way, I also changed the coils and the spark plug connector (damaged it while removing...). Why would this occur only at high RPM, I wonder?

 

 

 

- does you carb have an accelerator pump, and have you verified it is working

*** Yes it works and I have tuned it some time ago..(on the work bench).but to be honest am not so happy with the power when wacking the throttle at low rpm...response is also a drop in power. But its ok for my driving style..... :ride:

I guess can not the this pump as it occurs sometime after throttle is steady, full open....

 

The position of the camshaft is Now as in picture (Almost....), i believe that this is the proper  WR setting. 

 

For YZ the outlet is turned clockw. 1 teeth. This (YZ configuration) is more how it was...(however with wrong TOP Piston marker out of position, think it was counter clockw.), when there was not a drop in power....(before i worked on the bike)

2011-04-15_112217_yam_426_cam_time.jpg

 

 

One other thing. Yesterday I removed the Decibel Killer (in exhaust pipe)....to check if the mixture may be too lean....but removing this made not any real difference! Therefore I guess, tuning the main jet will not solve it.... :( 

 

Thanks!

 

BTW the bike starts fine, much better than it used to...Idle is perfect and runs ok when cold and warm in general...

 

 

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No. think its not,,,,

 

Double checked, and actually baby throttle also gives the loss of power at 6000 rmp

So, it doesnt only occur at full throttle...

 

I believe that the cam setting has a big effect on what the engine will do at different rpm. e.g. later closing of opening valve will result in very bad idling, but big performance in high rpm range. This has to do with the inertia of the gas flow.

 

I am going to set the exhaust to delay with 1 teeth, so it opens later and closes later, there will be a bigger overlap with opening inlet and better "cleaning" of the engines burned gas.

 

Anyhow, thanks for thinking with me. I keep the electrical issue in mind.

 

 

Cheers

Arjan

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 Please, see pictures of current valve timing setting of valve (marker | at TDC)

 

  • How do I count the amount of pins between inlet and outlet btw?

 

  • Does this picture seems ok for a WR?

 

                                Inlet is a little delayed and outlet dot is exactly how it should be according Yamaha spec.

 

 

Many thanks!

Arjan

 

PS I am gonne delay the outlet with 1 teeth...today.

 

 

 

 

valves 1.JPG

valves2.jpg

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Just changed the cam outlet one teeth time delay (clockwise looking on chain side)....

 

Results --> Damn!!! Still no change....Power still drops radically at 6-7000 rpm...Also motor sounds more rough/metallic (but this could be my imagination...)

 

Gonna set it back and maybe check the inlet timing or check if the TDC is correct as what the | says...need to buy a measure clock I guess?

 

Anyone any other ideas?

 

- Could it be that sparkplug cable is too close to other engine control cables...??? or a failure in the coils?

- ....

Edited by Arjan van Druten

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What about the valve springs, and/or valve guides.  Maybe one of them is not closing properly due to a weak spring, or binding with the valve guide?

 

Did you put the springs on the wrong valve, ie exhaust on intake or vice versa?  They have different part numbers on the microfiche , with what I would assume as a heaver spring on the exhaust and a lighter on the intake.

Edited by Pooley

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Hi Pooley, No did not change valve springs.

 

I forgot to mention one thing....I also changed the air filter....maybe this one is too oily and engine runs very Rich? 

I did read that a rich mixture could lead to drop of power at high RPM. I added the DB killer back on...as it will make the (final) mixture leaner I guess...

 

Engine valve setting now back to normal, runs fine at low RPM....(as before)...

 

Gonne try without air filter...Does anyone think this is a very risky thing? Or no problem for a few minutes (I guess)....

 

Cheers

Arjan

Edited by Arjan van Druten

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Just to complete the story, I guess Krannie was right.... :rolleyes:

 

I measured the resistance of the coils (I had to buy new ones because I damaged the old wire to the sprakplug) from the ignitor and the values differ from the specification in the manual. The voltage amplification is lower (ratio of windings 1st / 2nd coils is lower). 

 

I guess at higher RPM the spark capacitor does not have enough time to charge and allow the system to generate the voltage for the sparkplug, needed to spark and therefore the engine fully stalls... at 6500 or so.The engine runs again when the RPM drops, etc...

 

I use  it now as a kind of RPM safety restrictor. Have not changed the coils yet as they are expensive (100 $).

 

No further problems since as long as I stay below the 6500 RPM.

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Just to complete the story, I guess Krannie was right.... :rolleyes:

 

 

 

 

Well, duh......

 

Guess enough times and you get it right sometimes.

 

:rolleyes:  :jawdrop:

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The previous owner of my 03 WR450 picked up an R6 coil (coil on plug style) that he was planning on installing but hadn't gotten around to. I couldn't find any info on the swap, though. Didn't know if it was a common upgrade... looking at the fiche it appears to be coil on plug already, I hadn't remembered that before but maybe I didn't pay enough attention. The 426s appear to use traditional style coils, a quick google search found http://www.thumpertalk.com/topic/53170-450-coil-onto-400426-it-works-details-inside/ with some details. If I had to replace the coil I'd certainly consider upgrading.

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Is a bad coil a common problem? If so, any easy upgrades?

It think it was not a wear out issue. I replaced the coil with a second hand, I think it just was a different configuration coil (e.g. for newer or older WR models).

 

However, it could be a defect of course...In that case:

Typically if coils would break down, it occurs at the high voltage side by a mechanism called Partial Discharge (PD), which starts in very small voids in the insulation and slowly "eats" (called treeing) through the insulation untill there is a full electrical breakthrough (short).

 

It may indeed be that you first loose some windings and that the Voltage amplification goes down (So, the coil may "wear" out, but its caused by full internal (multiple) short failures. I assume than that the issue would occur at lower and lower RPM.It is likely that the process willl increase in spead as there are "hotspots" created. It also may be possible that you get a full open or short and suddenly have no spark at all...

(I have no experience with this [Yet  :rolleyes: ], just thinking loud)

 

I dont think there is any chance of having severe secondary failure effects (causing any additional damage to the engine).

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Well, duh......

 

Guess enough times and you get it right sometimes.

 

:rolleyes:  :jawdrop:

 

 

 

Yeah,

 

I discovered it more that a year ago, just did not post it :-) But it took me some time indeed. Should have measured the coils earlier!. :rolleyes: 

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It think it was not a wear out issue. I replaced the coil with a second hand, I think it just was a different configuration coil (e.g. for newer or older WR models).

 

However, it could be a defect of course...In that case:

Typically if coils would break down, it occurs at the high voltage side by a mechanism called Partial Discharge (PD), which starts in very small voids in the insulation and slowly "eats" (called treeing) through the insulation untill there is a full electrical breakthrough (short).

 

It may indeed be that you first loose some windings and that the Voltage amplification goes down (So, the coil may "wear" out, but its caused by full internal (multiple) short failures. I assume than that the issue would occur at lower and lower RPM.It is likely that the process willl increase in spead as there are "hotspots" created. It also may be possible that you get a full open or short and suddenly have no spark at all...

(I have no experience with this [Yet  :rolleyes: ], just thinking loud)

 

I dont think there is any chance of having severe secondary failure effects (causing any additional damage to the engine).

I had a coil go bad on my SV1000. It started by coughing every now and then, getting a little worse over time, until it eventually would cut out entirely on the front cylinder at times. It would be way worse if that happened in a highly trafficked tunnel on my WR than on the SV, where I don't wanna ride it like that but can at least get to safety.

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