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LayenPipe251

Spark Plug Reading / Jetting YZ250

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My 06 yz250 was running rich and fouling plugs so I changed the pilot from a 52-48 and moved the needle up from the 3rd to 2nd clip on the stock N3EJ needle. Not sure what the main was at before because the one I pulled out had no size identifiers. I put in a new main 178(stock). 

My plug before. (1 ride on new plug BR8EG - wiped off residual oil/gas on threads)

20190314_133952.thumb.jpg.9c252a5429bb19643f1110b944825130.jpg  

 

New plug after jetting changes.

20190313_151929.thumb.jpg.9282126b9e6083bc74608fdca34b936e.jpg

 

I was riding at 2500'-3k elevation. Why would I be getting such a drastic change? What do white ashy electrodes like that indicate other than lean and on which circuit. I noticed it was running lean when coming off being hard on the throttle (~ 1/2-3/4 throttle). When I let off the throttle it would ping ding ping ding down to idle.  What circuit is responsible when coming off the throttle? Needle? 

Bike specs:
FMF gnarly pipe with turbine core sparky 
vforce3 reed valve system
Premix is pump 91 octane 50:1 Motul 800 2T

 

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Wow, that’s a big difference for a needle clip and 2 PJ change. Doesn’t seem right. 

Go back to what you had a make one change at a time. I’d start with a 50 PJ. Is it possible something is out of spec after you reassembled?

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Posted (edited)

When you let off the throttle completley the slide drops down and the needle comes down into the main jet; leaving the pilot jet and air fuel mixture screw providing the fuel. That plug shows signs of high heat from the leaner jetting.

Edited by Duke_Moto
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Posted (edited)
18 hours ago, LayenPipe251 said:

When I let off the throttle it would ping ding ping ding down to idle.

As Duke suggested above, it sounds like you are experiencing surging from a too lean pilot jet/air screw setting.

 

By going leaner on the pilot, you may have just masked the real reason your engine was suddenly acting rich and fouling plugs :

-low compression

-worn/damaged reeds

-fuel/float level too high 

etc.

 

Keep in mind, using the stock factory jetting meant for 30:1

switching to 50:1 will be richer (more fuel / less oil per same volume flowed)

 

Motul 800 is also a high flash point premix,

less than ideal for trail/off-roading where engine combustion temps aren't that high versus MX racing applications.

Edited by mlatour
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Put the needle clip back to stock and set the air screw at 1 turn out and see what it looks like.

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Give this a try 

Main: 178
Pilot: 50
Power jet: 50
Needle: N3EW (N1EJ stock)
Clip position: 3rd from top
Air screw: 1-1/2 turns out (2 stock)
Notes: On a warm day it’s perfect but in cold weather or under a load, it tends to be on the lean side. For most situations, raising the needle will work, but if you add an aftermarket pipe, ride in sand or ride at elevation, you’ll need a bigger main or the N3EW needle.

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That plug looks lean and rich at the same time.  Rich at part throttle and lean at WOT I would guess.  Try getting a pic without the flash on, that second pic looks washed out. 

The ping ding stuff on decel is probably a high idle setting.  If you're running super rich at idle it requires more air to keep the engine running.  Leaning out your pilot circuit may have resolved that condition and now you need to back off the throttle stop to lower your idle back down and allow the throttle to close fully.

Might need a jet block gasket in that PWK too.  When these go flat you will not be able to jet the bike properly.

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Was it always running rich?

Did it run well with the original set up at some point?

What changed to cause it to suddenly run rich?

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