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2005 YZ250F misses on top end

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Hey guys, I'm trying to sort out a buddy's 2005 YZ250F.

The problem is that it cuts out on higher revs. What it sounds to me, is like the rev limiter is cutting the ignition at maybe 10K rpm instead of the full 13.5K.

Symptoms:

1- Starts/idles fine and runs great, but only until when it should be starting to sign happily in the fat part of the upper rpm powerband.

2- After the 10(?)k, it sputters just like my two 03s on the rev limiter, except the 03s are turning much higher rpms of course.

Things we've tried, with no difference in response.

-New spark plug.

-Disconnect neutral switch

-disconnect kill switch

-Tried MJs #182 (stock), 180, 175.

Air cleaner is recently cleaned and does not appear to be over-oiled.

The misfire condition is very repeatable and every gear gets the same response at the same rpm (judging by my ear-tachometer)

The bike has not always run like this. When it was new, it sang happily at upper rpms and was impressive on the power. Today = pooch.

Any ideas? Thanks in advance for quality ideas. :applause:

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It could be that the CDI box has gone bad. Im not too sure what happens when one goes bad or how much one costs. If you know someone with an 05, see if you can borrow their CDI box to see if it fixes the problem. It also could be something very simple with the electrical, who knows?

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I'd do a chop test and see how the plug looks. If it looks rich, then check to make sure the main air jet is clean.

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Mine is doing the same thing!!! I was told to lower my needle jet a couple of clips and test that. I haven't had it out yet. This happens to me mostly on wet days. I put my question out to the jetting section. Please let me know what happens as I shall do too.

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Also wondering: Does is seem like the clutch is slipping, or is it a miss or sputter?

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Barton, Thanks for the ideas. The clutch is definitely not slipping. None of the three MJs were plugged. Plug reading appeared that the jetting was in the ball park.

Yam259, If yours is doing it on wet days, that would indicate you have an electrical short which is sensitive to moisture. Check all wiring (give it some electrical grease on connections), and possibly suspect the coil.

BlueRider, I somewhat suspect the CDI, but it runs perfectly on the lower revs so it is difficult to suspect the CDI. A bad CDI is more typically- runs good, then nothing, then sorta good, etc. This bike is VERY consistent in its problem.

I'm going to try swapping a coil from one of my 03s tonight.... and then go water skiing & drink beer. :applause:

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It sounds like a lean miss to me...

You say you went to the stock MJ and two sized smaller, did you try any larger?

Yam259, If yours is doing it on wet days, that would indicate you have an electrical short which is sensitive to moisture. Check all wiring (give it some electrical grease on connections), and possibly suspect the coil.

Also, you richen a carb up as the humidity rises, then lean it out after it starts raining... Being sensitive to moisture in the air can also be fuel screw related.

For your problem, I'd start with a good cleaning of the carb with cleaner and compressed air, then if it still runs bad try upping the MJ.

If there is still a problem, I'll go with coil or cdi.

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I'm going to try swapping a coil from one of my 03s tonight.... and then go water skiing & drink beer. :applause:

i vote for the drink beer, go water tumbling and throw the coil in the lake idea!!!

i think the only way this is a fuel problem is if the MAJ is plugged (i doubt it) or the float is set so that it is starving for fuel at WFO.

my gut tells me to let a scuba diver find the original coil (and plug wire) and try new (or at least known working).

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Mine does the same thing also, and its also a 05. But mine will do it if its wet,dry,cold,hot. It only does it when you hold the throttle in one position in any certain gear. Did it since the first day I had it so I thought it was normal.

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We tried a whole bunch of things tonight, but no trophy.

1- Swapped coil w/ 03 component

2- Swapped CDI w/ 03 component

3- Dielectric greased up the connections.

4- Unhooked throttle position sensor

5- Tried #170 MJ

6- Verified clean carb

7- Checked valve clearance

8- Tested valve springs for similar stiffness, just in case one was softer and floating, or broken. Test OK.

I ran the 03 immediately afterwards and the 03 definitely turns more revs before limiter chops it out.

Tomorrow's plan-

#175 MJ

#3 Clip position

Maybe, #38 PJ

Do NOT work on it until it is too late to go W-skiing. (The water is PERFECT right now.) :eek:

Something very important to remember-

Today's ambient conditions- upper 80s, sort humid, 1200ft elevation

The bike ran very good with the stock jetting this spring when it was 35-50F. Ran REALLY good once late last April while riding in a spring snowstorm. That in itself tells me by the seat of pants, to jet down a couple-three MJs, a clip spot, and one size PJ. However, I am surprised to find this bike runs so badly w/ the stock jetting regardless of temps. Typically, the 4 strokes would run fine, but just be down on ponies when the conditions drive the jetting rich.

Stay tuned and feel free to toss in anything you think I've missed. Maybe, these 05s are just more sensitive to rich jetting. :applause:

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great, i have an 05'...i havent had this happen yet...but i guess we will see.....

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great, i have an 05'...i havent had this happen yet...but i guess we will see.....

:applause:

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bazooka,

gotta be electrical. a 170mj would almost prove it wasn't fuel related (unless it IS the float height). clip 3 might help, but at WFO the needle is not really "supposed" to come into play.

does this miss happen ALL the time at ALL engine temps (granted you don't want to rev it to the moon on initial startup)?

gotta be in the wiring if it is not the float height, fuel line or petcock restricting flow to feed the mj.

you sure there isn't a wasps nest in the exhaust pipe?

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Update...

Well, I'm not exactly sure what we did to cause it, but the bike runs better now. We had it out last Friday night for some more testing, and,,,, kick-me-in-head, it was running just fine. We all stood around with dumba$$ looks on our faces, and said, "Hmmmm,,, well WTH....? It WAS running like crap the other night, right???".

I'd really like to have found an identified problem, but no cigar. My best guess is corroded connections. :applause:

We agreed it was indeed running bad before, but is fine now, so at least that's not all bad. :eek:

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unplugging original, plugging in new cdi and stuff, unplugging it and then plugging back in the original probably cleaned the connectors enough to NOW make good connections.

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Thanks. I shall do that maybe spray some contact cleaner in there as well. Just curious where you ended up with the jetting? Or what jetting you have in there right now?

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Update:

Changed clip position to 3 and left all other settings stock, bike now revs out fine and hard. It is wheeling pretty easy now and doesn't miss and carry a higher gear with no problem. thanks to all and good luck to thosae who were having problems, don't be scared to try something new.

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