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NEED HELP!!!! 02 YZ426 keeps quiting at high rpm????

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Hey guys, im new to thumpers and i have a new 02 YZ426 i my girl friend (yamaha dealer) got me a awesome deal on. Now to the problem. The bike works fine down low and up to mid throttle, but, when i go through the gears at higher rpm it starts to backfire like crazy when i slow down then quits on me. I was out all day in the woods and it worked great but as soon as i got on a logging road and up to speed it backfired alot then quit. So i figured it might be when it warms up or something but then i just left the house for a little run to check and it still works great down low but up high it starts to backfire then quit. I adjusted the fuel screw but that only does the low to mid throttle??? I also cleaned the air filter as well. Any help guys would be really great. Thanks guys!

Mike :thumbsup:

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oh ya, i also took the fuel bowl off the carb and shot alot of carb cleaner up there too.

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If you have ruled out jetting problems, maybe you are sucking air through the case seal. I have had that same problem and could not figure it out myself. Dealer diagnosed easily and fixed for under two bills. But it may be a dozen other things too... :thumbsup:

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I had the same problem when I first got my '99 WR.

Changed the spark plug and it was fine.

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thanks, now that you mention it, i had the same with my old street bike. I'll give that a try!

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Spark plug, and check your jetting. Does it fire right back up after it dies? What altitude are you at. Dealers are notorious for leaving sea level jetting in the bike. that way when it doesnt run right, you take it back and they can charge you another hundred bucks for fixing your brand new carb.

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Here we go again.... blaming the dealer.

Its always the dealer or the "idiot" parts guy whos to blame.

How can a dealer possibly accuratly jet a carb for conditions that the bike is not presently in or having no clue where the bike will end up after the sale??

Instead of crying back to the dealer, why dont you dust off that valuable piece of literature they call an owners manual and start reading abount how the carb & its circuits work. You might actually learn something. Ya know if you look hard enough in the bag the owners manual came in you might just find a little baggie with a few main & pilot jets in it from the factory. Hmmmmm, I wonder where those go? :thumbsup:

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Here we go again.... blaming the dealer.

Its always the dealer or the "idiot" parts guy whos to blame.

How can a dealer possibly accuratly jet a carb for conditions that the bike is not presently in or having no clue where the bike will end up after the sale??

Instead of crying back to the dealer, why dont you dust off that valuable piece of literature they call an owners manual and start reading abount how the carb & its circuits work. You might actually learn something. Ya know if you look hard enough in the bag the owners manual came in you might just find a little baggie with a few main & pilot jets in it from the factory. Hmmmmm, I wonder where those go? :thumbsup:

ok, well first of all i tried the spark plug and it didnt work. I didnt receieve a manual or even a tool kit with the bike because it was a left over 02 show room model. Hence the great deal i got on it as i stated in the first post. Now back to the problem. Im at pretty much sea level (100' above). It does fire back up a few minutes afterwards and works pretty good. I was out yesterday and it seemed to do it if i was riding it for a while and it heated up alot. Any other ideas? Would it be the jets if it works perfect for a long while????

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Did the weather significantly change from one ride to the next when you said it started running fine? How about where you ride it, did the elevation change? If the jetting is on borderline it doesnt take much to make it run like crap.

It sounds to me like your pilot is just too lean. Get a zipy airscrew and and assortment of pilot & main jets. Your dealer should give them to you since thy didnt come with the bike like they should have. Ask them to order a manual from yamaha they can still goet one.

How does your bike accelerate? Are there any bogs or flatspots while going through the gears? It wouldnt hurt to take a look at the vlaves either.

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The elevation was within 100-200 above sealevel between all rides. It was pretty hot, around 32 nd the bike ran fine. We were on a small track and did a few laps at a time, but it seemed when i stayed out there for longer it started to act up again. It seems like its flooding inself when it gets hot. When it starts to backfire and bog out i pull the hot start and it revs higher??? It pulls fine down low with no bogs at all and has alot of power. Im stumped! Im going to drop over to MX mechanic tomorrow and see what he got to say..... :thumbsup:

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Before you start playing with the jetting; You say you're at sea level so the stock jetting should work OK as long as the temperature isn't extremely high or low. Maybe not optimal but you shouldn't have any major problems like you're describing. Personally, here is where I would start. 1) Clean the air filter 2) Remove the carb and shoot carb cleaner through every hole that you can find, followed by compressed air. While you have it apart, make not of which main jet is in it and post here. Someone with an 02' can tell you if that's the stock main for an 02'. If it is cutting up really bad when wide open then that main jet will be the most likely culprit. 3) Make sure you have fresh gas in it and also make sure nothing has built a home in your exhaust. You laugh, but I had mud dobbers build a home in my 2-stroke exhaust once and the damn thing ran spectacular down low, but wouldn't rev at all. Took me a few minutes to figure that one out. :thumbsup: Hope this helps.

One last thought; what is the temp where are are located? I see a Canadian flag so maybe you are in a temperature extreme area; if your bike is a Canadian bike then be sure to tell us because it may have come jetted differently than the US bikes.

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Thanks alot! Its a Canadian bike and all stock. The temp the other day was 32C. I'll try all that stuff and post up the results. Thanks for the help guys! :thumbsup:

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32C is 89F. The stock jetting in the US bikes should run fine at this temp near sea level. Check the jetting in your bike and compare it to a US model.

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Since it gets worse when the motor heats up it sounds rich.

Really rich if it makes the bike load up and stall.

Was the plug tip clean or sooty when you removed it?

Make sure the air filter is not clogged with dirt.

After you check that, take the bike out let it cool then start it and blast up the fire road, if it runs okay till the bike gets hot, I;ll bet the jetting is a rich. then you can go down 3 sizes on the main jet and try it again and see if it gets better.

don't keep riding if it gets worse, you don't want to melt down the motor with lean jetting.

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Blaming the dealer? Come on now. It is a simple question to the buyer as to where he will probably be riding the most, and if he is going there right away. If we sell a bike in Gunnison, Co to a guy with on Oklahoma address, we confirm with him if he wants his jetting for there. Explain this one....A guy lives in Montrose, CO. He buys a yzf250 from the montrose dealer. Dealer doesn't get it for altitude. The guys bike wont run in upper rpms. The way I see it with the markup, dealers should have the bike set up properly. So, in answer to your question,,,it is very easy for a dealer to accurately jet. And for looking in the owners manual, dont need it. Been riding for almost 30 years. Jetting, and carb adjustments are the easiest thing to do on a bike. I can take my bike anywhere in North America, and have it running awesome in less than 10 minutes. And that plastic baggie with your jets...don't cover you everywhere. I guarantee you one thing,,,what comes in that bag won't work here.

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ok, the bike is still doing it. I took the carb off and totally cleaned it again, changed spark plug, unplugged TPS, clean air filter, unplugged kill switch, tried from 1/2 turn on fuel screw all the way out to 4 turns and everything in between. The spark plug was black and sooty when i took a look at it. So all this and it still works fine low speed but at higher rpms it starts to backfire again and then quit. Anyone have any other ideas i could try?????? :thumbsup:

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