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marcmo0

Backfire,pop,pow, pop

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I just picked up an 02 yz 426. Pretty much stock. It has the stock jetting (162), and I run it at 7500ft. I took it for it's first ride and it was backfiring like mad. Any suggestions on jetting that bike at that altitude?

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Exhaust decel popping, assuming that's what you mean, is normal to some degree. Two to four pops out the back when you shut off is about right. If it's excessive, it could mean that your idle circuit is lean, but don't change that until you look it over for exhaust leaks at the midpipe clamp and the header bolts, or elsewhere. If you don't find any, back the fuel screw out a little.

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Exhaust decel popping, assuming that's what you mean, is normal to some degree. Two to four pops out the back when you shut off is about right. If it's excessive, it could mean that your idle circuit is lean, but don't change that until you look it over for exhaust leaks at the midpipe clamp and the header bolts, or elsewhere. If you don't find any, back the fuel screw out a little.

Gray, I disagree on this one, judging from the mental picture he paints both with the jetting (stock jetting at high altitude), and the sound of it.

He's wildly rich. My bike did this popping and burping when I first got it. He probably needs to lean out the PJ, MJ and Needle to some degree.

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You're probably right, Colorado Boy. :applause: Especially at 7500 feet. I gave that answer in a general context. His problem is almost certainly that he's rich overall, and he didn't even specify that he was talking about decel popping.

The motoman jetting chart would be a good place to start.

I just wanted to bring up the air leak thing, because it's so often overlooked in chasing a decel pop, and people will fatten up their idle too much trying to cure the wrong problem.

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You're probably right, Colorado Boy. :applause: Especially at 7500 feet. .

There's a first time for everything. :eek:

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Thanks guys. It is a pop when I back off throttle. And it seems to have to idle a little high to keep running. I did switch out the slip on today to an FMF Power Core 4, and I wanted to wait unti I put that on to start messing with jetting. I heard that these can be a bit tempermental to jet, is that true? Any tips to help make it easy?

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Sounds to me (as it did after only the first post too) like you're too lean.

If indeed you still have all stock jets and adjustments, and the stock jets seem to work at your altitude -- and I'm not saying they do, just going by the info already in this thread -- then I'm going to say you just need to back out the fuel screw a little. Mine was only .5 turns out when I got it and the bike wouldn't even idle when I cranked it.

I'd lightly seat it, and back it out 2 full turns and go from there.

If it's still too lean I'd go up a size on the pilot jet, and if it gets flat on top I'd go up a size on the main as well.

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Sounds to me (as it did after only the first post too) like you're too lean.

If indeed you still have all stock jets and adjustments, and the stock jets seem to work at your altitude -- and I'm not saying they do, just going by the info already in this thread -- then I'm going to say you just need to back out the fuel screw a little. Mine was only .5 turns out when I got it and the bike wouldn't even idle when I cranked it.

I'd lightly seat it, and back it out 2 full turns and go from there.

If it's still too lean I'd go up a size on the pilot jet, and if it gets flat on top I'd go up a size on the main as well.

How is this possible?

It has the stock jetting (162), and I run it at 7500ft. I took it for it's first ride and it was backfiring like mad.

It has stock jetting....for SEA LEVEL. Dude is riding at 7500feet and prolly 80'f. Would that create a richer or leaner situation?

It looks like stock jetting is based on Sea level and 70 degrees.

I'm telling you, stock jetting for my 03 was obscenely rich at 5900 feet when I worked out my bikes situation.

http://www.thumpertalk.com/forum/showthread.php?t=248849

His bike is rich and one or two steps on the PJ and MJ, most likely in a leaner direction will get it running perfectly.

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First check to see if you pilot jet is not plugged before you change the jetting too much, especially if the the bike doesn't want to idle.

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How is this possible?

It has stock jetting....for SEA LEVEL. Dude is riding at 7500feet and prolly 80'f. Would that create a richer or leaner situation?

Quite simply, its possible because everyones bike and conditions seem to be different. Go figure.

As lean as my bike was out of the crate, I bet even in your conditions you would have had to back out the fuel screw to get it to run.

It's also possible because the condition he describes just sounds like a lean condition to me.

Now whether it's lean from the fuel screw, too small of a pilot jet, or a dirty carburetor that simply needs to be cleaned out is anyone's guess.

And my previous response, as well as this one, are also both guesses.

Until I can stand on his back porch and feel the weather and other conditions for myself and see and hear his bike run, everything I recommend -- as well as what you and everyone else in this forum recommends-- are all purely guesses.

Opinions vary, don't take it to heart that I suggested something different than what you did. It's the difference of opinions and guesses that allow this forum to help people like him work through their problems.

:applause:

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Opinions vary, don't take it to heart that I suggested something different than what you did. It's the difference of opinions and guesses that allow this forum to help people like him work through their problems.

:applause:

You suggested something that makes no quantifiable sense. The reason I'm harping on it....is I had the same dilemma and for some insane reason people kept telling me it was too lean. There are several high altitude applications in the YZF and WR forums that indicate that OUR jetting MUST be leaner than stock, sea level applications. That is not wishful thinking. That's a fact. So why would you tell him to go richer?

You do know an overly rich jetting setup mimicks a lean condition pretty closely? Trust me. It does.

I'm not taking anything seriously other than I'm gonna save my brother a lot of headache and wasted time. I ride a nearly similar bike apart from its evolution into a 450, in identical conditions (altitude, trails and temp), he's described mechanical behaviour I'm very familiar with (after just working out my solution), what I'm saying is your opinion is wrong.

Having an opinion isn't a bad thing, giving bad advice that contradicts the facts of the case, is.

My setup

Model: 2003 YZ450f

Edit Revision:7/23/05

Timing:

Main Jet: 158...Stock 172 (seal level) or 162 (yamaha suggested High altitude rejet)

Pilot Jet:38 (edited)....stock is 42

Starter Jet: stock?

Leak: n/a

Fuel screw: Zyp Ty 1 1/2 turns out

Needle: #3 clip ( 1 up from stock, leaner)

Air box: Twin Air and Unis

Pipe/muffler: Stock header, FMF Q (older version)

Altitude: 5700 feet and up

Temp:current 40-80F

Humidity: 30%

I've had MJ from 172 to 155 in this bike.

I've had PJ from 45 to 35 in this bike.

It's now perfectly jetted and as you can see its MUCH leaner.

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Goalie,

How useful and or accurate has the spreadsheet turned out to be?

It got me close on both jets...within one size rich I believe. Where it helped was to get my PJ dialed down properly.

Worked pretty damn good gray...check this out.

172 should be 161

42 should be 39

Epic results. :applause:

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Thanks again to all, I went to the shop and picked up a range of main jets, pilot jet, and am going to tear into it tonight. Also, going to the 3rd from top clip position on the needle, currently at 4th. I think I fouled a plug today at lunch when I was taking out for a quick zip around the neighborhood. It stalled out, and I couldn't get it started. I thought I was pretty good at getting it started, so I am going to put in a new plug tonight too.

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moto goalie speaks the truth, i too went thru the same thing just last month. i ride 2000-5000' in 80-90'f (at the moment) high humidity. and my bike was acting lean,everyone said 'go richer' well after weeks of bangin' my head it was really rich way rich :applause: ended up with 40pj,160mj,DWRQ(leaner needle) 3rd clip and it finally runs(thanx moto goalie for starting that jetting thread) :eek: i couldn't believe it but there it is.

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Thanks again boys. Goalie, I used your bike for a guideline, along with the chart. Put in a 152 main, 40 pilot, 3rd needle clip from top, pilot screw 1 3/8 turns out. Running clean as a whistle. Can't wait to go for a ride!! I appreciate the help.

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Thanks again boys. Goalie, I used your bike for a guideline, along with the chart. Put in a 152 main, 40 pilot, 3rd needle clip from top, pilot screw 1 3/8 turns out. Running clean as a whistle. Can't wait to go for a ride!! I appreciate the help.

Try a 38 pilot, that is what I ran in my 426 at Rampart a few years ago.

I ran a 40 at 4,500 ft., and also ran 1.5 clips leaner. At rampart I went 2.5 clips leaner. So try another clip leaner (#2) also if you get a chance.

152 seems kinda small to me....

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