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2018 KX85 and 2018 KX100 Jetting

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Hey KX fans, 

I just purchased a 2018 KX85 and KX100 to replace our older ones for my daughter who races on a regular basis. I am installing a FMF fatty pipe and a FMF power core II exhaust on it. I want to get the optimal performance out of it so should I rejet it and put aftermarket reeds in it or leave the current jets and reeds in. I am running a 32:1 ratio with about 92-94 octane gas. We live at about 900-1000 feet with an average racing outside temp of 70 - 90 degrees. Thanks all....       

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On 9/6/2017 at 6:38 PM, KX Dad said:

Hey KX fans, 

I just purchased a 2018 KX85 and KX100 to replace our older ones for my daughter who races on a regular basis. I am installing a FMF fatty pipe and a FMF power core II exhaust on it. I want to get the optimal performance out of it so should I rejet it and put aftermarket reeds in it or leave the current jets and reeds in. I am running a 32:1 ratio with about 92-94 octane gas. We live at about 900-1000 feet with an average racing outside temp of 70 - 90 degrees. Thanks all....       

Keep in mind I'm not an expert, but I've heard that brand new bikes are jetted from the factory to run close to sea level , so at 1000 ft, you won't be that far off, what I would personally do, is do a proper break in, and check the plug every couple minutes. And make sure the coloring is good and go from there. I'm pretty sure it needs to be a good brown color. Wight= way to lean, black/oily= rich.

Like I said I'm no expert.  I just got my first bike all dileld in and its running good, over time you learn how to read the bike and the plug to tell how its running. Good Luck!!!!

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We ran stock jetting on my sons 14 85, and now on his 16 100 with good results, elevation around 1,000 ft.  Both bikes run great, no bog, minimal air screw adjustments for temp swings.

I'd leave the reeds alone, jetting will be determined when she is riding.

We are currently trying to get a 30mm Lectron dialed in for the 100, we tried it on the 85, my son wasn't wild about it, first track ride with it tomorrow on the 100.

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Thanks for the words Jack Wagon and Poldies4. I did notice yesterday that after the bike warmed up there was a bog right at the beginning of the throttle snapping open like out of the gate. I am messing around with the air-fuel mixture screw and seems to be clearing it up but is this the correct way to eliminate that bog?

Thank again fellas....      

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57 minutes ago, KX Dad said:

Thanks for the words Jack Wagon and Poldies4. I did notice yesterday that after the bike warmed up there was a bog right at the beginning of the throttle snapping open like out of the gate. I am messing around with the air-fuel mixture screw and seems to be clearing it up but is this the correct way to eliminate that bog?

Thank again fellas....      

Check the pilot size.  Should be a 42, my sons 85 came with a 38, which was way lean, no way to work that out with the air screw. 

Ideally you will land between 1 and 2 turns out.  If you are more than 2 your pilot is too rich, less than one you're too lean on the Pilot.  Start there, get the pilot dialed, then work on the main. Remember, one circuit at a time, don't try adjusting them both at the same time. 

Edited by poldies4
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3 minutes ago, KX Dad said:

yah the owners manual says 140 main and 38 pilot   

Weird, they are essentially the same bike, same carb, etc.  Are you familiar with how to test the pilot circuit?

I can send you my jetting setup for our 85 when I get home from work if you'd like. 

Edited by poldies4

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11 minutes ago, KX Dad said:

yah the owners manual says 140 main and 38 pilot   

Start here

Then read post #2.  Easier than me typing it out. 

http://dirtrider.net/forums3/threads/2014-kx85-jetting-problems.186009/

image.jpgNote that they are calling for a 45 pilot.  38 is way too lean.   Top line is stock, PC is for their pipe. 

Edited by poldies4

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So I checked the air mixture screw and to get that snappy/immediate throttle response for a hole shot I have to have the airscrew out barely a 1/4 of a turn from seated which I know is to lean but if I even go out a 1/2 or 1 turn it bogs when you drop the clutch for that gate start.       

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so if I'm reading this right I need a 140 and 45 but I'm running a FMF exhaust system so would I go with what they are calling for with their PC pipe which is a 138 and 42? They are also calling for a NAPE needle and my manual says it comes with a NRJC. Thoughts?    

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52 minutes ago, KX Dad said:

so if I'm reading this right I need a 140 and 45 but I'm running a FMF exhaust system so would I go with what they are calling for with their PC pipe which is a 138 and 42? They are also calling for a NAPE needle and my manual says it comes with a NRJC. Thoughts?    

1/4 out says it's way too lean, try the 42.  Don't worry about the needle or main yet.  They will be close enough for now. Get the pilot circuit right, then move to the needle and main.  

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What's funny is that my plug is black and oily which says to rich - lol    

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2 minutes ago, KX Dad said:

What's funny is that my plug is black and oily which says to rich - lol    

Plug chops only work for full speed runs on a fresh plug, you will find differing opinions on what plug color means.  Your air screw is the only definitive way of judging what the carb needs.  My standard carbed bikes run well, I personally pay no attention to plugs unless I know there is an issue.  YMMV.  

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Well I just rode it and it runs like a raped ape now so gonna leave it for awhile and see what happens before I tear into the carb. Thought?  

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5 minutes ago, KX Dad said:

Well I just rode it and it runs like a raped ape now so gonna leave it for awhile and see what happens before I tear into the carb. Thought?  

With your air screw only 1/4 turn out you are dangerously lean on your   pilot circuit.  I personally wouldn't do it.  Ten minutes to change the pilot vs expensive repair if you have a melt down, or at least the potential unseen damage which will inevitably affect how the bike runs and lasts long term.  I am certainly not telling you what to do, that's up to you.  I would hate to see the bike get ruined, or worst case have it

lock up on your kid on a jump face.  It happens. 

Edited by poldies4

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Also, lean bikes run really fast until they lock up, which is what happens. 

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ok I'm gonna have to put stock pipes back on cause I have to order the jets    

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A local shop should have a 42 pilot, if not, then I would order and wait.  Better to get it right and keep everyone safe, as well as the bike.  

If you have to order, order several sizes, 42, 45, do the same for the main.  You might need them.  

Edited by poldies4

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Well I just left FMF pipe on and turned air screw out 1 1/2 turns like book calls for. Its got a slight bog out of hole but if you throttle it right it rips out of the gate. Should be good that way/. Thoughts?   

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