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2007 KX 85 Tuning Help

Tio Pancho

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Having trouble tuning my daughters new to us KX85. It has an FMF Fatty pipe and Pro Circuit tip. I believe the reeds are carbon, but will be verifying that. Went to the FMF website and used that as a baseline. I assume the needle is stock as per the FMF website. Bought a jet assortment and cannot for the life of me get this thing to stop bogging on the take off. I am tuning at 1/4 turns of any screw and only one change at at time. No vacuum leaks that I can find. Sprayed starter fluid around when running. Any advice would be appreciated.

Anybody around the La Verne area that could help? I will gladly pay and bring it to you. Shops are a month out for a relatively simple fix and don't care to deal with them.

Thanks in advance.
 

red ant

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Was there porting done on it? It it was ported, could have been made to be even more of a screamer. Maybe throw some new carbon reeds in it. Those little 2 smokers like to bog easily. They are lil screamers and not much low torque---like none.
 

overdue

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Check the power valve, make sure it’s set right and operating properly
 

JUSTWANNARACE

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It is lean on the pilot jet!! Big gulp of air and not enough fuel in the hit!

Not the needle or the main jet.

What carb and what are the jet sizes in it now!.
 

DLC

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Check the plug to see what it looks like

google 2 stroke spark plugs, once you know the color of the plug you will be able to fine tune the carb

2A10A469-4014-4D4C-9D61-F81F9FF033FC.jpeg
 
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outboard_256

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sounds like its your pilot jet and/or air/fuel screw. Maybe needle position but the needle doesn't really kick in till 1/4 throttle and effects to 3/4 throttle. Main jet is mostly top end. Try and figure out if you are rich or lean then go a size up/down on the pilot jet and see if it gets worse or better.
 

Sharp Shooter

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It's common when a bike sits a while for the pilot jet to clog up. Will it idle? Anyway, I have a guy in Redlands if you want pro help.
 

sintax

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so i'm going to try my best, my coffee hasnt kicked in yet.

The guys above have offered some solid tips and i'm going to go a little deeper.

Prerequisite questions...

1) did you record all of the carb settings BEFORE you monkey'd with it?
2) has it ever run right since you bought it?
3) is it hard starting?

I'd suggest pulling the carb off of the bike, note every setting on the carb (even though i think it only has a air screw and idle screw), You'll want to count your turns in to get your settings. Next, cleaning it THROUGHLY, all the air ways, all of the fuel passages with GOOD carb cleaner, not that environmentally safe shit that wont even kill a plant. You'll want to pull the jets and needle out and record their values, and then clean them. I use carb cleaner and welding tip cleaner https://www.amazon.com/Cleaning-Carburetors-Sprinklers-Professional-Oxy-Acetylene/dp/B07X228R1L/ref=zg_bs_10704398011_6

you should see a very small, but clear circle of light in the pilot. If you have bad eyes, get a magnifier. This is the only way to get those jets cleaned, esp as small as the pilots are on those small 2stroke carbs.

Once that is complete, take a look at the pilot jet and needle jet (without getting too deep into tuning, for your off throttle bog, with a simple 2t carb, this is all youre using) youll want to find what "stock" is, and see how you fit in there. I like to start tuning from a stock, known good carb config and work from there. From what i'm seeing online, that should be around a 45 pilot jet in there with your air screw at 1.5 turns OUT, usually the needle starts in the middle and a fresh factory plug.

I'd be willing to bet simply cleaning those jets well will resolve your issue, 3/4 of the time its our shitty fuel and 2t oil that gums those suckers up.

Beyond that, post back your jet settings and we'll go from there.

Fair note, there are some larger issues like compression, and feathered reeds, or butchered port job that can make tuning a nightmare, if not impossible, but thats prob unlikely, and not where i'd start.
 
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sintax

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oh and if you're really done done, and want to throw in the towel, give a call over to Precision Concepts in Riverside. I'm sure they can squeeze you in.
 

ArizonaKevin

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I echo what was said above, if it will idle and bog as soon as you open the throttle my guess is a clogged main and partially clogged pilot. The pilot is still letting some fuel in to allow to it idle, but when you crack the throttle, the slide moves up, allows more air in and is supposed to pull fuel through the main. A clogged main will not let fuel in, so the bike will lean out, bog, and die.
 

sintax

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I echo what was said above, if it will idle and bog as soon as you open the throttle my guess is a clogged main and partially clogged pilot. The pilot is still letting some fuel in to allow to it idle, but when you crack the throttle, the slide moves up, allows more air in and is supposed to pull fuel through the main. A clogged main will not let fuel in, so the bike will lean out, bog, and die.
Typically the main jet doesnt come into play until way deep into the throttle rotation. This chart is a good visual for what i'm referring to.

1608572633733.png
 

ArizonaKevin

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Typically the main jet doesnt come into play until way deep into the throttle rotation. This chart is a good visual for what i'm referring to.

View attachment 953814
From a fine tuning conversating, you are correct that changing size of main only effects from 3/4 to WOT. However, if the main is entirely plugged and not allowing any fuel through, it doesn't matter what needle you have or how high you have it set.
 

MK1MOD0

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Having dealt with countless small carbs on bikes quads ect, I’ll say, pull the carb, disassemble, physically clean, then soak in carb cleaner over night. The air and fuel passage are tiny on these carbs. And with modern fuel they clog in no time at all. After reassembly, go with stock settings for the air and idle screws for a baseline.


edit. BTW when the idle circuit/ect is clogged, they do EXACTLY what yours is doing.
 

sintax

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From a fine tuning conversating, you are correct that changing size of main only effects from 3/4 to WOT. However, if the main is entirely plugged and not allowing any fuel through, it doesn't matter what needle you have or how high you have it set.
oh totally... not question there. But if the main is that clogged, you know that pilot is TOTALLY shot. Either way I think he should make sure everything is 100% clean in that carb before playing the tuning game.
 

Tio Pancho

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Thanks all. Quick response as I’m on lunch.
I have removed the and cleaned the carb . Jets were checked for blockage. Pilot jet was lowered per FMF Website. Also checked for vacuum leak with starter fluid. Needle was at 2nd position from top. FMF calls for 3rd so it was lowered.

Will bring pilot up and tinker again.
Will return with results.
 

DaveH

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Thanks all. Quick response as I’m on lunch.
I have removed the and cleaned the carb . Jets were checked for blockage. Pilot jet was lowered per FMF Website. Also checked for vacuum leak with starter fluid. Needle was at 2nd position from top. FMF calls for 3rd so it was lowered.

Will bring pilot up and tinker again.
Will return with results.
lowering the needle brings the main on later AKA leaning it out. this will make your problem worse if it is already starving for fuel.

also two strokes are hard on plugs.......put a new one in just becasue.
 

ArizonaKevin

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lowering the needle brings the main on later AKA leaning it out. this will make your problem worse if it is already starving for fuel.

also two strokes are hard on plugs.......put a new one in just becasue.
Correct that lowering the needle brings main on lower, but lowering the clip position has the effect of raising the needle bringing the main on sooner.

I hate jetting
 

sintax

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Correct that lowering the needle brings main on lower, but lowering the clip position has the effect of raising the needle bringing the main on sooner.

I hate jetting
hahah 100% , you gotta really THINK about what you're doing. A carb is a mechanical circuit and thinking about it that way is mandatory.
 

Flying_Lavey

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Wouldnt be a bad idea to check the compression on it as well. If the top end is too far worn out, it can cause a lot of symptoms that can mimic carb issues. Those little 80;s are so easy to kick over with a good top end, its kinda hard to tell until it REALLY bad.
 

COCA COLA COWBOY

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I bet your more confused now then when you started!

I’ve been in your same position. I can get them to run, but not perfect. Good mechanics can truly head the engines and make them crisp. I have two at the mechanic right now.

On a side note, my mechanic stated all jets are not the same. Some manufacturers are much better and accurate than others. Don’t trust the online numbers and recommendations.
 

sintax

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I bet your more confused now then when you started!

I’ve been in your same position. I can get them to run, but not perfect. Good mechanics can truly head the engines and make them crisp. I have two at the mechanic right now.

On a side note, my mechanic stated all jets are not the same. Some manufacturers are much better and accurate than others. Don’t trust the online numbers and recommendations.
that is actually a solid point, I only use factory Mikuni or Keihin jets.
 
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