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I've been helping a friend with his 2003 KDX 220. Upgraded to FMF Gnarly pipe with TurbineCore 2 silencer, Boyesen reed valve. The bike fouls plugs regularly. This is at 5000 to 8500 feet elevation. He's been using Amsoil mix oil made for 80-100 to 1 and mixes it at 90:1, so it's not the oil mix. The jetting was still stock - 42 pilot, 145 main. So I changed out the pilot for a 40 which I did not think would make much difference, but it idled noticeably better. However it still fouls plugs. I just switched the 145 main for a 140 but haven't had a chance to do an extended ride. What is the consensus?

2003 KDX 220
5000-8000 feet elevation
FMF Gnarly pipe
TurbinCore 2 silencer
Boyesen reed valve
Pilot: 42 -> 40
Main: 145 -> 140

Just for comparison, I'm running a 38pj, 150m, CEK needle at 2000ft at 40:1 mix. Riding at higher elevation like you mentioned I normally drop to a 148 main, and drop the needle one clip, with all else being equal, and my bike runs quite well in the 7000-8000ft range.

You didn't mention which plug he's running. I hope it's an NGK BR8ES. If running a BR9ES, or any other brand of plug, that will contribute to the fouling.

One thing he needs to consider by running such a low oil ratio is that he has more gas in place of that oil. That makes the bike richer compared to most recommended jetting for KDX's running more oil. His jetting will need to be smaller comparatively. So, in a way, it is the oil mix! I would drop to a 38 at the largest, a 35 would probably be much better, pilot jet. After you get the pilot jet sorted, then you can work on dialing in the main jet and needle. Follow the jetting guide here on the forum and I'm sure you'll get it sorted.

One last thing, have you done a leak down test on it to insure you don't have a leaking crank seal?

So if he was running 40 to 1 it would be leaner and maybe right on the money with the current jets. A little bit of oil runs richer and needs small jet sizes like mentioned above. Think about it in terms of of how much combustible fuel is in each drop of premix vs oil (rather than the more oil is richer, less is leaner way of thinking of it)
Also research the jet block gasket. If that needs replacing (theres a test) then you could drive yourself nuts trying to jet the bike.

Please give that engine some oil. Reevaluate after that.

https://www.amsoil.com/p/dominator-synt ... g-oil-tdr/
Mix 50:1 with fresh non ethanol gas.

you haven't mentioned the needle which is the most important factor

kdxdazz wrote: ↑06:21 pm Sep 27 2023 you haven't mentioned the needle which is the most important factor

Please explain. I haven't checked the needle, but since the jetting was stock, I'm betting it will be a stock needle in the stock position. How do you think it should it be?

kcomst wrote: ↑09:39 pm Sep 27 2023

kdxdazz wrote: ↑06:21 pm Sep 27 2023 you haven't mentioned the needle which is the most important factor

Please explain. I haven't checked the needle, but since the jetting was stock, I'm betting it will be a stock needle in the stock position. How do you think it should it be?

What he is referring to is that different needles affect your overall jetting due to them being larger or smaller in diameter. You can pull the needle and most will have what it is stamped on the side of it. A magnifying glass is helpful in seeing it.

kcomst wrote: ↑09:39 pm Sep 27 2023

kdxdazz wrote: ↑06:21 pm Sep 27 2023 you haven't mentioned the needle which is the most important factor

Please explain. I haven't checked the needle, but since the jetting was stock, I'm betting it will be a stock needle in the stock position. How do you think it should it be?

the needle maybe stock or maybe not, having a stock pilot and main does not guarantee a stock needle, the needle is responsible for feeding fuel at all throttle openings including idle so you can't really just solve a jetting issue with jets alone,has to be looked at as a whole. 1173 is stock for the 220 but an 1174 needle might be better for you at high altitudes. as stated 80:1 is crazy lean on the oil for all round riding,if you were just riding around at idle like a trials bike that might be acceptable
modding the slide from a number 5 to a number 7 is a popular mod but i would make that a last resort

You didn't mention how the bike was running...in spite of fouling plugs.

You could also have a crank seal leak that sucks in oil.

Please explain. I haven't checked the needle, but since the jetting was stock, I'm betting it will be a stock needle in the stock position. How do you think it should it be?

the needle maybe stock or maybe not, having a stock pilot and main does not guarantee a stock needle, the needle is responsible for feeding fuel at all throttle openings including idle so you can't really just solve a jetting issue with jets alone,has to be looked at as a whole. 1173 is stock for the 220 but an 1174 needle might be better for you at high altitudes. as stated 80:1 is crazy lean on the oil for all round riding,if you were just riding around at idle like a trials bike that might be acceptable
modding the slide from a number 5 to a number 7 is a popular mod but i would make that a last resort

Knowing the owner, who is the only owner, I'm positive the needle is stock. From my experience, there is something very simple causing the bike to foul plugs every ride. More likely jets and fuel mix, and for sure the clip position on the needle. The bike has such low miles I also don't think the needle or needle jet is worn. Also I'm going to check as some suggested for an oil leak between the crankcase and combustion chamber. I did change from 90:1 Amsoil to Motul 710 at 50:1, which is what I run in my Beta 300RR. I posted this on another forum and am amazed at how many people say the 90:1 Amsoil is more likely causing plugs to foul and 50:1 oil with lower flash point will actually cause LESS oil burning. But when you read why it makes sense.

Oh, another simple thing to check is float level. The stock setting causes excess fuel to enter the engine. Make sure it's set to 18mm.

Molly's 70 wrote: ↑02:51 am Sep 29 2023

Another good idea to check, thanks. I have had this happen 2-3 times. One year I decided it would be good to winterize all my bikes by draining the carbs. What I learned the next spring is that causes o-rings to dry out and crack. Now I leave the gas in. You can also test for this simply by blowing on the fuel inlet with your mouth while holding the needle shut with the float.

SS109 wrote: ↑09:25 pm Sep 28 2023 Oh, another simple thing to check is float level. The stock setting causes excess fuel to enter the engine. Make sure it's set to 18mm.

I looked at the float level but did not have a measurement. Thanks.

One thing I forgot to mention, the last 2 times I rode the 220 to test jetting (which now is at 135/35), it runs very smoothly at WOT, but in between (half throttle) it seems to stutter a lot and is not smooth. After just ten minutes of riding there is carbon on the plug. It idles just fine. Which makes me think, considering this diagram, that it has something to do with the needle (as someone mentioned before). I think I'm going to pull it out again today and take a look. Any thoughts?

That sounds like what mine would do if I lowered the needle clip one position. Idle and wide open good with a stutter in between. Try raising the clip one position at a time and test ride. Once you’re happy with the mid range it would be good to do a full throttle plug chop to make sure the 135 main you’re running is safe. It sounds like you’re almost there.
If it’s in clip 3 go to 2 and don’t be afraid to also test in 1. You may go back to 2 but it’s good to know what each feels and sounds like.

An update. I lowered the needle so the clip was in the #2 position and it still was raspy mid-range, so lowered it to the #1 position and it's still raspy, not smooth like you would expect. So...I contacted JD Jetting, gave them all my specs and am waiting for a tech to get back to me. I am starting to think after talking to sales, that with the FMF Desert Gnarly and Boyesen Reed, you have to upgrade to a different needle. Below are all the specs.

• FMF Gnarly Desert pipe

• FMF TurbineCore 2 muffler

• Boyesen RAD-14XA reeds

• 135 main jet, 35 pilot jet (down from 145/42)

• Needle lowered to position #1

• Float height adjusted to specs

• Float valve seals properly

• Power valve rotates normally when revving

• Non-ethanol fuel mixed with Motul 710 at 50:1

• Factory NGK BR8ES plug

• Air box lid removed

• Clean Air filter

• Altitude: 5000-8500 feet (but mostly @5000)

• Starts on first kick

• Idles smooth but hard to find a "high spot" turning idle air screw from 1-2.5 turns

• Wide open throttle feels smooth

• In between idle and WOT the bike still feels raspy.

kcomst, above John_S mentions jet block gasket. Go ahead & replace it. It is not expensive, eliminates a variable, & many of us experienced jetting problems that were resolved by replacement

JET BLOCK GASKET.... #1 priority.
With Boyesen reads, FMF Desert Pipe + Turbine Core 2 and air box lid removed, the stock jetting is pretty acceptable. I can be improved upon, but it really isn't too far off. A different needle helps a lot though.
If you're having that much trouble getting it to run even a remotely clean, you have other issues, and the jet block gasket is going to be the number one first suspect.
Shown on the Kawasaki parts diagrams, but it IS definitely in there. At this date, pretty much any KDX that has not had its jet lock and gasket replaced yet, should. Also, it requires a tamper-proof torx bit to remove, unless you want to cut a flat head slot in the head and replace it with a standard button head fastener.
You'll have to get the OEM Keihin jet block gasket from Sudco, JD Jetting, or Jets R Us. Trust Amazon or ebay gaskets unless you are 100% it's the official store of a Keihin dealer, and is in fact a genuine Keihin part.