'01 KX250

[Burnt1Dragon]

Hello Friends,
I need your wise opinions. When out on a low speed off-road adventure with my 10 year old daughter, I find the low speed jetting on my '01 KX250 very rich, near fouling of the plug.  I am unable to dial in the PWK38S with the air-screw within 2-1/2 turns out.  I would like to go to a leaner Pilot Jet.

The only "spare" Pilot Jet I have is a #45.  The stock jet is a #52.  In your opinions, would this be too much of a jump in jetting?  According to some of my research, this is 3 steps leaner, as PWK jets are available (leaner than stock) in #50, #48, #45.

My thought process is to run the #45 Pilot, and be able to make better use of the Air-Screw for fine tuning to weather and terrain.  Your input is greatly appreciated.

Mike

[gwcrim]

I'm having pretty much the same trouble after a fresh rebuild.  I went from a 50 to a 45 and it's still not to my liking.  I'm wondering if something in the carb might be worn?

[sandblaster]

Here is a link to a article I am working on for jetting with some basic theory..
http://www.oem-cycle.com/forum/viewtopic.php?f=5&t=109
I have not posted the article on this site yet because I am still working on various aspects.
I'm sure it could use a good proof read.....
Here is the part about your air screw and pilot jet:

Adjusting your Air screw
Enough of the theory, lets get into the meat of the article.
Here is a pic of the carb with labels for ID's.



Note: the air screw is usually located toward the air box side of the carb (Big end).
Why start with the Air screw?
The Air Screw is a fine adjustment tool that works along with your pilot jet for starting, idling and the initial phase of your power delivery.
However, as touched on earlier the pilot circuit affects the complete throttle range.
What, how could that be?
Think about it this way, when you are running at full throttle the main jet is delivering most of the fuel, yet, the pilot is also still delivering fuel.
Sure, it's not delivering as much fuel as the main jet but it still is contributing to the overall total fuel being dumped into the system.
Checking your idle screw first will give you a good indication if your pilot jet is too lean or too rich.
Since the pilot jet effects your entire fuel delivery system from idle to wide open it is important to start here.
Otherwise you will be chasing your tail round and round.
Adjusting the Air screw is simple and straightforward.
What I like to do is verify that my float height is set at the correct level.
If the float height is not set correctly your jetting will be off the entire range from idle to wide open.
As a side note the 90-04 KX500 float height should be set at 15-17mm or .590-.670".
I will cover how to check and adjust your float height in another article.
Once the float height settings are verified I clean my air filter, install a new stock spark plug, put in fresh gas, start the bike up, and ride it until it is fully warmed up.
Adjusting the air screw before it is completely warmed up is pointless as you will not know how it will run until after you warm it up fully.
Also, this procedure must be done at the elevation that you plan on riding it at and preferably under the same temperature and humidity conditions that you will operate at on average.
With the engine fully warmed up and running, turn the air screw all the way in (Clockwise). Don't over tighten it. Turn it so that it just bottoms out as that is good enough.
You may have to hold the throttle on a bit to keep it running or adjust the idle so that it is a few hundred RPM's higher then normal.
Once you bottom the air screw out, turn it back out (Counter clockwise) approximately 1/4 of a turn and then wait about 20-30 seconds so that the engine has time to catch up to that setting.
Keep turning the air screw (Counter clockwise) approximately 1/4 of a turn and then wait 20-30 seconds for the engine catch up, do this until the engine reaches it's fastest idle position.
Do not keep adjusting the air screw out past the point where the engine idles the fastest.
If you do your engine will probably run with a lag or be very mushy.
Now that the air screw is adjusted so that the engine is running it's fastest, shut the engine off and turn the air screw clockwise, counting the turns until the air screw bottoms out again.
The ideal air screw position is 1.5 turns.
If the air screw only turns say 1 turn or less then your pilot jet is too small and is running lean at idle.
Installing a new pilot jet is easy.
Simply remove the bowl, unscrew the pilot jet, read the numbers on the side of it and get the correct jet.
Now, install the next size larger pilot jet and start from the beginning, adjusting the air screw.
If the air screw turns 2 or more turns then your pilot jet it too big and the engine is running rich at idle.
Install the next size smaller pilot jet and start from the beginning, adjusting the air screw.
Here is a pic of the carburetor opened up:



The pilot jet is down deep in that hole and requires a flat blade screw driver to remove it.
Here is a pic of the pilot jet removed from the carburetor:



See those little holes in the pilot jet?
They must be clean and free of debris.
You should also be able to look down the center of the jet and see daylight through it with no obstructions.
If adjusting the air screw has no effect then most likely there is a plugged up portion of your idle circuit.
Remember what I said earlier? "Your carburetor must be clean inside and out"
That includes all the little passageways so disassembly will be required.
Eventually you should end up with a air screw setting that allows the engine to idle correctly and is between 1 and 2 turns from the bottomed out position.
There again we are shooting for 1.5 turns.

[Enginerd]

Here is a link to a article I am working on for jetting with some basic theory..
http://www.oem-cycle.com/forum/viewtopic.php?f=5&t=109
I have not posted the article on this site yet because I am still working on various aspects.
I'm sure it could use a good proof read.....
Here is the part about your air screw and pilot jet:

Adjusting your Air screw
Enough of the theory, lets get into the meat of the article.
Here is a pic of the carb with labels for ID's.



Note: the air screw is usually located toward the air box side of the carb (Big end).
Why start with the Air screw?
The Air Screw is a fine adjustment tool that works along with your pilot jet for starting, idling and the initial phase of your power delivery.
However, as touched on earlier the pilot circuit affects the complete throttle range.
What, how could that be?
Think about it this way, when you are running at full throttle the main jet is delivering most of the fuel, yet, the pilot is also still delivering fuel.
Sure, it's not delivering as much fuel as the main jet but it still is contributing to the overall total fuel being dumped into the system.
Checking your idle screw first will give you a good indication if your pilot jet is too lean or too rich.
Since the pilot jet effects your entire fuel delivery system from idle to wide open it is important to start here.
Otherwise you will be chasing your tail round and round.
Adjusting the Air screw is simple and straightforward.
What I like to do is verify that my float height is set at the correct level.
If the float height is not set correctly your jetting will be off the entire range from idle to wide open.
As a side note the 90-04 KX500 float height should be set at 15-17mm or .590-.670".
I will cover how to check and adjust your float height in another article.
Once the float height settings are verified I clean my air filter, install a new stock spark plug, put in fresh gas, start the bike up, and ride it until it is fully warmed up.
Adjusting the air screw before it is completely warmed up is pointless as you will not know how it will run until after you warm it up fully.
Also, this procedure must be done at the elevation that you plan on riding it at and preferably under the same temperature and humidity conditions that you will operate at on average.
With the engine fully warmed up and running, turn the air screw all the way in (Clockwise). Don't over tighten it. Turn it so that it just bottoms out as that is good enough.
You may have to hold the throttle on a bit to keep it running or adjust the idle so that it is a few hundred RPM's higher then normal.
Once you bottom the air screw out, turn it back out (Counter clockwise) approximately 1/4 of a turn and then wait about 20-30 seconds so that the engine has time to catch up to that setting.
Keep turning the air screw (Counter clockwise) approximately 1/4 of a turn and then wait 20-30 seconds for the engine catch up, do this until the engine reaches it's fastest idle position.
Do not keep adjusting the air screw out past the point where the engine idles the fastest.
If you do your engine will probably run with a lag or be very mushy.
Now that the air screw is adjusted so that the engine is running it's fastest, shut the engine off and turn the air screw clockwise, counting the turns until the air screw bottoms out again.
The ideal air screw position is 1.5 turns.
If the air screw only turns say 1 turn or less then your pilot jet is too small and is running lean at idle.
Installing a new pilot jet is easy.
Simply remove the bowl, unscrew the pilot jet, read the numbers on the side of it and get the correct jet.
Now, install the next size larger pilot jet and start from the beginning, adjusting the air screw.
If the air screw turns 2 or more turns then your pilot jet it too big and the engine is running rich at idle.
Install the next size smaller pilot jet and start from the beginning, adjusting the air screw.
Here is a pic of the carburetor opened up:



The pilot jet is down deep in that hole and requires a flat blade screw driver to remove it.
Here is a pic of the pilot jet removed from the carburetor:



See those little holes in the pilot jet?
They must be clean and free of debris.
You should also be able to look down the center of the jet and see daylight through it with no obstructions.
If adjusting the air screw has no effect then most likely there is a plugged up portion of your idle circuit.
Remember what I said earlier? "Your carburetor must be clean inside and out"
That includes all the little passageways so disassembly will be required.
Eventually you should end up with a air screw setting that allows the engine to idle correctly and is between 1 and 2 turns from the bottomed out position.
There again we are shooting for 1.5 turns.

Nicely written!  The two quick changes I saw when I skimmed this are for terminology only.  In the  photo showing the components in the bottom of the carb that are covered by the float bowl, the item labeled as the float valve is more correctly called the inlet needle (and seat).

In the picture of the pilot jet, the small holes on the sides of the jet are called the "Emulsion holes".

Again, let me say nicely done. 

[sandblaster]

Thanks!
I appreciate the feedback.
I will make some changes

[gwcrim]

Thats exactly how I went about it.  Still seems rich.  I'm going to have to take some time and do a methodical tuning.

I like that you recommend starting with the idle circuit.  I agree that it's the first place to start.  Lots of other people start with the main, but that just doesn't make sense.

[don46]

while the idle circuit is key, there are many factors that play into it, does your bike rev to the moon when started? you may say to lean, but, I would guess to rich, the reason, you have to crank the idle screw in t make it idle and when you start it it is typically colder and with the slide jacked up it will idle up, so in theory your bike should start and idle almost the same cold or warm, obviously big temp changes will have an impact. My experience with the 99-02 vintage motors found that the needlwe was at least 4 steps rich (at 3300') the pilot was rich as well as the main (I ran VP c-12 then which tended to run richer). you also have a power jet in your carb that isn't shown on the diagram supplied by blaster and throttle position switch, did I mention I also used a leaner slide, I think they came with a 7 and I used an 8. these are a pita to get right, but when right they are crisp, no racy motor on startup.

[2-Stroke Tom]

while the idle circuit is key, there are many factors that play into it, does your bike rev to the moon when started? you may say to lean, but, I would guess to rich, the reason, you have to crank the idle screw in t make it idle and when you start it it is typically colder and with the slide jacked up it will idle up, so in theory your bike should start and idle almost the same cold or warm, obviously big temp changes will have an impact. My experience with the 99-02 vintage motors found that the needlwe was at least 4 steps rich (at 3300') the pilot was rich as well as the main (I ran VP c-12 then which tended to run richer). you also have a power jet in your carb that isn't shown on the diagram supplied by blaster and throttle position switch, did I mention I also used a leaner slide, I think they came with a 7 and I used an 8. these are a pita to get right, but when right they are crisp, no racy motor on startup.

The Keihin shown in sandblaster's write up does not have a throttle position switch. The KX500s never had any electronic circuitry in their carburetors, while the 250s did. I know my '99 was the first bike that had wires coming out of the carb, and this scared me. It never failed though!

[don46]

actually the 98 cam with a power jet, in 99 kawie went to the 3d ignition and used the power jet and throttle position sensor.

[stevea100m]

I had this same issue with my 03 250 and I wound up going down to a 42 pilot

[Burnt1Dragon]

Excellent!  Thanks friends, for all the tech support.  Hey Sandblaster, awesome article.  After excercising different tacticts, test and tuning methods, including "ride and feel+choke", setting the idle circuit (as detailed by Sandblaster's article) and various youtube "tech" videos, I am confident that the Pilot (Slow) Jet is too large.

(See my original post) 

My PWK38S had the stock #52 Pilot Jet in it (Freshly Rebuilt).  As in the original post, I only have a #45 Pilot Jet available at the moment.  So my question was if that was a dangerous 3 step leap in the lean direction.  After even more research I found 2 "solid enough" reasons to give it a try. 

1.  The 1999 KX250 with the PWK38S and 3D mapping, uses a #45 Pilot (also recommended by MXA, if that matters).
2.  I consulted a local Bike Mechanic, who advised me that a change in the Pilot Jet in this range would not "hurt" my motor.  I would see "a definite difference".  Note: I did explain my circumstances for inquiring.

So...  I swapped out the #52 for the #45 Pilot Jet today.  The results are fantastic so far.  I am able to attain best idle within 1-1/2 turns out on the air-screw (Warmed up of coarse).  After a few quick cracks of the throttle, I was able to take the "Booowaaaahh" (lean) sound out of it by ultimately arriving at a total of 1-1/4 turns out on the air-screw.  Finally, I took it for a test ride on a tight little trail coarse I have set up on my property.  Nice and responsive down low.  I pulled through the gears on a straight away to see how the rest of the carburetors circuits were effected.  I felt that the bike pulled normal or better (easily losses traction, so hard to tell).  Sounds great too!

Finally...  I think the leaner Pilot Jet improved the areas that needed improvement, and had a positive impact if any in the overall throttle range.  However, I only have the one jet to try.  I don't think its perfect, just the right direction.  I think the best Pilot Jet for me is actually somewhere between the #45 and #52. I believe now it would be beneficial to order and try out a #48 and #50!

Back to the original post yet again though, I hope to put this experiment to the test this weekend, on a long and slow trail-riding adventure with my 10 year old daughter.  I doubt the bike will load up or foul plugs now, but you never know.

Final Note:  I ride in Northern Ontario at about 630' above sea level...  and yes, it's getting colder.

Thanks again Friends!  I shall keep you posted.  Keep it comming though!

Mike

[sandblaster]

Glad it helped.
If you see any place where it can be made easier to understand, let me know what your suggestions are and I'll make some changes.

[Burnt1Dragon]

Hey All.
Just wanted to post an update on my little jetting adventure. Just to recap, My bike was Loading up and Fouling Plugs on slow trail rides.

I last posted that I moved to a #45 Pilot from the stock #52. I was very happy with the results.

Now... Since I last posted the weather has taken a turn towards winter. It is now about 1-2 degrees celcius here in Northern Ontario Canada. Riding it today, I feared it may be a bit lean. So I moved the Needle Clip down to 3rd position from the top. (Stock is 2nd from top).
The bike was fine in tight twistys, but when I open it up, she was not happy. Kind of like it was misfiring. The strange thing was, it would not idle on its own and finally sputtered to a stop. It would not start again after that. I pulled the sparkplug to find that it had fouled??? I replaced the plug and the bike fired up no sweat. But sounded like absolute crap at idle and reving. So  I pulled out the carburetor yet again. Checked float hight, put the need clip back to stock and reinstalled the #52 Pilot. Fired up the bike, dialed in the idle as specified by Sandblaster's article and took her for another rip. The bike is back to its former glory.

So in conclusion, I have ended up going back to stock tuning on the Carburetor. This only confirms to me that the OEM jetting is rich enough to run in this cold weather. I still feel like the perfect Pilot Jet is somewhere between #45 and #52. Especially in the proper riding season. I will be studding a set of tires when the winter comes on full force and plan to ride some indoor again, so I will ride on!

Thanks for the advise. I will keep y'all posted. It would be great if anyone would like to shed some technical light on what I just experienced.

Mike