Please Explain: Pilot Jet Sizes

[Pigeonherd]

It's been a while since this change was made, but I still don't fully understand it, and multiple lost hours of googling still doesn't really make this clear to me....

When I took my '06 EX250R to the local shop for a tune, the owner said among other things he changed the sizing of the pilot jets on my carbs. I trust him inherently, as not only does he have 40+ years experience but is also the father of a good friend, so I'm not questioning the choice, I just want to understand the mechanism.

He said he changed them (it?) from 38 to 42 (I could have sworn this was 41 but google assures me these only come in even numbers)... Or was it 46? It has been that long, I really don't remember, I just remember thinking it was a big jump. But it's irrelevant...

My question is this: does going from a 38 to a 40-something make the flow of liquid happen faster or slower? Was this adjustment made to make my bike run richer or leaner? What, if any, of the differences between when I took it in and when I got it back would this have been responsible for?

[jkv45]

Originally Posted by Pigeonherd

It's been a while since this change was made, but I still don't fully understand it, and multiple lost hours of googling still doesn't really make this clear to me....

When I took my '06 EX250R to the local shop for a tune, the owner said among other things he changed the sizing of the pilot jets on my carbs. I trust him inherently, as not only does he have 40+ years experience but is also the father of a good friend, so I'm not questioning the choice, I just want to understand the mechanism.

He said he changed them (it?) from 38 to 42 (I could have sworn this was 41 but google assures me these only come in even numbers)... Or was it 46? It has been that long, I really don't remember, I just remember thinking it was a big jump. But it's irrelevant...

My question is this: does going from a 38 to a 40-something make the flow of liquid happen faster or slower? Was this adjustment made to make my bike run richer or leaner? What, if any, of the differences between when I took it in and when I got it back would this have been responsible for?

A 42 is richer than a 38 - so it flows more fuel.

Normally, on an unmodified engine I believe the 38 (stock) is fine. With a leaner Pilot Jet you would have your idle mixture screws turned out farther than you would with a richer jet - but both should end up producing the same (correct) mixture. Most often the mid-range is what is lean, and a lean stumble is caused by the Jet Needle sitting too low in the Main Jet. That's why it's common for 250 Ninja owners to "shim the needle" to raise it out of the Main Jet slightly and richen the mid-range to cure the stumble.

I would check that the caps on the idle mixture screws were removed when the carbs were off. That allows you to fine-tune the idle mixture for changes in temp or even type of gas.

Here's some info on that - http://faq.ninja250.org/wiki/How_do_...dle_mixture%3F

You may not need to touch them, but it's always good to have the option.

[ducatiman]

Keihin numbers their jets based on the diameter of the actual metering orifice understandably measuring in terms of mm.

38 = .38mm= 0.01496062992"

42 = .42mm= 0.01653543307"

So the larger number jet will in fact flow more fuel

Coming across many carbs, I frequently index (confirm sizes) using micro drill bits and DO encounter wildly "hogged out" jets on occasion, still stamped with its original Keihin size.

Keep in mind the pilot air correction jet (these are fixed and non-replaceable but can be hogged as well) in conjunction with the chosen sized pilot jets and adjustment of pilot screws contribute to idle mixture strength and flow.

Uncapping and properly setting the pilot screws on each cylinder on a warmed, otherwise properly functional EX is essential, basic tuning. Clean circuits within those carbs are an essential as well.

Monitoring both performance and fuel consumption are important. You want to find the balance there....ever smell unburnt fuel coming from the exhaust of an overfueled, overjetted bike? Ugh. BTW, excessive fuel washes oil off cylinder walls....not good.

[Motofool]

All excellent responses above!

Originally Posted by Pigeonherd

..........My question is this: does going from a 38 to a 40-something make the flow of liquid happen faster or slower?

Faster.

Originally Posted by Pigeonherd

Was this adjustment made to make my bike run richer or leaner?

Richer when idling.

Originally Posted by Pigeonherd

What, if any, of the differences between when I took it in and when I got it back would this have been responsible for?

Smoother idling.



The carburetors of your Ninjette are activated by the vacuum that the suction of the engine induces.

Twisting the handle opens and closes the two butterfly valves inside the throttle of the carburetors, which are there to regulate the amount of air+fuel mix that enters the cylinders.

The idle circuits are like two small carburetors inside your carburetors.
Handle not twisted open, butterfly closed, engine idling, vacuum is created, some air enters the circuit from the right air jet, fuel is sucked through the fuel jet, the mix flow is controlled by the screw, inside the cylinder the explosive mix goes.

The jet (brass cylinder with a tiny hole) of your question (responsible for restricting the amount of fuel going through when the engine is idling) is the one over the left, inside the bowl, represented in blue, partially submerged in the fuel puddle:

Link to original page on YouTube.

[Pigeonherd]

Originally Posted by Motofool

Smoother idling.

So this was an attempt to calm the my-engine-won't-start-without-massive-choke problem, or the with-slightly-too-little-choke-the-rpms-tank-upon-throttle problem? (this seems like a too-little-air issue) Or neither?
Or perhaps this was something he did after it died on him at a stop sign (not warmed up)...

Thanks for the responses, everyone. It's still a bit above me, so I'm not going to go futzing with anything any time soon (it's my daily commuter), but as mentioned by jkv45 a good understanding is nice, and that's what I'm after. I trust that it is best served the way he set it up, and I know he took the time to do the minute adjustments and test it and adjust and then test ad nauseam. I could not hope to "improve" upon that tune at this point in my riding career, so I'm not going to.

That video was extremely helpful!

Originally Posted by ducatiman

38 = .38mm= 0.01496062992"

42 = .42mm= 0.01653543307"

Shows me the shopmonkey needs a lesson or two... When I asked the young assistant-type guy whether the numbers referred to measurements he said nope, a number is only in reference to itself and the other numbers. XD

I wonder what percentage of that was "he didn't know" and what percentage was "he didn't think a girl like me could understand".... putz.

Originally Posted by jkv45

Normally, on an unmodified engine I believe the 38 (stock) is fine.

I don't know about engine mods, but the guy who sold it to me said he put on a bigger sprocket (and gave me the old one along with the service manuals).

Also, I live at 3500ft altitude, so there is that... I know logically that it shouldn't be enough to make a difference, but I do have other evidence that it is more than it seems.

But wouldn't you want it to go the other way for altitude?? If it really was going to make a difference, wouldn't thinner air unbalance the equation already and lessening the fuel in the mixture be the way to make up for it??

[jkv45]

Originally Posted by Pigeonherd

But wouldn't you want it to go the other way for altitude?? If it really was going to make a difference, wouldn't thinner air unbalance the equation already and lessening the fuel in the mixture be the way to make up for it??

Yes.

The real question is - how does it run now?

[Pigeonherd]

Still takes the whole choke to start up and I let it idle while I gear up (approx 5mins) backing off every time it starts to idle above 3500rpms... Still will straight up die if I try to throttle within this warming up period, choke or no....

But other than that, it's great.

[Ghostt]

What is the temperature when you have to use the choke? Do you need to use the choke all the time to get it started?

Higher altitude does require different jets per Kawasaki as stated below

High altitude adjustments require replacement of certain carburetor jets.

High Altitude Carburetor Specifications
Main Jet: #102
Pilot Jet: #35

After high altitude adjustments are performed. provide the customer with the Vehicle Emission Control lnfonnation Update Label and label installation instructions (P/N 99969-0614).

Advise the customer that by law, the Vehicle Emission Control lnforrnation Update Label must be affixed to any vehicle modified with the high altitude adjustments.

NOTE
If a vehicle with the high altitude adjustments is used below 4000 feet (1219meters), the update label must be removed and the original carburetor parts must be reinstalled.

If you need to use the choke all the time to start the bike on warm or hot days, then you have an issue with the pilot fuel circuit.

If your in need for Carburetor services I personally recommend @ducatiman

[Pigeonherd]

Originally Posted by Ghostt

What is the temperature when you have to use the choke? Do you need to use the choke all the time to get it started?

Higher altitude does require different jets per Kawasaki as stated below

If you need to use the choke all the time to start the bike on warm or hot days, then you have an issue with the pilot fuel circuit.

Well, recently it's been January in the mountains, first thing in the am and last thing before going home, so I'd say probably in the 30-45 degree range?

The high altitude thing here in CA is over a certain number of feet (5k? 8k? 10k? I'm not sure, but I know I'm not high enough to qualify), under which I've heard that sort of set up will get you in trouble with the authorities....

[jkv45]

Originally Posted by Pigeonherd

Still takes the whole choke to start up and I let it idle while I gear up (approx 5mins) backing off every time it starts to idle above 3500rpms... Still will straight up die if I try to throttle within this warming up period, choke or no....

But other than that, it's great.

You can only judge the carburation accurately when the engine is fully warm and the conditions are about "normal" for you.

If, after riding for 10 minutes or so at full temp, it idles at 1300 and pulls smoothly through the entire range - you are good. If there are any "flat spots" or stumbles along the way, adjustments need to be made.

I assume your mechanic synced the carbs after working on them. Did they mention that?

Are the caps off of the idle mixture screws?

[Pigeonherd]

Originally Posted by jkv45

You can only judge the carburation accurately when the engine is fully warm and the conditions are about "normal" for you.

If, after riding for 10 minutes or so at full temp, it idles at 1300 and pulls smoothly through the entire range - you are good. If there are any "flat spots" or stumbles along the way, adjustments need to be made.

I assume your mechanic synced the carbs after working on them. Did they mention that?

Are the caps off of the idle mixture screws?

After warm-up there are no troubles whatsoever.

Probably did mention, but like I said, it was a WHILE ago.... probably almost a year ago now....

Have not checked idle mixture screws as this falls into the range of things I'm not comfortable messing with. Unless I can see this from outside my bike, but this part of the process "-Remove gas tank." makes me think probably not.

[jkv45]

Originally Posted by Pigeonherd

After warm-up there are no troubles whatsoever.

Probably did mention, but like I said, it was a WHILE ago.... probably almost a year ago now....

Have not checked idle mixture screws as this falls into the range of things I'm not comfortable messing with. Unless I can see this from outside my bike, but this part of the process "-Remove gas tank." makes me think probably not.

Then you should be pretty close.

As far as the idle mixture screws go, you don't need to remove the tank to get to them. They are up inside a recessed circle in front of each floatbowl.

http://faq.ninja250.org/images/f/f3/...ure_screws.jpg

If you feel it's running perfectly there is no reason to make adjustments.

[Pigeonherd]

Originally Posted by jkv45

Then you should be pretty close.

As far as the idle mixture screws go, you don't need to remove the tank to get to them. They are up inside a recessed circle in front of each floatbowl.

http://faq.ninja250.org/images/f/f3/...ure_screws.jpg

If you feel it's running perfectly there is no reason to make adjustments.

If that black knob on the left is the idle-adjust knob, I feel like I may just know where this is, and could probably look next time I'm near my bike.... thought it looks like it may be a tough angle to identify anything from and/or take pics of....

[jkv45]

Originally Posted by Pigeonherd

If that black knob on the left is the idle-adjust knob, I feel like I may just know where this is, and could probably look next time I'm near my bike.... thought it looks like it may be a tough angle to identify anything from and/or take pics of....

That adjusts the idle speed - which should be about 1300 when fully warm.

[Pigeonherd]

Originally Posted by jkv45

That adjusts the idle speed - which should be about 1300 when fully warm.

So..... yes, then.

Lol. That wasn't a question as to that knob's function, I'm perfectly familiar with that knob, I was just saying if that's what it is, then I know what that picture is a picture of....

It's akin to you showing me a map of some town I've been to maybe once, and me going "If this is Town Hall, then yes, I understand where you're pointing me."