How do you know if the pilot jet is the right size?

[dinkyguitar]

Hi,

I'm finally riding my 87 KX 250.

I noticed that when the throttle is barely open while riding in gear that it sputters, then as I give it more throttle it smooths out and starts to pull. Could be normal for 2 strokes?

I've read that "On a 2-stroke - You should be able to ride in 3rd gear, throttle BARELY cracked open, and it should cruise smoothly. If it sputters and crackles, the pilot is too rich. If it bogs, the pilot is too lean." Is this true???

So I changed my pilot jet from 50 back to stock, 45.

The sputter was very slightly better...

She starts on first kick with both pilot jets and each were about 2.5 turns out.

On my 3 trip of testing, I had her in 3rd and cracked the throttle and God does she pull 

This is coming from someone who had a 92 Yamaha FRZ600 20 years ago.

These 250 2 strokes are dam quick! Even for an old bike like this.

BTW, the plug is about the shade it should be..still like this, but dry. This is my pic from when I had starting issues, but is basically still looks like this.



dinky,

[Foxx4Beaver]

typically yes, sputtering at very low speeds/rpm's indicates too rich...that's how I knew my 500 needed a leaner pilot.

not sure if you've seen/read this yet...but it's one of the better in depth jetting discussion's out there...thanks to Sandblaster for taking the time to type it up for everyone to use as a guide/reference.
scroll down to where you see "Adjusting your air screw", which is the 2nd post, that's where you'll find your answer...I'm sure your carb is different than the one Sandblaster used in this thread, but it's all the same idea...
http://www.oem-cycle.com/forum/viewtopic.php?f=40&t=550

it might be a good idea to do some WOT plug chops now....as that plug looks a touch on the white-ish side...you might be able to go up one or two sizes on you main jet

[queen of spodes]

WOT plug chops

Does a plug check have to be done with a load on the engine or could you start it and just hold it WFO on the stand to get the same effect?

[Foxx4Beaver]

WOT plug chops

Does a plug check have to be done with a load on the engine or could you start it and just hold it WFO on the stand to get the same effect?

never on a stand...
always with a load on...on a nice flat surface, such as rippin down a dirt road, sand pit, desert, or a paved road....otherwise your crank and rod bearings will HATE you!...there could even be a catastrophic ending.

[queen of spodes]

What gear and how many seconds should it be held WFO before chopping throttle and killing engine?

[Foxx4Beaver]

I've always wound 'em right out just until it gets past the "meat" of the top gear....or just until it stops pulling hard.
I've seen other guys wind 'em right out long after they've stopped pulling....but I don't really have any places where I can run a bike wide open for that long around here...not that I'd want to either.

[dinkyguitar]

So after changing my pilot jet back to stock, 45 it still sputtered when I have the throttle slightly cracked open...adjusting the air screw jet didn't make any difference.

Putting a 42.5 pilot jet didn't make any difference either.

I did however start using more throttle. Putting it in higher than normal gear at 3/4 to full throttle but not letting her get into powerband.

She didn't bog and kept pulling. It's the riding with the throttle barely cracked open when it sputters.

The plug at this point was looking a lot darker, more or less like a 2 stroke plug. And this is with a 360 main, stock is 350.

I'm starting to think it's got a vacuum leak somewhere because the idle seems to go up and down on while I'm trying to adjust the air fuel screw.

Like, it goes high, and then when I rev her it will sometimes go low, and then I'll rev her, and it will return.

If there is an vacuum leak it has to be the rubber intake boot to the carb. It's still original and this is the area I had problems with when I did the leak down test.

1 last thing, what coolant temp should this bike go up to? Yesterday when I adjusting her, it went to 187 deg., but once I start riding, it drops down to 140-150 deg.

Not sure if it the temp would just keep climbing on me.

dinky,
 

[Foxx4Beaver]

I remember the intake issue you had...
if I remember correctly, I think I suggested using some Yama-Bond on the mating surfaces.
Sandblaster did a thread a few years ago about how to re-furbish a rubber intake that was loaded with cracks, but I haven't been able to find it....but it pretty much consisted of thoroughly cleaning it, then sealing it with black RTV silicone.
This was meant for guys that could no longer buy new OEM intakes due to being discontinued, or being on a strict budget.

[dinkyguitar]

Thanks Foxx4Beaver,

The intake boot to cylinder head is sealed with yamabond...But I think it's the seal from the intake to the carb.

That's the last place I had issues with but somehow I was able to seal it using heavy grease when doing the leak down test.

I could try using a regular clamp instead of the stock one since that one is cranked as far as it could go.

My ultimate, would be to get the Boyesen RAD valve assembly....but funds are a little low right now.

I'll see if I can find some leaks using WD40.

dinky,

[dinkyguitar]

Put everything back to stock as far as pilot and main jet.

I did notice I had the needle in the 3rd position so I lowered it to the 2nd, as per the manual.

I took her out and went through all the gear.

I hit full throttle in 2 or 3 of them, got up to 50mph.

She pulled good without any bog or sputtering...it's just that dam off idle sputter.

Spray WD40 around the intake and didn't notice any surge in idle.

Messed with the air screw and idle was increasing as I turned it out.

I also noticed that a few times when I revved it, the idle would drop down, and then it would pick up either by itself or when I revved it a little.

But other than that, once I stopped messing with it, the idle seemed to be more steady.

dinky,

[Foxx4Beaver]

how's the carb slide?....is it loose and sloppy when it's in the carb body?...if yes, that "could" have you running around in circles trying to cure that off idle sputter.

[dinkyguitar]

Good question...

I'll check that too.

dinky,