2003 YZ250 jetting

[rairden]

Geez, it seems like every one w/ newer YZ 250's are having carburetor problems. Well I'm too. Mine is a 2001 YZ250. EVERY SINGLE WEEK when I go to start it up it will foul about 60 seconds later. I just realized from reading somewhere that I have rich jetting (even though I don't have bogging-EVER) since my bike starts first kick w/out the choke.

MY STATUS:
Elevation- 1025 ft.
Temperature- 70's (missouri)
Main: Stock(178) just bought a 175 though and havn't tested it out yet
Pilot: 48
Jet needle: N3EJ and 2nd clip position
Air screw: 1.5 turns out (stock is 1 turn out)

I think my throttle reponse is near perfect everywhere w/ no bogging. But then again this is my first bike 5 months in. I heard I could have a crankcase (transmission side) seal that has gone bad so tranny oil is getting to my plug making it appear oily every time.

Question : How do you want your idle to be. Do you want to play w/ your air screw until it reaches highest idle. What about the idle screw.

 

[intmach]

We also rejetted our bikes this weekend and set the float levels both were off. We also changed to the BR8es plugs.. Sometime in the last couple years we had switched to br9es.. No problems this weekend.. We are very happy with the results.. Both bikes start with out the choke first kick.. 01 and 02 yz250's.. Rideon brothers..
Zig

 

[sjmaster]

OK....how are you guys determining that the float is off? I went to the link someone here was kind enough to provide on Kihien tuning. I was glad I did as it cleared up the incorrect info in the Yamaha manual. The Yam book says to turn the carb upside down and measure the float hieght, ensuring that the tab is resting on the pin, but not depressing it. Well, this is misleading, since as soon as the carb gets past 45 degrees, and the tab is resting on the pin, it will fully depree the pin as 90 degres is approached. The kihien info clears this up (as I suspected) by saying you should tilt the carb over til the tab just touches the pin, then measure. Mine is right on the money at 7.5mm, when measured this way. Basically, the seams of the float are also level with the machine seam of the carb (surface the float bowl mates to) when viewed from the side.

Are you guys using this method?

I am riding today with 178 X 48 with clip on #1 groove posistion. My oil is R-50 @ 32:1 . I am only getting 3-5 drips out of the PV vent line, and no mess out of the silencer.

A shop I spoke to in Florida suggested riding it very hard, and reving it out a bit with fuel off before shutdown. (he was clear about not running it out of fuel, just getting it to lean run a bit) He thinks crap left over from the last ride (from being somewhat too rich) is being throw on the plug, and the temps are not high enough to deal with it, especially since the plug was already operating borderline fouling condition. Makes sense to me...

Later, John

 

[intmach]

John, We did it the way you mentioned also, but i set it to the lowest height. I think the manual says something like .220 to .300. I set it to shutoff the fuel at the earliest setting. If you turn the carb over without the bowl on( so it is positioned like it is on the bike) Then hook up the fuel line and turn on the fuel you can push the bowl up and shut the fuel off just like it would if the fuel was raising the flaot. On the 2001 the fuel would still leak out just a bit all the way up.. This is kinda strange because the needle and seat are in good condition and there was never any fuel coming out of the dump tube.. We adjusted that one till the fuel would have been 1/4 of an inch from the top of the dump tube.. (it took a couple trys)
If you adjust it like the maual says it will probably overflow everytime... The 01 has a 180 main 48pilot second clip pos.. stock pipe
02 has a 152 main 48pilot third clip pos.. FMF fatty
we jumped up one main size thinking it was going to be colder this weekend and it turned out to be like 75deg all weekend. Better to be fat than thin.. :-)
Zig

 

[sjmaster]

I tried something simular to what you described....I just tried blowing air in with my breath so I could feel if it was shutting off. There is 2 foot of fuel sitting above that valve, and if i remember correctly, each foot of water exerts a pressure of 7.2 PSI. I will have to check my books. That would be a fair pressure to shut off with such a "weak" spring as the one in the tip of the needle. I wonder if the strenght of this "tip" spring is the problem? Too much deviation from approved specs by Kiehien? The jetting arrived at by the engineers at Yamaha would hopefully be based on a properly operating carburator; if the needle and seat are not shutting off fuel at the correct hieght (due to a weak tip spring), than the jetting will be altered.

Before I go out today, I am going to see if I have a stronger spring of the same length. Yes, I have tons of miniture springs....I take everything apart before I discard it. heehahah

 

[okieguy]

Update on fouling:

Rode 2 days last week (life is good). Looks like the float adjustment fixed the fouling problem. Did lots of trail riding and some motorcross. Bottom end seems crisper. Plug reading was perfect. It never even loaded up a little on the slow trails.

Current jetting 48 pj, clip #2, 178 mj, as 1.5

sjmaster: Try bending the tab on the float down just a little. If you don't like it you can always put it back. I thought it was very difficult to get an exact mesurement on the float level.

I can almost guarantee it will fix your problem.


Good Luck
Guy

03 YZ250

 

[Buckholz]

Boy is this the mis-info posting of the year:

1) Foot of fuel height is maybe ~0.3 psi (density of fuel ~.7 /33ft water per atm*14.7 psi/atm)

2) Pilot/slow jets meter fuel, so bigger numbers result in more fuel, smaller numbers less

 

[sjmaster]

Yep, I was way off on that. I said I have to check my books!! 17 years since I got out of college.....but it seems like the float is the common problem. I will set up the same test tonight with the bowl off and see what she does.

If the needle does not shut off the fuel with the tab pressing the pin in say 1/3 of it's total travel, I am going to chalk this one up to a weak spring / lack of consistency on production runs by Kiehien.

I have not had a chance to ride, but tomorrow afternoon looks good. I will certainly report back the hopefully good news.

 

[jsmoto]

Finally got a chance to ride my stock 03 YZ250 but with the 48 pilot jet. Start-up was a bit tenuous, as the rpm raced up higher-than-usual as soon as she lit off. After coming back down she stumbled during warmup, and sounded like it would have fouled the low hour BR8EG plug had I not been working the throttle. 52 miles of singletrack with AS at 2 turns out w/o fouling, but from time-to-time it felt like it would load up had I not pulled the clutch in and whipped up the rpms. And this was cooler weather, ~50F, which should have leaned it. Still lots of spooge from PV vent line. Otherwise, bike ran fantastic.

I get the impression there is really two things happening here, and likely related. Too fat on the bottom, and leftover oil fouling at startup. The fat bottom may be a result of the float height problem (I'll check over winter) and/or pilot jet size. Wouldn't correcting the float height affect other jetting besides the bottom ?. If my float height checks OK, then I'm trying a 45 pilot. This may then help with the oil accumulation problem at startup. As for 40:1 vs. 32:1, this also sounds tempting. Never had fouling problems with other bikes using 32:1, but perhaps this bike is more sensitive. One more thing, doesn't going to 40:1 w/o changing jets effectively richen the engine ?.

 

[intmach]

You guys need to make sure in colder weather you choke the bike to start it especially if you have went smaller on the pilot. Your bike is revving high because it is lean for a few seconds until the fuel starts flowing at the correct rate. If your bottom end is jetted correctly make sure you choke it to start it in temps below 60 deg. This will save your piston and bearings some wear and tear. You may only need to leave the choke on for a couple of seconds to get it to idle so don't leave it on long enough to foul out a plug..
Yes, you are correct a 40;1 mixture will lean out the whole throttle range from the 32;1 mixture.. (thats without any jet changes)
Zig

 

[jsmoto]

Intmach, thanks for the good advice, but I think you meant to say that the 40:1 fuel/oil mixture will effectively richen (as in greater fuel/air ratio due to less oil per volume of fuel +oil ) the engine compared to a 32:1 mixture, with the same jetting ????.

 

[intmach]

Yap, Sorry for the confusion......
Zig

 

[sjmaster]

As I think I mentioned, I raised the float about 2mm higher than the stock highest setting. Rode for 3 hours today, and everything was perfect. Motor had a little more power on the bottom, snappier all around....want to spin the rear tire much easier now. Only saw two to three drops of oil come out of PV vent while taking breaks, none in the garage afte sitting for several hours now (not sure how important that really is) Plug is now dry and dark brown, and reads open on my Fluke 87.

Have not started it again yet, but I am confident this will solve the problem.

One other note, is it normal to hear a mechanical noise difference between clutch in and out while sitting still with motor running? I am guessing there is more noise when clutch is engaged (in nuetral) due to one more shaft rotating. It is not a bad noise, sounds normal, just different.

Thanks again, John

 

[sjmaster]

Well, here's one for you to think about. The carb does not sit level on the machine.....it is canted up in the rear 10 degrees or so. Now I realize these things bounce around a lot anyway, but maybe the factory/ book setting forgot to take this into account?

This will just drive me nuts, because I want to know if it really is just a float level specification problem, OR is adjusting the float just fixing the symtom of some other problem....yeh, call me nutz...just ride the darn thing.

Seems like most dirt bike carbs are canted up in the rear a bit anyway...guess that is the norm??

 

[rairden]

I don't know the answer to your question. I'm sure that it's safe to say that the manual did take it into account though.

sjmaster, as you may have read before(in the 3 YZ250 threads about jetting) I'm going through the same thing you are. Do you bend the float tab down (if you were looking at the carb upside down) in order to raise the float height.

So your float height is now at about 9.5mm. Mine is at a low 4.5mm(thats how its always been). Have you tried measuring it like keihin.com says to?

So what's the floats purpose? The petcock is turned on and gas flows down into bowl of the carb. That causes the float rise w/ the gas and "float" ? What the hell does that do.

 

[merlinnn]

the float controls the needle and seat, when the float drops it allows more fuel in to the bowl, once a certain level has been attained it closes the needle and seat restricting the flow of fuel into the bowl. Similar setup to your toilet. I think

 

[jmitchell]

I have a 2000 yz 250, I'm also having the same fouling problem. I've been just about everywhere with the jetting. The only thing that seemed to help was droppping the needle one posistion. The only problem with this is that it seems to also hurt my bottom end acceleration. Like everyone else the bike seems to run best on the stock set up accept for the plug fouling.

 

[sjmaster]

Did you try adjusting the float? I have been very busy the past two weeks, but I am happy to report the fouling is OLD NEWS. Sat for a week after riding all day on the "lowered" fuel level. Sucker fired right up and ran for two hours straight. It does have more power too...spins the rear tire much easier, but I will get used to it in a few more rides.

 

[rairden]

Man it seems like every YZ250 needs the float level adjusted. As you may know I have an '01 YZ250 and just fixed my float level from about 10mm to 15mm(the way keihin.com said to; the manuals way is wrong...they need to fix that next year).

I fouled a plug every-time I started/warmed up the engine. Now, as soon as I start it...less smoke and the throttle is nice & crisp right off the bat. Ridden it twice now. I now know that was it.

 

[bclapham]

i leaned the floats on my 01 also- its still rich though, even with the clip in #1 slot and a 45 pilot. i am thinking about trying one size smaller powerjet since that leans from 1/4-3/4????? any other ideas????

 

[rairden]

To correct you bclapham, the power jet(according to my manual in front of my face right now) meters/controls from idle-1/2 throttle. It shows how it is most influential at about 1/8-3/8 throttle opening.

BTW, the stock powerjet on '01 YZ250's is #50. That is the leanest out of the group. Leanest--#50, #55, #60--Richest. If your bike is european, the stock jet is #55, you could go leaner(to 50).

My plug is a dark mocha color(first time). That is after letting it sit for a couple days. And I was just farting around in a field.

 

[fuzzy]

Funny thing is the bike sure runs good until the dreaded foul.

5 dolla on the fact that it will run even better once it's jetted right(not fouling)! Probably got a lot of hidden throttle response in there to be had. Not sure about the '03, but Yama's are characteristic of having a *******ly strong mid-range hit, but you won't feel this if coming off a super-rich pilot circuit. Here's a simple, crude jetting methodology to follow:

1. Get yourself a decent pile of pilot/main jets (generally) leaner in size to your factory jetting. Most factory jetting will be good in 40deg weather at sea level :)
2. Get your bike to operating temp!
3. Perform WOT plug chop and get the main correct via needle clip adj, or different main jet (search on WOT chop if not sure what this is)
4. Now w/ your air screw set at 1.5 start installing leaner pilot jets until you get a lean bog.
5. Switch back to the richer pilot you had before you got the bog

Fine adjustments can now be made to your AS for throttle response, and I seriously doubt you will foul a plug at this point. You will have better off-idle power. You will need your choke to start the bike cold. Your yama will wheelie at will when getting on the pipe now. For woods use may I recommend diving into needle profiles to help with the midrange surge--this will also more than likely affect your pilot-main circuits so perform steps 1-5 again accordingly.
Enjoy your proberly jetted bike! I won't even head into the woods until my AS is adjusted perfect--you will crave(and depend on) that perfect throttle response!
Jetting will change due to air-temp/quality, fuel, and elevation.....This being said any advise like this xx pilot 'sounds' too lean will mean nothing once you learn how to jet your carb. "What's in your carb? The correct stuff..."

 

[fuzzy]

The above is meant for a carb w/ proper float level...

Oh yeah, on those PWM's, your powerjet will affect the main circuit as well....Enjoy :)

 

[tbrooks]

guys go to the basics and stop over thinking this. Fuzzy has it right, I only change my plug during over haul and do not have any problems. my yz has been ported, case matched, rad valve and spark arrester. any change in motor componets or alt over 2000ft or weather (summer-winter) will require jetting. my scoot is 164 main, 48 pilot, needle 3rd pos this jetting will not work for you without motor mods and conditions similar to me. what I am saying is follow Fuzzy's methods and enjoy the new found response and power of a properly jetted motor Tom

 

[portish]

its funny reading all this. a few weeks ago i read this post and thought ive had my 03 for nearly a year and not had any hint of fouling, until now, i endoed the other week and i guess the bike landed pretty heavy and now its fouling real bad, so i guess it'll be the float height thats out. owell here goes...