2003 YZ250 jetting

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#1
Fouled 4 or 5 plugs on my 2003 YZ250 this summer, normally at start-up or shortly afterwards. Choke is turned off as soon as possible after she fires, and it still has fouled. If I get passed start-up, she runs w/o fouling for the remainder of the day. Factory calls for 1 turn out with the standard 50? pilot jet, and I've been running mostly at 1 - 1.5 turns out on air screw. I'm at 900 ft. elevation and most of the riding has been above 70 degs. F. Also experiencing lots of spooge from the power valve chamber vent line. I recently ran at 2 turns out and it appeared to clean up the exhaust smoke, and ran well. Other than this fouling, the bike is fantastic, and it's supposed to have the best carburetion out there according to the magazines. Anyone with similar experiences ?. The bike is stock. I'm not sure whether to open the air screw further (> 2 turns), or go with the next smaller pilot jet.
 

bclapham

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#2
try a smaller pilot- i run 45, but a 48 might do it an also try leaning the needle by one clip and see how it runs (do them separately).

the magazines are full of crap, no more so when it comes to jetting- only YOU can jet your bike so you have to experiment and pay attention to the conditions

good luck
 
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#3
jsmoto,

Did you try the 48 main jet? My 03 is doing the same thing. Always fouls on start up, then runs fine the rest of the day. I have not got around to going smaller on pilot, hoping cooler weather would fix it.

Guy
 
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#4
Do you run the carb dry (shut off petcock) every time you shut down for the end of the day? This will help reduce fouling first start in the morning......
 

bclapham

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#5
sorry Buckholz, IMO thats a bad idea since it will let you crank bearings run dry and cause premature engine wear.

ive never fouled a plug on my YZ. why? because i changed the jetting!
 

Studboy

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#6
You need to jet that thing.
I've had the same plug in my '02 since January!!!
I'll put a new one in with the rebuild though, just to feel good. :) :) :)
My guess is that you are rich on the pilot, as that is where you usually foul plugs.
 

smarttoys

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#7
I've never fouled a plug on my '02 YZ250 even when it was running a little rich on the pilot. I've never fouled a plug on any of my bikes for that matter. I usually just change the plug when I do a top end. I have a load of good used spare plugs that come in handy at the track when other people need them.
 
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#8
You might also check the float bowl height. If the float level is to high it will foul plug just like you are seeing.
 
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#9
Thanks for all the great info. I did install the next smaller pilot (48), started the bike up as usual, warmed it up, and it then proceeded to foul on me. This was not a new spark plug, but the plug was at low hours. This was on a cooler day as well, which should have helped, if the problem is pilot-size related. Installed a new plug but haven't had a chance to ride with the 48 pilot since the leaves will be falling soon. I'm relatively sure that if no float bowl problem is found that I'll be changing to the 45 pilot over the winter.
 
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#10
I went to the 48 piliot on my 03 yz250 but still loads up went starting but have not fouled a plug yet but close. The dealer said check the float. I think that is the next step.

The 48 seems a little lean.

guy
 
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#11
I've been on Hondas the last three years, the jetting on my 03 YZ is a dream compared to the never ending Honda saga. I'm at sea leve (Fl.) I've never had plugs fouling . I did find the mags were innacurate on a few things though. For anyone interested, I tried the DEP pipe and silencer, I'm very pleased. It actually delivered what the mags said. I went with a 52 tooth rear because I was killing my clutch, I don't have near the problem with the torquey four bangers now either.
 

stormer94

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#12
Freaky, I came here looking for information on this very topic. I too am fouling plugs on my '03 yz250. If I can make it through the warm up, it's usually pretty good (stone stock on every part, as delivered by Yamaha, with the exception of an FMF gnarly pipe). Usually starts without choke on the 3rd kick when cold. I ride at about 2200-2400 feet of elevation. Running 91 octane pump gas with Yamalube "R" mixed 32:1

I would say that evil exhaust pipe spooge is at a minimum for me. just a little bit around the tip of the pipe after all summer running.

HOWEVER, Yesterday I fouled 2 plugs, one on start up, and one about 3 hours into the ride. Both were new plugs, fouled the 9, stuck in an 8, fouled it on the trail 3 hours later.

Here's the question. The plug I fouled on the trail I have a pretty good feel for the exact scenario that lead to it. I was on some twisty little 2nd gear stuff, and I had been holding the throttle about 1/4 for about 3-4 minutes in 2nd, not bogging, not in the powerband, not revving, just happily motoring the engine in a good spot on a flat twisty trail. I got to the end of the trail, had a heckuva hill to climb to get out of the trail, and it sputtered to the top and promptly fouled 5 seconds later when I was on the pipe trying to get up this hill. What needs to be changed? I thought the needle was mostly a top end adjustment. My top end feels fine, the bike is crisp all through the power band, and I have no complaint at all (short of the fouling plug thing). What should I adjust?????????

I can't help but think a few more minutes at the lower RPM would have wiped out the plug anyway, but the hill finished it off first. I'm perplexed. Also, both of yesterdays fouled plugs looked good in color, but were soaking wet. (they should look good, they were both new, need to buy plugs in bulk).

Thanks,
-Bob
 
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stormer94

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#13
Unrelated, but kind of, thought a few you might relate. I had a plug fouling problem with my last Yamaha sled. I'd foul a pair of plugs every 30 miles, no kidding. By the time I got the sled to 600 miles I had a coffee can full of fouled plugs, nearly 40 of them. Nothing we tried worked, and I MEAN NOTHING, the dealer was perplexed as well. Finally Yamaha sent a factory guy out. He took my sled engine APART in a way that would make you cry. He mic'd everything and examined EVERY LAST part. Finally came to the conclusion that some small hardened steel shaft in some part of the injector was out of tolerance due to an incorrect hardening procedure. They replaced the part and the sled worked fine ever since. It was among the most frustrating times of my life. Ride it for a few hours replace 6 plugs, take it to the dealer, nothing wrong... (repeat over and over again for months)

Who knows, we could be having some kind of freaky thing like that with our YZ250's... :think:
 

stormer94

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#14
So how do you know if your pilot jet is to lean once a change is made?????

I'm aware of the haul butt kill the engine and coast the bike to a stop method of checking the plug for a lean top end needle setting. How do check to make sure you are not to lean down low??
 
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#15
Sounds like our yz's are doing the same thing.

If you go too lean on the pilot the idle will hang or it will bog off the bottom.

I am running the 48 pj, clip #3 and 178 main. The plug it looks fine even a little light.

Then it will foul either at startup or after riding the trails for a while.

I have moved the clip to #2, will let you know how it goes.

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

Guy