yammieman

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Jan 5, 2013
63
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hey everyone.

Lately ive been fouling a lot of plugs. I had recently gotten the bike back from a top and bottom end rebuild done along with them re-jetting carby at a bike shop about 3 weeks ago and ive had to change a plug after every ride ever since they did.
Went to start the bike today and it would not start at all. I took out my spark plug just to check it and it was covered in wet oil. Never had that problem before to that extent..only been since they re-jetted it and since ive gotten the bike back from them.
What are my options in terms of richening up or leaning out the oil....mix my oil/fuel at a different ratio?? or re-jet??
Its a Yamaha YZ 250(2 stroke) motocross bike and currently my needle clip position is set to stock(2nd from top) and my main jet is set to 42 and I always been mixing at 40:1(250ml to 10 litres) and using the stock BR8 spark plug(but don't want to go too hot of a plug)
I was thinking about moving the clip on the needle down 1 spot and setting it on the 3rd clip position instead...but not too sure if it would be safe to do so or or what the impact would be?..or should i just leave it at stock and mix a different fuel/oil ratio? I mostly use the bike for trail/bush riding now days...but still sometimes go to MX tracks.
 

ellandoh

dismount art student
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Spanky's jetting guide:

A correctly jetted carb makes a tremendous difference in the torque, midrange pull, top-end pull, and over-rev of your engine. If you have never jetted your bike correctly, you will almost certainly gain some performance at some point in the bike's powerband. A cleanly jetted pilot circuit can be the difference between having to clutch the bike out of a turn or not. The needle can make all the difference in the world for the power of the machine in most situations, as it controls the throttle range that most riders spend most of their time using. A correctly sized main jet could mean the difference between being able to rev out high enough to not have to shift one more time at the end of the straight, or the power falling flat on top and requiring you to make that extra shift.
Are you fouling plugs? Many people will tell you all sorts of band-aid fixes, from running less oil, to running a hotter plug. Both are incorrect fixes for plug fouling. It's all in the jetting.
The only way to know what jetting changes you will need is by trial-and-error. No one can give you jetting specs, because every bike is different, every rider has a different style, and jetting is totally weather dependent. Unless the person telling you what jets to use is riding an identical bike, on the exact same track, at the same time, his recommendations are meaningless.
Jetting is fairly simple, and is a useful skill to learn if you ride a two-stroke and want it to perform at it's best.
It's very important that you start with the pilot circuit. The reason is simple. The pilot circuit affects the entire throttle range. When you are at full throttle, the main jet is the primary fuel metering device, but the pilot is still delivering fuel as well, adding to the total amount of fuel that your engine is receiving.
Before you start to rejet your bike, you need a clean air filter, a fresh plug (actually you need several plugs to do plug-chop tests for the main jet), and fresh fuel. One important detail: Make sure the engine is in good mechanical condition. If your engine has a worn top-end, fix it first. Trying to jet a worn out engine is a waste of time. The same goes for reeds that don't seal properly, and a silencer that needs re-packing. Worn reeds will mimic rich jetting, and worn rings will mimic lean jetting.
Before you start the jet testing, install a fresh plug. Set the float level to the proper specs, an incorrect float height will affect your jetting all across the throttle range.
Warm the bike completely, and shut it off.
As already stated, start with the pilot circuit. Turn the airscrew all the way in, then turn it out 1.5 turns to start. Start the engine, and turn the idle screw in until you get a slightly fast idle, or hold the throttle just barely cracked, to keep the engine idleing. Turn the airscrew slowly in, and then out, until you find the point where the idle is fastest. Stop there. Do not open the screw any farther, or your throttle response will be flat and mushy, and the bike may even bog. This is only the starting point, we will still have to tune the airscrew for the best response.
Now is the time to determine if you have the correct pilot installed in your carb. The airscrew position determines this for you, making it very simple. If your airscrew is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet.
Once you have determined (and installed it if it's necessary to change it) the correct pilot jet size, and tuned the airscrew for the fastest idle, it's time to tune the airscrew for the best throttle response. Again, make sure the bike is at full operating temperature. Set the idle back down (the bike should still idle, despite what you read in the Moto Tabloids), and ride the bike, using closed-to-1/4 throttle transitions. Turn the airscrew slightly in either direction until you find the point that gives you the best response when cracking the throttle open. Most bikes are sensitive to changes as small as 1/8 of a turn.
The airscrew is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the airscrew to compensate for changing outdoor temps and humidity. An airscrew setting that is perfect in the cool morning air will likely be too rich in the heat of the mid-day.
Now, it's time to work on the needle. Mark the throttle grip at 1/4 and 3/4 openings. Ride the bike between these two marks. If the bike bogs for a second before responding to throttle, lower the clip (raising the needle) a notch at a time until the engine picks up smoothly. If the bike sputters or sounds rough when giving it throttle, raise the clip (lowering the needle) until it runs cleanly. There isn't really any way to test the needle other than by feel, but it's usually quite obvious when it's right or wrong.
Last is the main jet. The main jet affects from 1/2 to full throttle. The easiest way to test it is to do a throttle-chop test. With the bike fully warmed up, find a long straight, and install a fresh plug. Start the engine, and do a full-throttle run down the straight, through all gears. As soon as the bike tops out, pull the clutch in, and kill the engine, coasting to a stop. Remove the plug, and look deep down inside the threads, at the base of the insulator. If it is white or gray, the main is too lean. If it is dark brown or black, the main is too rich. The correct color is a medium-dark mocha brown or tan.
Once you have a little bit of experience with jetting changes, and you start to learn the difference in feel between "rich" and "lean", you'll begin to learn, just from the sound of the exhaust and the feel of the power, not only if the bike is running rich or lean, but even which one of the carb circuits is the culprit.
The slide is also a tuning variable for jetting, but slides are very expensive, and few bikes need different slides, so we won't go into that here.
Keep in mind, even though this article is intended primarily for two-strokes, four-strokes also need proper jetting to perform right, although they are not quite as fussy as their oil-burning cousins. The only real difference in the two is with the pilot circuit. Two-strokes have an air screw that you screw in to make the jetting richer, and screw out to make the jetting leaner. Four-strokes, on the other hand, have a fuel adjustment screw that you screw in to make the jetting leaner, and out to make it richer.
 

yammieman

Member
Jan 5, 2013
63
0
thanks for that. i read through it...but still not 100% sure because bike runs well at 1/2 to full throttle and doesn't revv too hard like its going to blow up or anything and I have great throttle response...the only problem i have is the fouling of plugs...so does that mean i should move the clip down a notch?
it is set to stock so not sure why a stock recommended setting would have problems.
 

RM_guy

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Damn Yankees
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Nov 21, 2000
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I have found that jetting problems for the general population usually show themselves at the lower throttle settings. People say they are at full throttle when they ride but that's not always the case for most. That's not saying the jetting may be off at full throttle but I'd start with the needle and air screw, especially since you ride mostly trails.

Sounds like you are running rich. Try raising the needle clip one position and ride it to see if it helps. If it does, raise it one more level. You know if you have gone too far if you get a dead spot and WAA sound then drop the clip one spot. Don't worry about seizing when testing the jetting on the needle or air screw you won't be running it long enough at a steady throttle setting to hurt anything.

The guide above is a very good one.
 

yammieman

Member
Jan 5, 2013
63
0
cheers mate. ill play around with the air screw and take it for a little ride.
when they say too rich, does that mean too rich on fuel/air or too rich on oil??
 

BSWIFT

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N. Texas SP
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Nov 25, 1999
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Very important to make one change at a time. Spanky's guide works. Did my 05 in 05 and still running the same plug. Took about 2.5 hours and was well worth the time.
 

yammieman

Member
Jan 5, 2013
63
0
thanks for all the advice and help guys. I did it and got the plug a perfect crisp brown colour just by following the above guide and doing nothing but fiddling with my air screw and moving it out 1.5 turns to where it idles best and isn't running too rich. Although i didn't really ride it instead just let it idle then checked a few plugs i used for testing and all seemed a perfect colour. or should i richen it up a fraction so that when i do ride between closed and quarter throttle it won't be too lean or just leave it?
 

BSWIFT

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N. Texas SP
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Nov 25, 1999
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Ride IT!
 
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