Spark Plug Problem?

[bmoore4w21]

Well, I just bought a used 01' KX 250. The bike is almost 100% stock and appears to be in excellent shape. From what I can tell no engine work has ever been conducted due to the perfect shape of all bolts and screws in and around the engine. With that said, I just recently took it out for my first ride. I had ridden it for about and hour and a half when I went for full throttle up a gradual incline. This is when the engine failed. It sounded as if I had forgotten to turn on the fuel. However this was not the case. I changed out the spark plug and replaced it. Once again I was off and the engine started on first kick. Later on the same thing happened again. I'm not an expert but I know that this is not normal. I have read some threads and responses about jetting and other plugs. But I don't know exactly what should be the first step. I guess I should also mention that I have been putting in Champion plugs that I bought at the local Autozone. They were crossed referenced with the original NGK plug that was in it. I'm not sure if maybe I need some different plugs or if the jetting needs to be changed. I also noticed that the articles about jetting ask about the top end being worn and the piston rings being shot. I'm not exactly sure how to determine that as well. I also am using a 40:1 fuel mixture with some special Bel Ray oil that was recomended by a follow KX 250 rider. Please help me on this one because other than this little problem I had a great ride.

 

[Se77vN]

Maybe a jetting prob. May have too big a main jet. Did you pull ou the 2nd plug and see what it looked like? Also, get a few NGK plugs....it has been my experience that champion is crap. Also, you can try just leaning out the mixture to see if that helps...BTW....(this may be a dumb question but did you have the choke on during the ride after the first plug change?) I have seen it before.

 

[bmoore4w21]

No the choke was not on in either case. The second plug was wet and appeared to have a little carbon on it. If this is a jet problem how do I go about fixing this. Is this something I can buy easily and cheaply or does this require a lot of time and money. I'm going to fix this regardless but I just want to know what I'm getting in to.

 

[nickyd]

explain to me how the engine failed. dead cut out. or did it go BAAAAwwwwwwwwwwwwwaaaaaaaahhhhhhhhhhhh....

 

[bmoore4w21]

The engine made that sound pretty much. It sounded like it does when you generally run out of gas. It didn't just cut off completely it just sturted out. Like its normal higher pitched sound to a deeper sound. But I was on the throttle both times it happened.

 

[nickyd]

what does the air filter look like? And the silencer packing?

 

[bmoore4w21]

The filter is somewhat dirty but nothing that bad. I've seen alot worse than what it looks like. I'm sure a good cleaning wouldn't hurt though. The packing looks good from what I can tell.

 

[Rcannon]

Was there any water on the airfilter...perhaps you washed the bike?????

This is abotu the only way I can kill plugs in my yz 250.

 

[bmoore4w21]

Nope I washed it when I left, after the plug problem.

 

[highflyernick]

Is the spark plug cap really easy to get on, if it is then it could be your spark plug cap. The same thing happened to my 01 KX250. Sounded like it was out of gas but really the spark plug cap was broken and was only letting half a spark in sometimes. Check it out.

 

[viking20]

Even though the bike looks great , you may have an issue with the carb....Be sure to clean it , the float or float valve could be sticking.
Next time the bike dies on you , try to tilt the bike , and see if fuel comes out of the owerflow tubes.
Check that these tubes are not not clogged with dirt.
If you are really fowling plugs , make sure the jetting isnt too rich , especially the pilot jet / air screw...This will rarely cause a fowled plug if the bike is ridden in the powerband on an mx track , but the more the bike is ridden at low revs the easier you will fowl plugs.....Be sure the plugs you are using are really whats supposed to be in there , maybe its the wrong heat range.
If you decide to change the spark plug cap , remember to get one with the proper resistance , you can fry the bikes ignition system if you dont.
Do a search for jetting , lots of threads on this subject already...
Good luck !

 

[ellandoh]

this is my favorite jetting article ive found


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.

 

[bmoore4w21]

Ok thanks for all replies. It turned out that the spark plug cap was loose. It actually came off while changing out a plug. I cut a little off and screwed it back in. I was on the the bike for 6 hours non-stop without any problems. Thanks for all replies.