Carbon Fouled Spark Plug (i think)

[bdh825]

Well I'm fairly new to the whole riding thing and this sight. I bought a 1996 RM 250. The perfomance is outstanding - a really strong bike. I have had it out riding twice it thick and heavy woods, not to metion 100% humidity too with it being 92 degrees.

The first time it fouled I had no idea why the bike quit on me. I brought it to my Suzuki dealer to get it checked out and to get a full service on the bike. I got the bike back running strong. I was also told to run the bike hard and keep it opened up.

The second time out I did ride the bike extremely hard and it quit again. This time I was pretty sure it was the plug. I put a new one it and it started right up. I analyzed the other that my Suzuki dealer put in. It had a black soot caked all over it. From what I have been reading it causes the electrical current which causes the spark to be bypassed, therefore not allowing the bike to run.

What could be causing this to happen? How could I correct the problem? All help would be greatly appreciated.

Thanks

 

[Rhodester]

It sounds like you are running way rich. I'd start my experimenting by turning the air screw out a little bit ( 3/4 turn) dropping the needle (raise the clip 1 notch) and going 2 main jet sizes leaner. Study your results and go from there. I assume that your filter is clean as well as your silencer. If not give them a good cleaning.

 

[joh9356]

Yup, Rhodester is right, lean out the needle and main jet.

 

[bdh825]

Thank you.

 

[ellandoh]

this is my favorite jetting help ill cut & paste for you when you get to dialing it in :cool:

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.

 

[bdh825]

This helps a ton, but as embarrassing as this is.. I'm not even familiar with jetting. I have been told that it is a simple process, but I physically don't know what to do to get correct jetting for me.

I know it is in the carb.. right? And I know an aggressive rider can have it richer while some one who doesn't ride as high rpm has to have it leaner. I would but myself right in the middle.

So where do I start? ......... I feel like a moron

 

[Rhodester]

Get the factory manual and study up first. That will take some mystery out of the process. Don't feel too bad...everybody has to start someplace.

 

[bdh825]

Still not right..

Ok, I rebuilt the carb, but fresh mixed gas in (I use R50 oil), and gave the it more air (with 100% humidity I figured it needed it), and put a new spark plug in again.

It started up and sounded perfect. After I rode it around a bit and killed it and let it set for about an hour. When I came back to it, it acted as if it wasn't going to start, thought I finally got it started again. I rode it around a bit opening it up more. (I checked spark plug and seemed fine) The bike set for about 24 hours and today I couldn't even get it started.

Could this still be a jetting problem? Maybe a new plug again? Or should I maybe try mixing my gas leaner or richer? (I use 32:1) All suggestions are appreciated greatly.

 

[Rhodester]

What is your temperature when it won't start? Are you using the choke or not? If it's warm to hot out try it with no choke. The air screw can certainly effect starting. You may have to screw it in a little bit and experiment. Be sure to log your results. Memory tends to fade. Your notes will start to show you patterns developing that you will learn allot from.

 

[bdh825]

I would assume it was the temperature that it was outside. (about 85-90 degrees) I tried choking it, I tried screwing in the air screw, (though were I had it seemed to work last time) and it is still not even trying to start.

-Thanks for your help

 

[bdh825]

Its Alive!

After I rebuilt the carb I have been making a stupid mistake. When starting it with the choke on I was giving it a twist of the throttle... Very Stupid... I remember to NOT TO DO THAT and it started right up. As far as everyone helping me with the spark plug problem it has saved me lots of time and money.

Thank You All