CR125 fouling plugs

possibly crank seals
You can pull the ignition side and see if its leaking, clutch side you can't.
Had a yami do it.

Could be a crank seal....also check your float level, and make sure all your carb passages aren't clogged....Maybe run a hotter plug and don't putt around it....those bikes are made to ride them like you stole it

what about the float height? could explain the hard starting problem. leaner needle? Have you done a plug reading after your son has been riding it around when it doesnt foul a plug? hotter plug? Prolly all things you have checked already but if not I would.

On a side note I have new renthal 13 front sprocket for a cr125 that I will never use. If you want it PM me your address and I will send it free. Just reimburse me the shipping after you receive it. Can't imagine it would be more than a couple bucks to ship.

Swap the 02 carb over to the 04. If that fixes the issues, then you know where the problem is. If not, you know that it's something besides the carb. Also try the 04 carb on the 02 and see if the problems follow the carb.

If they are putting around, lean your needle and also use a higher ratio Oil-Gas mixture. If the throttle isnt pegged most of the time I would closer to a 40:1 instead of the standard 32:1 or 34:1. Depending on the pipe on the bike you may want to go to a hotter plug as well like mentioned above, but again if they arent grip-it and rip-it riding, then the hotter plug is still going to foul easily.

Change the clip position

I would check the jetting on both carbs and compare. Needle number, clip position, pilot jet, main jet (not much affect on putting around), and air screw position.

Since you mention the bike is very cold natured I'd lean toward the jetting or right side crank seal issue. Does the bike have black spooge coming from the exhaust? More than the one that runs good?

My experience with CDI and Stators is they're either good or bad. I've never seen an ignition system cause a bike to run poorly, only not run at all.

here is a great jetting guide I used when I had a cr250:


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 air screw 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 air screw for the best response.

Now is the time to determine if you have the correct pilot installed in your carb. The air screw position determines this for you, making it very simple. If your air screw 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 neccessary to change it) the correct pilot jet size, and tuned the air screw for the fastest idle, it's time to tune the air screw 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 air screw 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 air screw is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the air screw to compensate for changing outdoor temps and humidity. An air screw 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 begine 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.

for float height, a good rule of thumb is about parallel with the carb. Anyway, the tool is basically a glorified ruler. What is the float height spec?

If he is putting around on it, that is probably allowing the plug to get wet. If the bike gets parked with a wet oily plug, it will be fouled after sitting for a while. The bike needs to be jetted for how it is ridden. Start by raising the clip on the needle. If it is all the way up, drop one size on the pilot and put the clip back to the middle.

Man, a #400 main sounds pretty big for a 125. Is this the stock jet that came in it new? I would suggest comparing the stock specs against what is actually in the carb at present time,for starters.

av-gas and rejet !!!!

How many turns out do you run your airscrew? Does the bike blow white smoke when he cracks the throttle form idle?

Setting your airscrew: turn throttle to just above idle, turn airscrew in until bottomed out, turn screw out half turn, half turn again, slowly adjust screw out until idle starts to increase, keep going, when you get to the sweet spot the bike will get a ba ba baba bababawaaa, when you get waa or the highest rpm you're good. Kill bike and count how many how many turns out you are. This will determine if your pilot is rich or lean. Under one turn you're lean, need richer pilot, I doubt this is the case. Over two and you're rich, need leaner pilot. If the bike is huffing white smoke when you crack throttle try raising the clip on the needle, lowering needle, leaning mixture. When these settings are right you should be able to bike on the stand crack the throttle wide open from idle and not bog. Smoke and a bog could also be an oil side crank seal. A lean bog could be the ignition side seal. I have seen ignitions get week over aperiod of time and cause similar complications, usually start by usung a hotter plug and then get to the point where you just can't seem to get or keep the bike lean enough.

OOps, I posted before reading the responses. Good call CJMX!

Ive had this problem in the past. Adjusting the Clip position should help. If it has an FMF or Pro Circut pipe call the pipe manufacturer and get the suggested jetting for where you live. Keep your fuel and your fuel mix ratio the same its one constant you can always contol

Go to a hotter plug. My cousin had the exact same problem. He switched to the next hotter spark plug and I believe he is still using that same plug about a year later. Try the simplest solution 1st.

Just curious as to the outcome of this?

A couple of other things to look at would be the needle and seat and reeds. The tip of the needle can get a grain of sand or un even wear and will not let the needle seat properly which when its run wide open its isn't that bad but can load up in slow speed situations. Inspect the reeds and make sure they don't have any frayed edges or if they aren't sealing, this can also make a rich condition at low RPM's. I might have missed it but what did the color of the plug look like?

I know some people say a stator either works or it doesn't , but i had a cr125 that fouled a plug every day, if i put a new plug in it would run all day, but the next day after warm up it would foul. I went through everything,{ jetting, hotter plug, crank seal, testing the resistance of the stator}, it tested within spec, finaly i tried a friends stator and that was it, a weak stator from the factory. After that i played with the stator with an ohm meter hooked up and heated it with a hair drier and watched it go out of spec.