This is my second bike it is a 99 KX125. The person I got the bike from I guess had given the bike to a local shop for a tune up and some repairs and then defaulted on payment so they gave his back completely disassembled, essentially a frame with 2 boxes of parts and a motor torn down to the crank.
I bought the bike for cheap in 2019 and began reassembling and semi restoring the bike to a ridable state. I did a top end kit from namura on it, new reeds, replaced all bushings for the swing arm, new chain, new rotors, new brakes, cleaned and polished everything and painted the frame, new clutch and basket, new levers and grips, plastics, rebuilt the rear and front shocks, new rear wheel tensioner inserts as the old ones were stripped, prolly alot of other things I'm forgetting too.
After the rebuild I got to take the bike out 2 times for a total of maybe 6hrs riding before the season was kinda over and life got in the way, knowing i wouldn't get out again that season I prepped the bike for winter, draining the gas and bowl and emptied the coolant system out as I had used a light antifreeze water mix I knew wouldn't stand up to our -30f winters so I wanted it empty. Well the bike was stored at a buddies house and at some point the following year he needed to move the bike or something and decided to fill it with water and start it up and never drained the water back out. Fast forward 4 years I get the bike back and go to put gas and coolant in it to start it up and find the water in the radiator! I ask my buddy about it and to my horror more or less its prolly been in there for 2 or 3 years over winter in -30f or below temps tons of thaw and freeze cycles here in Michigan, the bike isn't visually leaking any coolant or oil externally and rads seem fine. I decided to start the bike up and it started right up but seems like it has some extra white smoke. My mix is rich also cause I wanted extra lube after what I had assumed was 4 years sitting which could be part of it.
First thing I did when starting it was check the sightglass on the crankcase as it warmed up, everything appeared good it was clean oil with 6hrs on it. Then I popped the radiator cap and noticed it was bubbling and when I raised RPM the bubbles increased 
I'm assuming this is exhaust gas entering the coolant system from a crack. After finding this I closed the rad and gave it some revs to see if the smoke would clear out from the exhaust and checked the sight glass again, this time the oil appeared aerated and possibly beginning to get milky?
I obviously don't want to junk the bike after putting so much work and money into it, but i'm not sure what my options are for repair. As of now im assuming I have a crack somewhere between the combustion chamber and coolant system based on the bubbles? so in the head? And also if the oil in the crankcase is getting milky(Not positive untill i drain it) then there's another crack from the coolant system to the crankcase allowing mixing?
Here are my questions:
1. bubbling in radiator= exhaust leak from cracked head?
2.Is there a common place these motors crack when froze? hoping to narrow down my search if I crack it open.
3.If I find a crack or cracks is it repairable? I'm not apposed to some shade tree fixes if it might last a few years and I can get some time on the bike. Not sure if the cast stuff is weldable or maybe jb weld if it's in the right area? lmk
4.If a crack is not repairable what are my other options? New case halfs or head depending on where the damage is? and swapping everything from the cracked engine to the new case?
It’s not really worth stressing over until you find out the issue. Sounds like you have a crack in the cylinder wall or head for sure. And possibly cracked case. Maybe you get lucky and it’s the water pump seal that failed and let water into trans.
Is the bubbling a cloudy color or could it be the water boiling? If the water is in a state of boil, revving the engine might cause a more furious boil - it's a surprisingly direct correlation to the heat produced in the engine.
I'd say drain and flush the radiator, then fill with the appropriate coolant mix. Drain and replace the transmission oil, then ride and look for obvious water leaks. Check the tranny oil immediately after you ride for 5 minutes or so. If it's milky, you know you have coolant in there. You can also drain the coolant and check for oil in the coolant.
It's possible you could have had water freeze in your engine without harming anything, and also possible the water never quite froze.
Trying not to stress it too much till I hear from you guys with experience. Dont really know anyone that knows anything about dirt bikes and the only info I can seem to find is mainly referring to street bikes and people running water and water wetter.
Yes the water appeared to be turning cloudy as it bubbled and the bike was not up to temp yet I only ran it for a minute or two tops so I don't think it was even warm enough to boil. The bubbling deff increased when I blipped the throttle seemingly correlated to the rpm. I would compare the amount of air coming out of the coolant to someone blowing into a drink with a straw
Ok I will do a fluid change and a ride and report back with my findings.
unfortunately the garage it was stored in is a detached uninsulated building and during winter we regularly get down to -30f sometimes even colder and we have alot of freeze and thaw cycles every winter where the temp will swing from above freezing to below.
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I wouldn’t even bother with all that. Just start tearing it down. It seems pretty clear you have issues with the head or cylinder and possibly the cases.
Is it using coolant? If you bleed the air out of the system, ride it, let it cool and check it and it isn’t low I think it’s fine. I can’t say I’ve ever had the engine running on a bike with radiator cap off but that could just be air in the cooling system. I wouldn’t be tearing it down just yet.
Don’t listen to anyone telling you not to tear it down. Pull the top end off and maybe side case. Shouldn’t have hurt the bottom end but not impossible.
Correct me if I’m wrong because I may have missed it, but he didn’t say he was loosing coolant. If it isn’t using coolant, what is there to fix?
It sounds like it is pushing compression gases into the system and putting fresh fluids in would be a total waste. It should not be bubbling in the radiator at all really.
He said the smoke was white. He’s burning coolant. The quick check is to pop the head or pipe off and look in the cylinder.
Call me crazy but smoke from the exhaust of a two stroke doesn’t really stand out as anything unusual. If it was burning coolant it would have been low when he checked.
There very well could be an issue but I think it would be a good idea to verify that and pin point it before you go ripping this thing apart looking for something that isn’t even there.
Bubbling in the radiator could be from air trapped in the system. I’m assuming your buddy didn’t take the time to bleed the cooling system when he dumped water in there to move it. Also, this may also be cause by the vibrations from the engine running. Go grab a water bottle and shake it- you’ll see some bubbles quickly rise to the top.
Your gear oil being milky after running it may not be an issue. You stated the bike sat all winter. When I start my bikes to change the oil after they have sat all winter the oil tends to look milky. This is caused by condensation from sitting all winter and temperature fluctuations.
If you opened the radiator cap and saw water in there, I’m assuming it was full, so I would guess it sat all that time and didn’t go anywhere, telling me there isn’t a leak.
You can do one of two things- tear this thing apart looking for an issue that potentially isn’t even there, wasting time and money on gaskets. Or you can put fresh oil in it, fill and bleed the cooling system and run it and monitor the coolant level. If you’re not loosing coolant, you don’t have a problem. Coolant entering the the combustion chamber will either cause the engine to consume it, or push coolant out of the vent tube. Coolant entering the crank case will of course cause milky oil. All of the above scenarios will cause a drop in the coolant level. So if you run the bike and the level does not drop, your fine.
Now, if you do notice a drop in the coolant level or coolant being pushed out of the vent tube (a small amount of this is normal by the way, especially if the system has been overfilled or the bike gets very hot), I would try to isolate the issue before tearing it apart because the issue might not be visible to the naked eye.
I would recommend getting the bike hot to build some pressure in the cooling system ( unless you own a cooling system pressure tester). I would then pull the spark plug and put the piston at the bottom of its stroke. If you don’t have a borescope, some auto parts store might rent one, if not, they are relatively inexpensive and are an awesome tool to have. Using a borescope, inspect the cylinder. Looking for coolant pooling on top of the piston or running down the cylinder walls. If you see this you could then isolate it to the head or cylinder itself. If you don’t see any coolant here, I’d remove the reeds and inspect the base with the borescope.
If your loosing coolant and the crankcase oil is getting milky, then clearly you know the issue is on the water pump side of the crank case, and at that point your disassembling that side and inspecting.
Again, I would not recommend doing any of this until you verify it is loosing coolant.
I was kinda thinking the same thing something deff not right about bubbling in the rads
TBF I didnt run it long enough to see if I'm losing coolant, basically as soon as I saw the bubbling in the rad and what looked like the crankcase oil getting milky I shut her down to limit any damage I might do. I assume if air can get into the coolant system coolant can get out too?
Correct, I am familiar with a little smoke from 2 strokes but this seemed more dense, not as bad as a car with a blown head gasket but more than what I would consider normal.
Thank you! I really appreciate your detailed response. I wasn't even thinking about it having air in the system from being filled improperly!
I will start it up tomorrow and get it up to temp and see if I can confirm and mixing and if the coolant level remains stable. I also did not know that oil could become milky after sitting over winter, I'm assuming that's from condensation gathering inside the case during temp swings? Really hope thats the case with the case oil looking milky after running. Replacing the jug or head would be ideal not thrilled at the idea of having to replace the case.
Thank you everyone for the advise I will update this week with my findings and some pictures of the fluids
Just to help put your mind at ease a little bit… I went out and started my two stroke with the radiator cap off and there were bubbles in the coolant. Like I mentioned earlier, this could be cause by the vibration from the engine running and doesn’t necessarily indicate a concern.
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