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Hi guys, having a little trouble with my 250X.. any advice appreciated.

Last summer I noticed I was leaking a bit of coolant. Not a massive amount, 100ml per ride. But i wanted to sort it out. Visual checks showed no signs of a leak, no damp patches, so i figured it's probably the radiator cap or the head gasket.
First I changed the radiator cap, it's the cheaper of the two options. No difference. I let her heat up till the pressure filled the expansion tank and boiled out the overflow. Still no signs of a leak.
Second I replaced the head gasket. I had a new OEM top end rebuild kit in the garage so i replaced the piston & rings while I was at it. No change. Still leaking.
Still no external signs of a leak. Weep hole looks good. So I replaced the water & oil seals in the waterpump anyway. No difference.

I thought at this stage maybe the radiator has a hole that's leaking a fine mist under pressure .. so got some 2nd hand radiators and fitted them. (My old ones were a bit bent and battered). Still no change. Still leaking coolant.

So over the months the leak has been steadily getting worse. Now it takes about 400-450ml every ride. If I stop after 30mins it takes 400ml. If i stop after an hour It takes nearly 400ml.

Kinda exhausted my ideas to fix it. Next step will be another head & base gasket change.. thinking maybe it's leaking into the combustion chamber? any ideas. Still no signs of an external leak and entered into an enduro in two weeks. Will not be doing any events until it's fixed. Only use she's getting at the moment is experimental use to try find the damn leak.

Thanks in advance. . confused

When you had the head off , how did the piston look ?? , was it completely clean of carbon (or mostly clean , not spotless) , or was it "normal" and sooted with a carbon buildup ??

If the piston was really clean , it would mean the coolant was leaking into the combustion chamber , if its was covered in soot/carbon , then there is no leak within the cylinder


Also when you ride what type of riding is it ?? , is it REAL SLOW tight single track ?? , or are you able to get speed up and ride thru the gears ?? , or somewhere in between ?? , as you will get some coolant loss due to steam , from going real slow all the time , if the bike heats up enough during a ride to steam over from lack of air going thru the radiators , you will get "some" coolant loss , as the overflow bottle is not a sealed unit , it still has a overflow , and coolant can and will escape during extended periods of low air flow , since air flow is the ONLY cooling the coolant gets unless (like me) you have a cooling fan on one of your radiators

In other words , do not expect it not to lose "some" coolant on a ride , especially if its a slow hard , or hot day ride , but if its a major loss and the riding was easy , then there would have to be another cause

Have you inspected the overflow tank for leaks ?? , maybe a cracked seam ?? , i run a 250X bottle on my 450R , and i still get a slight loss of coolant , but not enough to cause overheating even after several rides , and not nearly as much as i would without a overflow tank

A cup of coolant loss per ride seems extreme , if it was a head gasket and its not coming out the exhaust (white sweet smelling smoke) , and your piston does not look exceptionally clean , and not turning your oil milky colored , then it has to be going out the overflow

Thanks for the reply Jim.

I replaced the piston in November and still have the old one. It's black, carbon, normal looking.
The mating surfaces of the cylinder barrel and head had no noticeable lumps or bumps. I gave them a gentle rub of 2000grain sandpaper to clean them up and help highlight any irregular surface areas, and used new honda OEM gaskets in the rebuild. All seemed well.

Riding wise, there's some singletrack, some forest, and some bog/marsh that we cross, but there's plenty of forest / logging trails in between the rough stuff, and she gets up through the gears and blows wind past the radiator. Don't have a fan, don't require one.

There's no signs of a leak in the expansion tank, but it's not a sealed unit so wouldn't be under pressure there anyway. There aren't any signs from the radiator or hoses either. Nothing from the weep hole. A pressure test would be useful though.

I'll order new head and base gaskets this week and strip, examine, and refit the cylinder. Thinking either a gasket has failed, and coolant is escaping into the cylinder and being burned off, or the cylinder or head might be cracked. Hoping for a dodgy gasket, obviously.

Do you know, when the cylinder and head are off, in addition to the usual gaskets, are there any O rings required?

Only O-Ring would be the water outlet on the front of the head , and make sure the dowels are in place for proper alignment

As i mentioned , even though the tank is not under direct pressure , the tank can still be evacuated by means of the overflow tube that comes out of it in the event of a boil over , , as once the tank reaches capacity , all excess fluid , goes out the drain tube down below the shock ,then when it cools the radiator sucks back in whats left ...follow ? , so you would not necessarily see coming out while riding , unlike a 250R that would dump over the header directly out of the radiator and make steam that you would notice , , so you can still lose some coolant in slow areas , not sure if that is the case , but its a possibility

That was one of the unusual things I noticed. I was losing coolant from the radiator but not from the expansion tank. I'd return from a ride and have to put 100 - 200ml (or more) into the radiator directly, even though there was coolant in the expansion tank. Opening the radiator cap, with the engine stone cold, there was a 'hiss'.. and although I would normally expect it to be a release of air pressure, in this case I thought it might be a hiss of air being sucked in, filling a vacuum in the top of the rad. I figured that as the heat and pressure increased, and the coolant escaped out the overflow, it somehow failed to return during cooling, creating a vacuum. I know it's the same tube, flowing in the other direction, but it was my version of logic at the time.

Since then, I tried out the DRC 1.6 radiator cap. I let her warm up till the expanding coolant filled the expansion tank and boiled out the overflow. It only took about 10 mins at idle to get there. She cooled quickly and as the temperature and pressure reduced, coolant returned to the radiator normally, and the level of the expansion tank returned to normal. When she'd cooled enough and i opened the radiator cap she was as near to full as to make no difference. I figured I'd eliminated the expansion tank and rad cap as suspects, and moved on to the gaskets, and then from there to the water pump seals, and radiators.

I tested her out the other day, getting her through three or four heat cycles on a short 3 mile looped trail. At the end of each lap I checked and refilled the radiator from a full 500ml water bottle (I'm using plain old water now, it's cheaper than silkolene) and she took about 300-400ml at the end of every lap. When I did a double lap she didn't take much more.

I'm still guessing that there is an internal leak, that leaks quite badly when the system is full, and under normal operating pressure. When she's lost the first 300ml or so over the first couple of miles, I guess the loss of fluid in the system equates to a loss of pressure and the rate of coolant loss decreases. I've never needed more than 500ml, and that would've been probably after 30miles+ of trails so I guess that once I've lost that much coolant, the pressure is so low that either
1. I'm not losing it anymore because the pressure is too low, or
2. the level of coolant in the system is below the level of the leak.

2 above is unlikely as the bottom of the radiator is still higher than the top of the head. So I'm back to a blown gasket.

Regardless of the reason, I'm at risk of seizing the piston if I run her without cooling, so this has to be sorted quick. Parts will be ordered wednesday, and the bike will not be run again until they're fitted

Thanks for all the help Jim

Yea no problem , but its definitely strange , i would think if its a leak , the less water would equal more pressure because the more air taking up the space the water occupied , so to me it would mean less water = more pressure , but the issue remains is the water has to be going somewhere

Try this ,refill your system , then take a old plastic drinking water bottle (16 fl,oz/500ml) and place it under the bike where the overflow tube is , place the overflow tube inside the bottle and tape the bottle to the tube , then do your ride/laps , and see if any ends up in the bottle , this will eliminate overflow loss , or rule it as where the water ends up

The issue is the water has to GO somewhere , so its either mixed in with your engine or tranny oil , or its going out the muffler , or its going out the overflow , you need to pinpoint the exit location , so you know where to look for the cause of the issue , if you know where the water goes , then you can trace the path back to how it got there ....follow ??

tis confusing alright.. I'll strip down the head and pull the cylinder & piston during the week to have a look at her internals.

Will update on what I find. Have a 4x4 with fuel issues to look at first but that should be an easy fix

Had the day off work so replaced the fuel pump on the 4x4, and then got stuck into the bike.

The mating surfaces of the bottom of the head and the top of the cylinder seemed free of any dips or high spots. They got a gentle rub of 2000grain paper but it didn't show up any obvious issues.
- the scores around the top of the head (inside the combustion chamber) are from when a metal clip got sucked in the intake and bounced around in there before jamming a valve open and making me push for a while. The valves seal is good so considered it cosmetic damage. Bikes been good the past 3 years like this





Top of the piston - this one's been in there since November - has about 20 hours on it


The old piston>> unknown hours, a lot more than 20 though


New cylinder base gasket


and head gasket


There was nothing unusual about the old gaskets. no distortions or tears. no obvious signs of a failure or leak

ran her for a few minutes after all was put back together and everything seems ok. Top of the radiator was heating up to operating temp without any issues. moved around the bike with a torch looking for any steam or water escaping and found nothing. Will have to take her out next week for a proper spin and see if the problem re-occurs

Well hopefully , whatever it was you fixed it and you can ride again , the top of the piston looks to have oil ?? , if so maybe Valve Guide Seals are leaking a bit , its definitely got carbon buildup , so no sign of coolant getting into the combustion chamber , otherwise the piston would be clean

And yea , it looked like something cut loose in there , good thing you got away with minimal damage

If it does overheat again , do as i mentioned and fasten a bottle to the overflow tube and see if that is where your loss is coming from

Have you ever carried out a pressure test on the cooling system? Is the system still pressurised when you take the radiator cap off to top it up?

So, after replacing the gaskets, today was my first chance to get her out onto the trails and warmed up. Unfortunately, she is still losing coolant.

When the radiator cap is removed there is a hiss from pressure release

Did a bit of trial and error with mates riding along beside me. We found that at idle. or low rev/high gear/reasonable speed on the road, coolant loss is minimal.. but as soon as you wind her on the coolant (water) gushes out leaving a very nice black line behind me on the road.

Now, the coolant is not overheating and leaving under pressure in that way. It almost seems as though the cooling circuit is blocked and the coolant can't circulate and so pressure builds between the water pump and filler cap, but not on the other side of the loop. How else do we explain that the radiator loses approx 400ml, which appears to go to the expansioon tank. This is all pure guesswork at the moment.

We opened out our tool bags and checked the hoses. All seemed to be good. We removed the fuel tank and seat and traced the hoses between the radiator neck and the expansion tank, and the expansion tank overflow hose. No kinks or blockages.

Next we disconnected the expansion hose from the radiator and drained the expansion tank completely. Then, with the radiator full to the brim, and the expansion tank completely empty, I rode her up and down the street, not being shy on the throttle, and we found that in the space of 500metres, the expansion tank was full and overflowing, and the radiators were half empty. We tried this with two different radiator caps and got same results.

Tis wrecking my head.. but on the plus side - this is the first time we've been able to find a visible sign of where the coolant is going to.
All hoses will be checked and possibly replaced. Expansion tank too if required. Will see if I can get a pressure test done and will report back.

Fixed. The locating dowels between the head and barrell were preventing the two parts from seating correctly. Removed from the bike, with no head gasket, the head and barrell could get no closer than 0.25mm apart. Filing a very small amount of material off the dowels allowed the parts to come together correctly and it's now all reassembled with fresh gaskets again. Been out for a road test on Sunday and a trail test Monday and there's no loss of fluid.

Great news !!

It sounds like the dowels were/are out of round , possibly from grabbing with pliers , etc , when doing the last overhaul ??

Maybe order a new set (they are cheap) and when the next rebuild time comes along , you can swap them out !!

Hi Sprocket, Gianni here,
I have the same issues as you had, just to check,
My bike uses water at a round 50° it starts to spit from the neck overflow , changed radiator, caps, done pressure test and co2 test all ok but uses water
I am considering to do what u did , is it a big job and costly.
Thanks Gianni