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Hi guys, Ireland had sunshine today, and seeing as it might be the only sunny day of 2014 I figured I'd take the 250x up some forest trails. . It was her first run since I did a bit of head work on her last month and everything went fine, of course then I got home a few hours later, shut her off, and noticed a small coolant leak. Balls.

She was seeping coolant from the front of the head gasket, slow but steady. There's no visible contamination of the oil or coolant, when the bike had cooled a bit I dipped the rad and it was only about 1/4 full. Topped it up and she took about 400ml. Expansion tank had loads left. Topped up the oil too - it was at the bottom of the normal range.
Anyway, obviously I need to replace the head gasket, and I'll get to it next week.. in the meantime my question is ..

If theres loads of coolant in the expansion tank, and theres a small leak from the motor, I would've thought the expansion tank would keep the radiator full.. but it didn.t .. . . why not? airlock?

One of 2 things. Either it didn't cool enough to create enough vacuum to draw overflow coolant back into the rad, or much more likely, that if coolant can get out, air can also get in. If the air can get in before sufficient vacuum can draw reserve fluid up, then it'll never get back to the rad.

hmmm.. must examine all my hoses. The leak is literally just a seep. The gap it's coming out is so small that it can't really be seen, and if the small trickle of coolant wasn't magically appearing where the head meets the barrel you'd think it was soaking through the aluminum. If air is getting in, I'd say it's getting in somewhere else.
Never mind, I'll sort the gasket, examine all the coolant hoses, and get her up to operating temp and inspect for leaks before going too far next time

I can't say then. When you opened the rad cap did it suck a lot of air? Could also be that with that much air space it just couldn't establish enough of a vacuum while cooling off. With my VTX if the rad cap doesn't function properly it will go so far as to collapse hoses. Could be the rad cap isn't allowing fluid back. It's been known to happen with the bigger bikes, so maybe....

suck? . I though the little hiss was air escaping. I let her cool s bit before opening and it was only a small hiss. like with one 's'.. like that time you open a soda bottle the 2nd time.
cooling systems to me, are kinda like suspension. it's some kind of dark art. magic from a bygone age. I can replace a valve shim or a chain or anything with nuts n bolts. but magic airlocks and vacuum something or other. sure ya may as well be talking some mad gypsie language. I'll throw a new head gasket at her and change the oils and flush and replace all the coolant. after that I'll say a prayer to saint somebody and hope for the best

LOL

Your plan is probably best.

None of the following will solve your problem, but hopefully will help you to understand how this works. If you just don't care and just want to see it work thne please feel free to ignore the rest of this post.

The cooling system is simplicity itself, especially in what we are discussing. The radiator cap is basically a 2 way valve. When the pressure builds, it releases vapor or liquid (if wanting to overflow) into catch reservoir. The tube into the reservoir goes to the bottom. The reason for this is that once the fluid and any air that's in the system cool, it will contract and create a vacuum and pull fluid back out of the reservoir into thew radiator. If you take a plastic drink bottle and fill it with hot water, empty it and then cap it, you will notice that the bottle will collapse a bit as it cools. This is because it is creating a vacuum internally as the air inside cools. There's not much magic in that.

If you heard air or vapor escape as you opened the radiator, the air had not yet cooled to a point where a vacuum was created and there was no vacuum to draw fluid up from the reservoir to the radiator. As the radiator is well above the reservoir, as they always are, it takes some force to motivate the fluid to make the journey to uphill to the radiator. Normally it would be that the radiator is at a lower pressure than ambient (a partial vacuum). The difference in pressure would need to be about 24" of water or about 1 psi (give or take) lower pressure in the radiator than your current ambient air pressure (about 14.7 psi at sea level).

[quote
The cooling system is simplicity itself, especially in what we are discussing. The radiator cap is basically a 2 way valve. When the pressure builds, it releases vapor or liquid (if wanting to overflow) into catch reservoir. The tube into the reservoir goes to the bottom. The reason for this is that once the fluid and any air that's in the system cool, it will contract and create a vacuum and pull fluid back out of the reservoir into thew radiator. If you take a plastic drink bottle and fill it with hot water, empty it and then cap it, you will notice that the bottle will collapse a bit as it cools. This is because it is creating a vacuum internally as the air inside cools. There's not much magic in that.


That there is an excellent decription ! Nice job.
If you heard air or vapor escape as you opened the radiator, the air had not yet cooled to a point where a vacuum was created and there was no vacuum to draw fluid up from the reservoir to the radiator. As the radiator is well above the reservoir, as they always are, it takes some force to motivate the fluid to make the journey to uphill to the radiator. Normally it would be that the radiator is at a lower pressure than ambient (a partial vacuum). The difference in pressure would need to be about 24" of water or about 1 psi (give or take) lower pressure in the radiator than your current ambient air pressure (about 14.7 psi at sea level).[/quote]

Did you torque the head bolts up properly? i had a leak after i did my re-build and i realised i hadn't torqued the bolts up correctly......as i'm assuming you put a new gasket on when you did the head? i wouldn't think it would fail unless it was a dodgy one or not installed properly.

Harmsy

Ya I torqued her up ok but I left the old gasket in. then I spent a few hours fitting a new head gasket yesterday, so hopefully all should be good (the old gasket was in a fairly sorry state) Will try taking her out this week if the weather is half decent, get her up to temp and check for any signs of leakage.
have managed to convince myself that it'll be fine, which completely frees up my mind to worry about other things, like women, and beer.
If anyone wants me, I'll be in the bar