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'01 YZ250F losing coolant under load

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Hello everyone! I'm new to the Thumper Talk Forum, however I have been digging around looking for the answer to a very specific and ongoing problem I have, and have decided to post this. I'll give the full story of my bike's problem and see if there is anyone out there who has had this problem. Last year I replaced the piston, rings, and cylinder on my bike in spring. I installed the high compression stock bore Wiseco piston and a used stock cylinder (the coating on my original cylinder was bad). The new/used cylinder is clean, no scoring, and everything went together well. After a few rides, the bike started losing coolant and I discovered that it would literally shoot coolant out the overflow tube when you twisted the throttle open. I assumed the head gasket was blown, and I was correct. I found it odd that a new gasket would go after only a few rides, so I checked out the flatness of the head to find that it had .003" of warp. Under the advice of Eric Gorr of Forward Motion I lapped the head until it was flat on a lapping table. I also lapped the gasket surface on the cylinder to make sure that was flat. Assembling the head to the cylinder (not bolting it down, though) with no head gasket showed that there was no mis-match between the two. So, I installed the new gasket and the problem of losing coolant disappeared. However, after a few weeks of riding I noticed the coolant level slowly dropping again, however nothing drastic. Winter came and passed, and when I pulled the bike out to ride this spring it kept losing coolant, more and more each time. I replaced the head gasket again, lapping the head and cylinder to make sure there was no warpage. Once again the bike is fine for a few rides then starts to lose coolant more and more each ride out the overflow. I have since had to do this two more times, however when I take the old head gasket out there are no signs of burning or blowby, even though the most recent time I could get coolant to shoot (about 10 feet!) out of the overflow when I load down the engine, even when it is still cold (choke on). Coolant was entering the combustion chamber when the bike was sitting, fouling the spark plug. However, removing the head revealed no burning of the black rubber coating on the head gasket. This time I took the head and cylinder to a shop where they checked them for flatness and said they were well within spec, both in terms of flatness and surface finish. I also watched them as they did a dye-penetrant (Magna-Flux) test to check for cracks in the gasket surfaces, and no cracks showed up. I pressure tested the intake and exhaust ports and that showed no leaks to the water jacket. Both visual and dye-penetrant inspection show no cracks in the dome in the head. I know that somehow, some way, compression is leaking into the water jacket, and I have run out of ways to tell where it is coming from. It seems like a head gasket because replacing it solves the problem for a few rides, but neither the head or gasket show any signs of leaking. If anyone out there has any ideas, please post them. Sorry for the long winded post, I just didn't want to leave any details out. Thank-you in advance for any replies!

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it could be a bad rad cap, but it seems like youve narrowed it down to combustion pressure.

there are very few places that the pressue could be coming from, the head gasket being the most obvious. what about the base gasket?

mabe there is a clog in the cooling system causing pressur to build up and if you also had a bad rad cap it would probly spew coolant pretty good.

thats weird whats goin on, i bet your gettin pretty frustrated.

anyone else have ideas? youve stumped me :thumbsup:

good luck

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