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Coolant pouring out of overflow as soon as bike starts

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Hi, I've recently purchased a 2 stroke 2007 Honda CR 250.

The bike has been standing for 6 months.

I topped up the coolant till just below the neck and replaced the radiator cap.

As soon as I start the bike the coolant pours out of the overflow pipe, if I rev, more seems to empty out. Haven't driven the bike as I'm scared it might lead to bigger issues.

Any idea what might be the problem or what I can do before I go a mechanic and pay through my teeth

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Sounds like a warped head. Exhaust gasses are making their way into the coolant system which is causing it to bubble in the radiator. It's simple to check this. With the bike cold, take the cap off the radiator, start the bike up and watch the coolant. If you see bubbles within the first few minutes then you have exhaust gasses getting into your coolant.

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Bad head gasket possible.  Replace with OEM honda and plain head on peice of plexiglass or something flat. Check simple stuff first ofcourse but probably that🙂

Edited by Motox367

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If it was fine last time you rode it there is a very good chance it's just an improperly installed or defective rad cap.

If it's not the cap it's a blown head gasket/warped or cracked head/cylinder.

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Too much blinker fluid it sounds like.

Likely you filled it up too much.

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2 hours ago, woods-rider said:

Sounds like a warped head. Exhaust gasses are making their way into the coolant system which is causing it to bubble in the radiator. It's simple to check this. With the bike cold, take the cap off the radiator, start the bike up and watch the coolant. If you see bubbles within the first few minutes then you have exhaust gasses getting into your coolant.

There is enough turbulence in many cooling systems that this test is not conclusive.  Some will shoot coolant out with the cap off as soon as the engine starts.  Most will just froth and bubble and make a small mess.

Exhaust gases will definitely cause overpressurization of the cooling system, resulting in coolant loss through the overflow without overheating.  Only caveat here is if the cap itself is defective and not building pressure.

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I know on my 88 cr500 it was doing the same thing as the head gasket was blown. The head seemed ok but took it to a local machine shot to have it milled. The guy couldn't figure out how to mount it to his equipment or didn't want to figure out how so I got it back. Ended up buying a pro design cool head which uses o-rings instead of the normal head gasket. I haven't looked back the cool head sealed up nicely. Problem solved.

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Sounds like bad head gasket, thinking maybe your exhaust gasses are getting through the gasket and into the cooling sleeves. Your lucky it’s a 2t! Just get a torque wrench and whatever size it is, pop the head off, old gasket out, Honda oem one in, torque head down and your good to go. Also those coolant caps should resist coolant release I believe to 12 psi, if the new ones a Chinese special, it could very well just let all the coolant out.

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