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Discussion Starter #1
My CBR h is spewing coolant from the radiator cap at idle as soon as it gets to operating temp. I replaced the cap thinking it was the issue but still dumping coolant out the cap. I got to looking and it seems the lip on the radiator filler neck is bent down so I straightened it the best i could and put a rubber stopper in under the cap to totally block of the cap and put the cap over it. Still blew out coolant. So i ran a compression test thinking blown head gasket but all cylinders read great compression. My question is simply by me blocking off the reservoir and cap but putting in the stopper would it create enough pressure to blow by the stopper and out the cap or is there excessive pressure coming from somewhere else? If it is normal pressure that means i have to junk a perfectly great radiator over a stupid bent lip or is there any other fix you guys can help me think of.
 

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My CBR h is spewing coolant from the radiator cap at idle as soon as it gets to operating temp. I replaced the cap thinking it was the issue but still dumping coolant out the cap. I got to looking and it seems the lip on the radiator filler neck is bent down so I straightened it the best i could and put a rubber stopper in under the cap to totally block of the cap and put the cap over it. Still blew out coolant. So i ran a compression test thinking blown head gasket but all cylinders read great compression. My question is simply by me blocking off the reservoir and cap but putting in the stopper would it create enough pressure to blow by the stopper and out the cap or is there excessive pressure coming from somewhere else? If it is normal pressure that means i have to junk a perfectly great radiator over a stupid bent lip or is there any other fix you guys can help me think of.

Is it running hot or does it do this even when cold?
 

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Discussion Starter #3 (Edited)
It only does it after it get to the half way mark on the temp gauge or a hair above it. I can take the stopper and cap off, top it off with coolant and sniff it for exhaust fumes with my test kit I get nothing and the coolant stays put until the bike warms up then it starts to push it out and steam. I can't think of a way to seal it off so it will stop gushing coolant. It doesn't run hot until it had sat and ran for about 20 minutes but by that time I'm sure it has leaked all the coolant out is why it starts to run hot. The fan kicks in at the 3/4 mark like it should so at least that's working. Water pump is pumping great but the cap will not keep the coolant in. I need a way to repair rather than replace if possible. I have everything you can think of at my shop to make repairs just can't think of a way to do it. I can tighten the cap and see the gap under it because of the bent lip but i figured the stopper which fits in super tight would be enough force to keep the coolant from pushing out unless all i am doing is makeing the system over pressurize because the pressure has no reservoir to release into. Or should it not have that much pressure I'm not sure. It only has 15000 miles on it but the previous owner dumped it out of his truck when he was loading it up to bring it to me I gave $500 for it and it came with a box of brand new never installed complete body work. He told me it had a bad ignition switch and starter which both turned out to be a blown main fuse so this guy was an idiot of that I have no doubt.
 

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Sounds like you have a bad filler neck, specifically the part you bent back.
You may have to replace the radiator or perhaps replace the neck if you can find the part and a radiator shop that will do it.

It only does it after it get to the half way mark on the temp gauge or a hair above it. I can take the stopper and cap off, top it off with coolant and sniff it for exhaust fumes with my test kit I get nothing and the coolant stays put until the bike warms up then it starts to push it out and steam. I can't think of a way to seal it off so it will stop gushing coolant. It doesn't run hot until it had sat and ran for about 20 minutes but by that time I'm sure it has leaked all the coolant out is why it starts to run hot. The fan kicks in at the 3/4 mark like it should so at least that's working. Water pump is pumping great but the cap will not keep the coolant in. I need a way to repair rather than replace if possible. I have everything you can think of at my shop to make repairs just can't think of a way to do it. I can tighten the cap and see the gap under it because of the bent lip but i figured the stopper which fits in super tight would be enough force to keep the coolant from pushing out unless all i am doing is makeing the system over pressurize because the pressure has no reservoir to release into. Or should it not have that much pressure I'm not sure. It only has 15000 miles on it but the previous owner dumped it out of his truck when he was loading it up to bring it to me I gave $500 for it and it came with a box of brand new never installed complete body work. He told me it had a bad ignition switch and starter which both turned out to be a blown main fuse so this guy was an idiot of that I have no doubt.
 

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Discussion Starter #5
I will try to find a compatible neck and I can weld it myself. I wonder if a honda car would have around the same diameter neck. If not I will go to a local bike boneyard and try to pick one up. If that fails I will eBay another complete unit.
 

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You might be able to do a reverse look up by seeing what vehicles fit that same radiator cap. Worth a try anyway.

I will try to find a compatible neck and I can weld it myself. I wonder if a honda car would have around the same diameter neck. If not I will go to a local bike boneyard and try to pick one up. If that fails I will eBay another complete unit.
 

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Are you sure its over flowing because its not circulating properly? most car/bike radiators flow in one direction through one end of the engine out the other and through the radiator. often goes from left to right of the bike with the radiator cap being on the right side... have you checked to see if there is anything clogging the system causing it to over pressurize and leak at the cap? might be time for a system flush before you think about replacing the radiator.

a good way to spot clogging in coolant system is get the bike up to temperature and then feel the radiator hoses... if one is hot and one is cold... chances are you got our self a clog or a bad or maybe over worked water pump
 
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