What is the deal?........coolant geyser.......86 250R

[InPiEcEs]

I got my R buttoned up for the first real actual run time.
I put the exhaust in place, bolted the carb on, and put the cooling system in place all the way, with new overflow hoses, and the radiator, and cooling hoses are fine.
After running in the garage for 7-10 minutes, tops, at idle and a little throttle blipping to keep it clean, it's blowing out the overflow hose like a geyser.
I pulled the head, thinking it was the CR gasket, I threw in, but that's all good. The water pump is all good, with new seals , and all, and no crud in the cooling system........it's been cleaned up nice. Radiators are spotless inside and out.
What would cause this? Did it get that hot that quick?, Temps here have been in the 50's.

[InPiEcEs]

Forgot to mention......it has a 36mm PWK, prejetted( close but obviously never right on), and a PT High Rev, FMF silencer on it.
The coolling system is good and clean, but as mentioned, coolant is blowing out the overflow hard, after 10 minutes max running in the garage, like a warm-up.

[2strokeBandit]

Well if the engine got warm which i imagine it did. Its just the coolant expanding. Is the cap on good and tight. Is the cap rated for the right psi. If thats the case i would let it run a little more out and see if it stops. then you know how much should be in there when the engine is warm. It has to have some room to expand

Just my 2 cents

[Mr. Sandman]

It sounds to me like one of the hoses is plugged or your head gasket is flipped over and the cylinder & head passages are not flowing as they should. Or, your water pump/impeller is not turning.

[x.system]

Sounds like you have air in the system, fresh build? did you run it without the radiator cap to get all the air out of the system? I always do a couple heat cycles without the radiator cap, or just sit the cap on it and leave it loose.

[ATC-Eric]

Yeah, sounds like you have blockage, or the impeller is not turning, and the thing is just boiling out.

[InPiEcEs]

I think it just needs to burp out like x.system said.
I had it running today for a bit with the cap off, and my coolant dropped a little.
So, I topped it off, and it seems to be fine now......thank god!!
I just never saw one shoot coolant out the hose like that before.
Thanks for the help everyone.