03' 450f, coolant from rad cap rev'd in 3rd/4th.

So i have a very odd situation.   I have an 03' yz450F.    recently i replaced the impeller shaft, impeller, bearings, and both seals.    I rode two rides before i had to put it up for the winter.


Upon taking it out, after winter storage, 5 kicks and it started right up, i let it idle 2-3 minutes to the point of getting its usual cherry hot.  And shut the bike down and went inside for my gear.


I took it up the road to my buddies house, and upon stopping i had antifreeze all over my pants, crotch.     Coming back i noticed it was spewing out of the radiator Cap and it was almost frothy.

During winter storage since i had just done the water/pump etc, it had fresh 50/50 mix.  


I did a few tests, and noticed that 1st and 2nd gear when the bike was revved high, it was ok.   Only when i revved it high in 3rd or 4th, did the antifreeze start spewing outta the radiator cap, which makes absolutely no sense to me.   I can rev the bike in neutral, and it wont spew fluid out of the cap.    Considering the impeller is crankshaft driven, and not gear drive, what the hell is causing this?


I'm assuming a blown head gasket?   The guy i bought the bike off, said he had replaced the rings and gaskets.   So i'm a bit worried if it blew another head gasket.  Perhaps he warped the head, torquing it wrong?  


Guess i'm just asking for a few spots to check.  I already bought a new radiator cap thinking that might be it.  no go...still does it.


my local mechanic i dont think has a leak down tester....but he does have a cooling system pressure tester. 

Should i have him pressure test my cooling system? (i'm a bit afraid he may force antifreeze back into cylinder, if he does this).


suggestions folks?    or should i just get a new head gasket, pop it in, and grab a mirror and check flatness of head?  (i've never worked on 4 strokes before really in the cylinder area)



EDIT:  This bike runs flawless, carb issues worked out, etc.   but for some reason, i keep getting  coolant issues, its beginning to annoy me lol.   getting sick of smelling anti-freeze.

Oh, and can anyone suggest or rather link me a good deal on a descent Leak Down Tester? =) 

Edited by grayracer513

Probably an head gasket.  The engine works harder pulling 3rd or 4th than it does 1st or 2nd, and spends a longer time doing it.  Don't over-analyze.

Thanks gray. Yeah i have a tendency of over analyzing. I imagine he just put in a new cheap head gasket....and those are crap.

I ordered new OEM head gasket. Base gasket. New head bolts and washers. New exhaust gasket and midpipr gasket.

Im a bit worried about pulling the head off. Ive never messed with timing chains and all that befote.

Any suggestions of thinks i should do as i take head off? Mark spots on cams and timing chain? Im very nervous about this. But i figure i gotta learn somehow.

You need a service manual:






Pay careful attention to the head torque procedure:



Marks on the chain won't help.  Set the cam timing according to the manual instructions.  Not that hard.   Check the valve clearance before you pull the cams and DO NOT move the lifters from one valve to another.  Leave them in the head until there is a need to remove them, then lay out each lifter and lash shim in its proper orientation in a safe spot on the bench.

So im going to need feeler guages.

And two torque wrenches; one capable of reading accurately at 10-75 ft/lbs, and another capable of reading accurately at 3-12 ft/lbs (near zero to 140 inch pounds) .

i dont think i'm going to touch the lifters if i dont have too. 


Do i have too?   basically i wanna get the head and cylinder off.....bring it into my mechanic, have him check to make sure head isn't warped.....then just reassemble.


it looks like i have to take the cams out, but not the lifters?  shouldn't they stay in place?


i'm sorry for all the noob questions Gray.




Also, i saw the tightening sequence.  threw me off a bit.  It talks about tightening them to a 1st torque spec, then 90 degrees, and then loosen them, then tighten to new torque spec + 90 degrees.   But then it says final is 180 degrees?

So i'm guessing i do that, but then when i'm ready and they are tightened 2nd time + the 90 degrees on all 4 bolts.  i tighten them a further 90 degrees on each one?

They can stay in place, yes, as long as you never turn the head upside down to, say, clean it, or anything. ;)


Probably worth having him pull the valves, inspect them, and install new seals.

They can stay in place, yes, as long as you never turn the head upside down to, say, clean it, or anything. ;)


Probably worth having him pull the valves, inspect them, and install new seals.


what would be a average cost of him doing so, parts wise? (not counting labor).   and time wise?    



i'm at the point now. of debating whether to keep the bike, i might just sell it after i fix known issues, and get a newer two stroke 250.   i'm really not liking the maintenance on these 4 strokes so far lol.    

Seems i'm working on it, far more then i'm riding it (which would be fine, if i was installing cool aftermarket parts, but i'm not!)  LOL!!  :ride:

Edited by soulsedg3

The seals are about $8 each (x5).  If it doesn't need any other work, he might not charge much to install them.

The seals are about $8 each (x5).  If it doesn't need any other work, he might not charge much to install them.


awesome. thanks gray, you truly are the best!!

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