YZ hybrid getting too hot

[gtilley45]

I thought I had this taken care of but I went to a race a couple weeks ago and it happened again. Apparently, I'm too slow and out of shape to push the bike very hard but a buddy of mine's bike was down so I let him ride mine. He's pretty fast (was running pro am production a couple years ago) and when he pulled off the track, the overflow tank was full and coolant was squirting out of the cap. When the bike cooled off, all the coolant emptied out of the tank and went back into the radiator.

I've been reading that a lot of people put high pressure radiator caps on the dirt bikes to help with this so I'll probably go ahead and get one of those since they aren't too costly. The bike already has the boyeson water pump. The only things I can think of is the radiator and maybe the bottom hose set up. When I had this quad apart for a rebuild, most of the fins on the radiator were bent but I straightened them the best I could. Also, the bottom radiator hose gave me some headache. The one that was on it wouldn't work with the exhaust I'm using now because the hose would hit the header. So I am using the water spigot and bottom hose from a YFZ and it goes up and over the header. What has me wondering is that the spigot is a tad taller than I'd like but not too bad, but it still has a little bit of a bend in that bottom hose where it connects. Is it possible that the bend in the hose could have caused a coolant flow problem that could have caused an air bubble which pressurized the system?

Sorry for the long post, just trying to give some details.

[89trx250r]

Did you let the bike sit and idle and warm up etc? I know it sounds silly but on my buddies Walsh yz if he doesn't just let it warm up for just a minute and start ripping it will start to overheat because it doesn't suck enough air on its own...You gotta remember these motors are coming out of dirtbikes which have 2 radiators. They may both only be as big as our one but they are much more efficient. I was just putting this out there as a possibility not saying its the cause. After just going to review your build thread your hose doesn't look too bad at all nothing that a healthy impellar couldn't manage. Goodluck bud!

[gtilley45]

I usually will crank it and run it long enough to check the oil and stuff like that. But I don't let it sit and idle too much. It's always got hot if it isn't getting enough air flow to the radiator, the CRF hybrid I used to have did the same.

I thought of that about the water pump too. But I was still wondering because the flow goes from top to bottom, right? So I was wondering if the pump just got the coolant back to the top of the radiator and gravity pulled it through to the bottom of the radiator or if the pump basically pressurizes the whole system.

[gtilley45]

Also, you said you looked at my build thread. Does your buddies Walsh YZ have the same set up as mine as far as cooling? I have been seeing the guy on here that has made Walsh replica radiators and thought about one of those, but I like the filler tank for the radiator up by the shock towers and was just wondering if your buddies was like that. Other than a little extra coolant, I don't see it being much better other than just being cool. LOL

[laeger400]

I might just make some more radiators I am for sure
making one for my Walsh.

Jerry

[gtilley45]

Well if you do, I'd definitely be interested in one.

[laeger400]

I will let you know. I am really busy with work and family stuff.

Thanks Jerry

[gtilley45]

I understand. Plus it's that time of year when everything gets crazy.

[hontrx265r]

Its your head gasket.. pretty much guaranteed. I've run built yz's with stock radiators with really no problems. The smallest pin hole leak in the head gasket can over heat the bike and boil over in minutes. Not sure when or who built your motor but OEM only on headgasket.. just my 2 cents but I've had a couple..haha. Also it will not smoke, run odd or really show any other signs of trouble. You can check your plug, if you have yellowish/green residue or any tiny crystals on the electrode are usually signs.
Someone sent me a picture of a broken exhaust valve yesterday (ltr which i'll post later) to which I wondered how/why until I saw the head and knew right away it was leaking and he over heated it bad just to start with.

[gtilley45]

I appreciate the answer. I'm about to go out and do some maintenance on the quad and go over a few things. I'll pull the plug and look at it while I'm at it. To be completely honest, I'm not sure when the motor was rebuilt last. I know it has "TC national 440" engraved in the head but that's about all I know. I would like to freshen it up and go through it real well, so I might try to do that over the winter.

[hontrx265r]

I actually thought for some reason you were running an 03-05ish 450 knowing its a 440 and probably a big bore not a sleeved down 450 your even more likely to be a head gasket with the 426 done to 440. You can't get oem gaskets obviously that are 2mm over. I still had better luck opening up an oem gasket with a drum sander cleaning it really well and still using the oem on my big bore motors.

[gtilley45]

It is an '03 YZ450F motor. But the cc limit was still 440 for 4 strokes so they put a sleeve in the cylinder to de-bore it and keep it under 440cc's. But whenever it gets rebuilt I'll just put a new OEM cylinder on it.