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2016 YZ250X overheating

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Chasing my tail on an overheating issue. Bike is a 2016 YZ250 X, that my son uses for racing cross country. I only ride it after maintenance on it around the yard.  We run Motul 800 at 40:1 and 750ml of Maxima MTL 80. Bike has 130hrs and a fresh OEM top end installed around 100hrs and everything at that time looked very clean.

 

At a race two weeks ago, while doing my once over, I noticed the bike down around 150 to 200ml of coolant. Topped it up and sent him off. During the race the clutch obviously got hot and was slipping bad. Coolant was down around 150 to 200ml again. The week after the race I put a new Barnett clutch in with the 250X springs, and took the head off thinking it was the orings. Nothing very obvious, plug was a nice tan colour, good piston wash on the piston. Possibly looked like coolant getting past the oring? Didn’t check the flatness of the head. Ran it in the woods for 2 hours in practice, and coolant level didn’t drop, and all seemed good. This past weekend it was a hot and muddy race with lots of 2 ft deep ruts, roots, and very steep hills. People were dropping out like crazy. The bike overheated on the 2nd lap, and clutch lever was very tight indicating very hot. Bike needed around 700ml coolant to top up. Not sure it was all blown through the rad cap or ingested in the motor, or both? Oil was black and burnt smelling, and typically always came out the same colour it went in prior to these two races.

 

Checked the water pump and it was all good, and took the rad cap off and saw lots of flow. When revving it up it was hard to tell if it was cavitation, or cylinder pressure. Not sure if I am chasing a clutch issue because of too much drag, a cooling issue, or both. This time I will check the cylinder and head on a granite flatness table. Any suggestions would be greatly appreciated at this point. It should be simple, but maybe I am overlooking something.

Unfortunately these two races are going to kill his season.

I have a 2018 250X and will start comparing part to it. Possibly something as simple as a rad cap??? I will probably give him my bike for the next race till we get this sorted and feel very comfortable we got it.

 

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Does it pour coolant out the overflow? It could be as simple as a bad rad cap and it’s not building pressure. Swap it over and see if it does the same thing. 

If it’s using that much coolant, you should be able to notice it billowing white smoke when it’s getting hot. That’s almost a liter of coolant! 

Also, the weep hole on the water pump isn’t leaking or clogged? Is the oil milky?

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I am thinking rad cap also. No weeping out the water pump. oil was not milky, just lots of clutch fiber.

 

Unfortunately since I wasn't riding, and he didn't notice till in the race where it was to the point where it was very low. He did say it was blowing coolant at that point. But I am sure in a race condition, with the real bad conditions he wasn't paying attention till it was too late. We rode it days before after inspecting and replacing the head orings and it didn't use a drop of coolant. It was more of a fast paced trail ride though. 

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18 minutes ago, KissTheMud said:

I am thinking rad cap also. No weeping out the water pump. oil was not milky, just lots of clutch fiber.

 

Unfortunately since I wasn't riding, and he didn't notice till in the race where it was to the point where it was very low. He did say it was blowing coolant at that point. But I am sure in a race condition, with the real bad conditions he wasn't paying attention till it was too late. We rode it days before after inspecting and replacing the head orings and it didn't use a drop of coolant. It was more of a fast paced trail ride though. 

I would swap the cap and see if it works.

If not, let him use your bike for the races? Unless you race as well, but he’s your son, so do whatever.

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Replace the radiator cap first.

kid sounds like a clutch abuser tell him he will be faster if he lets out the clutch all the way.

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If so much clutching is required in current form,

perhaps consider a flywheel weight and a change in final drive ratio.

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We have never had an issue till just recently. Oil always came out spotless. We are original owner of the bike. He is very fast. It just seems something has changed recently. Perhaps it is as simple as a rad cap. We will have to do a small test one by one to see what fixes it. May as well start with the simple and most logical fixes first.

 

Thanks for the suggestions

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i'd put new pump seals in it just to cover it as a possibility and inspect the propeller shaft for bad grooving as well, make sure you put the seals in the correct way, people put those in backwards all the time and wonder why it never seals.   Other than that change some parts, rad cap, head gasket, run it hard and hot on a dry trail or dirt road and pull over and look for anything wet.  Make sure your drain tube by the cap isn't crimped closed or plugged up as well. 

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Posted (edited)

Make sure that there isn't coolant leaking out of the the weep hole between the cases on the front of the engine below the exhaust outlet.  There is an o-ring in the coolant passage between the cases that can deform and get pushed into the weep hole causing a leak.  It happened to mine, had to split the cases at 220 hours for a $7 O-ring.  This is why I don't run higher pressure radiator caps anymore.

Edited by borlo

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