overheating yz400?? HELP

I Have a 99 yz400 that i have converted to mostly trail riding. the only thing i have changed is clarke gas tank, and the sprockets to 13-53 i have had the bike since 99. first owner. when i rode it on the track it never over hearted. but when i get in the tight stuff she over heats rather easy. my riding partners ride 426s, nad another 400 and they hardly ever boil over. my bike runs very strong still starts very easy. no problems other than the overheating. i had the radiators rebuilt a couple years ago, and not much change if any. the bike has had regular oil, and filter changes for maintenace and has required little else the bike has been bullet proof with the exception of overheating. I understand that when its working hard and not moving fast it will get hot, but it seems to probably boilover 3 times to 1 to my friends riding the same stuff. any suggustions would be appreciated. thank's

WHat are you using for coolant? Also, might want to check the radiator cap. If the spring is weak, it will boil over sooner.

I have a 99 YZ400F and about 2 weeks after I bought it, the bike overheated.

I replaced the radiator cap with an pit posse 1.6 psi radiator cap and flushed the entire system and replaced the Fluid with Engine Ice.

its been about 7 months now and I have not overheated once (all I do is trail ride)

I also rebuilt the water pump and replaced all of the seals, the impellor Shaft, and the water pump bearing.

I would say start with the new fluid and a new radiator cap and if you still have issues i would check to make sure the water pump is working okay.

It was a pain in my A#$ but after I got it all fixed, I now know my Yz400F is not going to leave me on the side of a trail because of overheating (other issues is a completely different story:bonk: :crazy: )

I hope this helps!!!!

Patrick M.

I suffered with the same issue on my 400f.I used engine ice coolant and fit a wr overflow bottle under the rear fender an the problem was a fix.I did not have to tamper with anything else.

engine ice and a new cap cured mine too

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