3 replies to this topic
  • boricua1

Posted August 11, 2011 - 03:23 PM


Does anyone have the slightest idea on what the operating temp is for a 99 yz 400f? the last two times that i have ridin it it has either boiled over or its reached 250 degrees and does not idle. So far i adjusted my pilot screw and have that dailed in and and also did the jet clip but on the clip i had to move it to the sixth slot down from the top and theres only seven slot. I have it to idle but the idle screw is still all the way in. You think i just need to jet a jet kit or wat? And would rejetting it make it run cooler?

  • grayracer513

Posted August 11, 2011 - 04:23 PM


No, it's very unlikely that jetting will make it run cooler unless it's so far off that it runs badly because of it.

If you do a lot of low speed riding in and around tight trails or spend more than a minute at idle, that's your culprit there.

The 400 had smallish radiators, and that makes matters worse than with the rest of the YZF line. Radiators from a WR, or from a YZ426 will help, but you should also make sure your radiators are clean and the airflow unobstructed by debris.

Temps will range from 140 or less at an easy 30-40 mph cruise upward to a boil on a tight trail.

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  • yzf1999

Posted August 14, 2011 - 12:51 PM


water wetter helped my 99 400f...pure in the whole 12 oz. bottle...mine never boiled over and woods riding was pretty much all i did with that bike.

  • yzf1999

Posted August 14, 2011 - 12:53 PM


oh..i also made that bike a 426 with a kit from thumper racing...never changed the radiators and it never boiled over

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