new to me yz400f

13 replies to this topic
  • 90atk

Posted March 31, 2014 - 04:30 PM


just got a 99 yz400f i really like the bike. ive been told they overheat when riding slow. how can you tell if the bikes overheating? i dont want to mess up the bike. thanks.

  • lkeffersyz

Posted March 31, 2014 - 04:34 PM


Well if its overheating the radiator cap should overpressure release some fluids out of a semi-clear hose the runs from the fill hole. Also your pipe will glow up around the head port at night the whole header would glow

  • 90atk

Posted March 31, 2014 - 04:54 PM


ok so itll be obvious. thanks. do you guys run that special anti freeze thats supposed to keep it running cooler?

  • lkeffersyz

Posted March 31, 2014 - 05:41 PM


In the summrr run a little less so its thinner. Also I would consider I higher psi pump maybe in the future if you have cooling issues. If not don't sweat about it

  • 90atk

Posted March 31, 2014 - 06:12 PM


Ok i have another question i wana do a compression test ive always done them cold but the book i got with the bike says to warm it up then check compression. So do i check it warm or cold?

  • lkeffersyz

Posted March 31, 2014 - 06:14 PM


Start it choke it count to 10 and then check
Its what I've always heard and done and its supposed to be accurate
A cold dry cylinder leaks a little a oil wet and warm one is your compression you have when running

  • 90atk

Posted April 01, 2014 - 04:38 PM


anyone know what the red knob on the left is for? im guessing the one on the right is hot start. or are they both hot start?


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  • lkeffersyz

Posted April 01, 2014 - 04:45 PM


The one under the choke is hot start for sure but idk about the other I've never had a yz with that


  • grayracer513

Posted April 01, 2014 - 05:47 PM


That's a hot start, too.


You have a YZ400F with a YZ426 carb on it.  A hot start is simply a way of allowing extra air into the intake stream to help starting when the engine is at full temperature, and tends to "flood" over any extra fuel at all.  On the 400, the push/pull air valve was bolted on the frame and a hose run from the clean side of the filter to the intake port.  On the later bikes, the valve and the air path are built into the carb.


The external hot start assembly, hose and all, can be removed.  Just be sure you cap off both ports where the hose connected and secure the caps against pressure (in case of a backfire).

  • 90atk

Posted April 02, 2014 - 04:50 AM


ok is there any down side to having the 426 carb on a 400? the bike runs good.

  • 90atk

Posted April 02, 2014 - 04:52 AM


also can these bikes be coasted down hill for a while with the engine off? or will it need oil pressure?

  • grayracer513

Posted April 02, 2014 - 06:38 AM


ok is there any down side to having the 426 carb on a 400? the bike runs good.


No, it's a newer, more advanced carb.


also can these bikes be coasted down hill for a while with the engine off? or will it need oil pressure?


I wouldn't coast it more than a half mile or so, and then only right after it has been running.  The trans is pressure lubed, and even if it was splash lubed, it couldn't oil itself by driving only the output shaft.

  • 90atk

Posted April 02, 2014 - 05:44 PM


ok thats what i figured. is there any way to put more tension on the throttle? it seems too easy to turn it. everytime i hit a bump and my arm moves it revs the throttle. maybe i just need to get used to it.

  • grayracer513

Posted April 03, 2014 - 06:48 AM


Without opening up the throttle linkage on the side of the carb and leaving it exposed, I don't see a way to add a spring to it.  OTOH, if it's really a lot lighter than other contemporary 4-strokes, it might be that the throttle return spring on yours is not properly in place.  Compare with another bike.

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