YZ450 A little too hot

7 replies to this topic
  • 450GN

Posted July 21, 2007 - 01:41 PM


well this is on a 04 yz450

this is my friends bike that we have just overhauled...

and it has a tendency to run hot it seems...

idling for over a minute exhaust is glowing red... i know its not good to let idle but this turns red excessivly fast

and the bike does not want to start up once it gets warmed up that is the main problem... if it stalls... well it takes about 10-15 mins of cooling till it starts up again

and it has proper coolant...
proper oil level...
everything seems correct... maybe jetting problem?

well one more thing... my friend actually loosened the carb throttle cables and then readjusted... maybe they are too tight or too loose? but throttle tube feels perfect

  • grayracer513

Posted July 21, 2007 - 03:34 PM


How does it run when you ride it? If it's hard to start hot, the first thing you might check is the idle mixture. Set the fuel screw up and see what happens. Recheck your cam timing if the performance seems off.

  • Octane250F

Posted July 21, 2007 - 07:11 PM


Its normal for the exhaust to glow red if its just sitting idling because of how tin the walls of the exhaust are.
You shouldnt let it sit and idle for any longer than 30 seconds. It has no cooling fan, so when it sitting in 1 spot it wont take long for it to overheat.
If I let my YZ 250F sit and idle in about 30 seconds the headpipe would start to glow red hot and it will start to push a little coolant out of the overflow.
About the hot starting issues, its normally for a 4-stroke to be hard to start when its hot. Are you using the hot start lever? Id check the valve clearances and make sure they are right. Valves that are out of adjustment can cause starting issues. I know you said you guys just overhauled it, but its possible the valves are out of adustment.

  • CaptainKnobby

Posted July 21, 2007 - 07:34 PM


I would say it might be in the timing maybe. My 06 runs pretty well in the hot summer months. I have a DRD full exhaust system and have never seen my head pipe glow or even the antifreeze boil out. Although however I used some oxyigenated gas last weekend for the first time and the temp that day was around 88 degrees outside and after the second moto that day when I came back in I noticed the gas bubbling up the gas vent hose for about a half minuite and then it quit. I have all ways been able to restar mine after I stall it at the track with the hot start and a few times with out it. I would just go back and check the valve clearence to make sure..... my mechanic friend that owns the yamaha shop that I bought my 06 from said when you check the valves just make sure you can get the smallest feller gauge under neath them cause the valves rarely get loose they just get tighter.

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  • 450GN

Posted July 22, 2007 - 02:57 PM


valve timing looks right on.. but motor is lacking SLIGHTLY in compression maybe i am crazy and it is off slightly...

check valve clearance before motor was torn apart and they were all in spec

i richened his fuel mixture screw 1/2-1 turn and it is a little better he said for restarting but still not as easy as before

i took out his decompression plug from the carb and reinstalled since i thought it was a little twisted and again it helped slightly

again performance is excellent... it is running now faster than ever and feels perfect... just that once you stall it or kill the motor... you have to wait about 10 mins to start... really wierd

  • grayracer513

Posted July 22, 2007 - 04:17 PM


i took out his decompression plug from the carb and reinstalled since i thought it was a little twisted and again it helped slightly

OK, now we're getting somewhere. There is no decompression device in the carb. If you mean the plunger connected to the cable that ends up at the lever on top of teh clutch, that's the Hot Start. Be certain that the plunger has free play, and moves freely, and seats completely in its bore. If not, you'll have an air leak, and teh bike will run lean.

  • 133_Bradley

Posted July 22, 2007 - 04:37 PM


I got a 04 Yz450 with a JE high comp. piston and I just started running engine ice coolant and I dont use the hot start lever no more.

  • 450GN

Posted July 22, 2007 - 07:42 PM


yes yes thats the plunger grayracer...

i am not very good at the names of somethings... as you can tell

but yes that is what gave the most improvement but i still think something is not right...

ok... on his bike... start it up and when it is warm.. you pull the hotstart lever and motor revs... then you release and it hangs for a min before the rpms come all the way down

like there is a hesitation...

and on my bike... you release the hot start and the rpms snap back down to normal idle

so i think there is a problem here... maybe just the cable needs better adjusting but i am starting to think that this is the problem

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