Wr 450f "05" Wont start when hot

8 replies to this topic
  • 4esh_numa

Posted July 26, 2010 - 02:47 PM


Long story short.... i recently broke my leg and have been unable to ride my bike for approx 2 months and get my cast off tomorrow. besides that my bike has been harder to start and is getting harder and harder.

It starts first kick when cold but when i run it and let it warm and then shut it down it fails to start again.

I dont know much about motorbikes and would just like to know what possible problems am i looking at and if anyone knows the cost's?

Does anyone know any good bike shops i can take it to on the Southside of brisbane?

Please help it would be much appreciated.

Kind Regards


  • CanadianWR450

Posted July 26, 2010 - 03:22 PM


First thing I would do is check your valve clearances.

  • 4esh_numa

Posted July 26, 2010 - 03:44 PM


First thing I would do is check your valve clearances.

Will do, does anyone know approx how much it will cost to get the valves done in AUS.

I have one question im not 100% sure but could it be the valves as it does start cold. I do understand metel expands when hot.

Thank you for your input CanadianWR450.

  • William1

Posted July 26, 2010 - 05:23 PM


Valve clearence issues only affect cold start (unless the valves are really, really bad).

Being your bike seems to spend time sitting, I'd first replace (noty merely try to clean) the pilot jet. Then I'd do the following:

Fuel Screw/Pilot Jet
Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method. Do it with the bike fully heated up.
Gently turn the screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly). Once warmed, slow the idle to the lowest possible speed.
*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***
Turn the screw in until the idle becomes rough or the bike stalls.
if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.
If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.
Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.
If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.
If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.
If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,800 rpm, but go by the spec in your manual.

If, after this, it is still hard to hot start and the hot start knob does not do it for you, you can try to close the fuel screw 1/2 turn less than optimum, but it may be harder to cold start (unless you give it some wrist action and baby it for the first 15 seconds.)

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  • 4esh_numa

Posted July 26, 2010 - 10:09 PM


Thanks a heap William1

I will give this ago and say it is not this, is there any other possibilities it could be?

please people give me your thoughts.

Kind Regards,


  • KevB

Posted July 27, 2010 - 02:32 AM


My WR400 did the same thing, checked the valve clearances, turned out the Ex Valve closest to the Cam chain was too tight, which when hot meant the valve was leaking, which in turn meant no compression on a hot startup... as soon as it cooled down, it started first kick again. I described the problem to my brother-in-law, who is a bike mechanic, and he said, without batting an eyelid, was to change the exhaust valve shims as they're too tight. When I checked, he was right...

  • William1

Posted July 27, 2010 - 03:00 AM


Thanks a heap William1

I will give this ago and say it is not this, is there any other possibilities it could be?

please people give me your thoughts.

Kind Regards,


Well, it could be the crank positoipn sensor, bad coil or CDI, though I doubt it is. If it were these items, you'd of noticed a high speed mis-fire that would get worse and worse over time.

When cold, your engine needs a rich-ish mixture. As it heats up, you need a leaner one. This is why, all jetting adjustments are made on a fully heated engine and then you use the choke (actually an enricher) to add some extra fuel to get you through the warm-up.

When valves get tight, they are tighter when cold and get looser when the engine heats up. The alloy of the head expands more than the shank of the valve stem. So a cold engine, the head is 'smaller' and therfore the valve protrrudes further. So it is not your valves.

My money is on a carb issue, either the wrong pilot jet/fuel screw adjustment or possibly float height. It can also be a worn needle jet emulsion tube - the part the needle slides in and out of-the part the main jet screws into.

  • 4esh_numa

Posted July 27, 2010 - 04:27 AM


Thank You KevB & William1 again.

When i get my cast off i will have a look at these things most probably on the week end and report back as soon as i find the problem. In the mean time keep posting your thoughts.

Thanks again guys for all your help.

  • 4esh_numa

Posted July 29, 2010 - 01:39 AM


Hey guys,

i think i might of solved the problem. i gave my bike a service, changed the oil and air filter as well as the fuel. took it for a little run. turned it off and it restarted it first kick and she starts then repeated the process several times and she fired up again.. so either it was the fuel,oil or somthing to do with the air filter. by the way i'm still getting my valves done just to make sure nothing is wrong in that area.

So i hope its not just a temporary fix.

Thanks Heaps everyone.

Kind Regards,



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