2007 yz 450 will not start when hot but will bump start

I have a 2007 it is a great bike runs like a champ and starts up easy when cold but if I ride it for more than 20 min or so and get it good and hot .. it will not kick start but it will bump start and run perfectly fine. The valves are good and and it has been doing this since the day I got it not sure if it's in the carb . Decompression on cam or electrical issues . Can anyone help? If it helps I do have full FM exhaust and boysenberry quick shot on carb, that is the only after market stuff to do with the motor . Any help will work just don't have the money to keep chasing the problem

What size pilot jet?

Check also fuel screw setting.

I assume you must choke it to start when cold, every time, right?


Hot engine, hot start and bike should fire right up unless the pilot/fuel screw is off.

The jetting I am not sure I think 175 on I will have to check when I get home. I've had many people tell me it was the carb . As far as starting I do have to use the choke

Just checked it the jet is a 175. And as far for the air fuel mixture screw it is set and 1 3/4 turns out the spec is for 1 1/4 but I have no popping and best response at 1 3/4

Probably the valves. When this happens to me that always seems to be the problem.

I agree with the above, check valve clearances. 

Valves are dead on that was my first go to. They are all perfect. Had the local dealer check them and then I checked them and there good. Not only that but the bike runs good no backfiring or popping . That the thing that weird and even when I bump start it after its hot it runs perftect. And if I let it sit long enough to cool off it will start the first kick every time

175 is the main jet, not the pilot jet.

Ignore 'popping'. If the bike is jetted to prevent popping, then I bet the pilot is huge.

To prevent popping, ensure there are no exhaust leaks and ride it like a race bike and not a trail bike.


Not the valves. Valves manifest with cold start, not hot start issues.

Sorry yes that is the main jet I will have to pull the carb off to check pilot jet size .. there are no exhaust leaks on the bike . I do ride the bike like it should be rode I run the 450 b class so I'd like to think I run it hard lol. I will check the pilot jet size tonight and put it on here

Is just weird because I mean if it is a car issue shouldn't I be noticing some issues or something when the bike is running ? I'm just lost and I appreciate the help guys . I can rebuild the whole motor all the way through but the whole jetting area is not my specalty

Most bikes, once running, will idle fine even if too rich.

Pilot jet is right next to the main, only slightly harder to remove.


You do use the hot start when firing up when ho, right?

Yea I do pull hot start when trying it . When bike is cold 2 kicks and fires, bike gets semi warm Mabey ten minute ride I pull hot start and usually 3 or four kicks it fires . But after I get it extremely hot Mabey 3 or four laps in a moto you could kick it as much as you want and nothing but when ya bump start it it's fine. But you don't bump start it it has to cool down 100% to fire again any heat in the motor it won't do a thing

And the hot start does work to

pull your plug and post the results. Id almost guarantee your too rich. My bike does the same thing when its hot it takes a few kicks to get goin even with the hot start lever in, 1-3 kicks when its cold. I just pulled a black plug out but I had let it idle a lot on that plug, so I just put a fresh one in and ill pull it after next ride to check again. 


also a 07 yz450

Edited by Hakkalugi

Let me know how the plug looks .. I'm going to run open practice tomorrow so I will check it after the first run tomorrow

No more help needed I sold the bike today after looking into it the original owner of the bike had it jetted for straight 112 octane I think that may have been my problem

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