2007 yz450 won't start after warned up?

Ok guys....bike fires FIRST kick when cold....idles just fine....runs GREAT!!! After it's warm, if it dies or whatever, WILL not start back up UNTILL it cools back down!!!! I need more ideas....I'm out of them!!!

-New piston n rings

-Fresh hone

-New valves/valve job

-Valves in spec, checked 4 times

-Correct timing

-Brand new stator (old one had a burned spot so I changed it, thinking it was the problem)

-I get 75# of compression (I know it's not accurate because of the auto decomp) but shld be 80-100???

-Carb is clean

-Fresh mixed fuel (91 and ethanol treatment)

I checked all my electrical, everything spec'd....I replaced the stator because of the burn....still does the same thing!!!!

Possibly one of my rings turned on me and it's not enuf compression to start it hot????


Maybe timing is off per cam chain stretch???

Idk guys, any help would be awesome!!!! Thx much.

Hey. Will it start with the throttle cracked part open? 

I can t comment on compression.

When it is warm, does it idle ok?  Will it bog on quick throttle opening?

All signs of jetting issues, that may be worse when warm. Sometimes the idle gets adjusted around a partially blocked pilot jet.


Good luck!

Won't start at all....I've tried blippin the throttle as I kick it after it's warm!!

I'm going to go back thru my jets....anything I'm missing??

Pilot jet

Starter jet

Leak jet

Main jet

What journals do I need to make sure are clean? I've sprayed carb cleaner thru and it comes out in slide area!!

Cld the acc pump be an issue??


Did you hold the throttle part way open, not blipping, Hold about 1/4 turn or so the whole time while kicking.

The jets can still allow spray to go through and still be partially blocked.

Pilot being the primary one. Some varnish coating will reduce size considerably and still let spray through. Carb cleaner does not remove varnish/buildup.


Strange that bike runs good while warm, but wont start.  Could be compression issue too.

Maybe your carb is the culprit

Yep I've tried the throttle 1/4 open. :/

So I cleaned carb again and made sure my hot start was working properly....fired it right up as usual!!!! Went and rode it hard.....came back and shut it off, WONT start!!! I took the air filter out and spayed a touch of ether in there, just to see if it would fire!!! (Eliminate the carb questions) no start!!!!!

Gotta be the cdi huh??? After it's warm it won't start. Cold starts rite up!!!

I hate to ask, but have you checked for spark while it is hot and refusing to start?

Presumably the starter jet plunger is operating properly and not sticking to leave the starter jet 'open' a tiny bit. I assume it needs the starter plunger pulled to start when stone cold?


If the stator hadn't already been changed my money would be on that. In my experience however (had it a couple of times but on 2T "flywheel magneto" systems), if the windings totally break down under heat then you lose spark when hot and the engine stops - not just unable to start again. Might be a weak spark enough to keep running at idle but not fire up at kick start speed though. As well as checking for a good spark it might be worth checking stator coil resistance and voltage output hot v. cold.


It could be some other ignition component/circuit affected by thermal expansion. I know of no way to check the CDI module other than a substitution test but I guess there may be recommended resistance figures for the pickup(sensor) which might be OK cold but not hot? 


My tuned 04 can be challenging to start when hot but I've got the technique a bit better now. Hot start lever pulled and closed throttle until it fires works for me.

Gray.....yes I have checked the spark...it still seems to jump pretty well but it might not have been soon enuf after I shut it off. Motor was still plenty warm to touch tho, Cldnt hold ur hand on it...

72...yes cold start I just choke it hit the throttle once or twice and it fires first kick!!!!! Then once it's warm, I can shut choke off and it goes rite to an idle and runs perfectly!!!! Go ride it and it never misses a beat!!!! Get it good and hot (normal hard riding) and shut it down.....NOTHIN!!!!!

I swapped an 06 CDI onto it today....same story, started right up....ran it warm....stalled it on purpose.....NOTHING!!!!!!

CDI is out....now what!!!!???! Lol.

Did the bike ever start hot? if so, what has changed since then?

Did you check for 3-6mm free play in the hot start cable?

I bought it blown up...

No I have not checked that. Thx.

@gray....I re checked the spark while hot...NONE!!!

I switched cdi's-nothing

I changed out my harness-nothing

Unplugged kill switch-nothing

So I re checked my stator ohms...it is brand new!!!

But my source coil 1 is higher than spec!! Top spec is 1080... I'm at like 1270

The source coil 2 is also high.

Top spec is 66....I'm at around 90

Will this make it not start??

My flywheel wouldn't be causing bad readings would it???

Thx guys I appreciate it.....

I would write off the readings as tool/user error and try a new or borrowed ignition coil.

Out of curiosity, have you been able to bump start the bike when hot? That could start a weak spark system (2nd/3rd gear bump starting)

Also when cold and just warming up, will it restart?

I would think the piston compression only gets better when warm, I cant think of what else would effect compression (valve leak) that would not show as bad running.

Does the plug look wet when hot and trying to restart, would it somehow be flooding out when hot trying to start it?

I am following this one, really weird, Good luck

Just went back thru the carb....pilot jet was plugged again!!!! Cleaned everything good put it back together....thought for sure I had it figured out!!! Went and ran it....came back and.....NOTHING!!! Lol.

I haven't been able to try a good bump start....no hills by my house!!

Tried checking the spark a couple times....can't seem to get the plug grounded out good enough!!! Can't see the spark....but when it's cold just after trying to see spark it starts!!! Where is a really good spot to ground out??

I'll check the plug now for fuel...

No excess fuel on the plug....not really ANY fuel on the plug to speak of!!

What would cause it not to pull fuel up after its hot????

No excess fuel on the plug....not really ANY fuel on the plug to speak of!!

What would cause it not to pull fuel up after its hot????

I plugged up tank vent. Try it with the cap off the tank. I do not think a vapor lock is your problem but you never know until its ruled out.

I have taken the cap off...

Thx for the thought tho boss.

Some still think my carb is dirtier than I can clean....I need to have it sonic'd!!

Still sounds like something expanding and shorting out in the ignition system when hot, but there's no doubt that a soak in an ultrasonic bath reaches the carb parts other methods cannot reach. Mine has paid for itself several times over now.


I used to find it useful to check for a spark directly at the end of the HT lead with the cap removed on older air cooled stuff as you could wedge the end of the lead between cylinder fins. This also eliminated the possibility of a bad cap. I'm not sure how modern CDI systems react to operating without the plug cap though. I seem to recall it was a considered a no-no on BMWs years ago.


Trouble is, by the time you have got the cap off the lead, everything may have cooled down enough for the spark to come back anyway. In fact, the time taken to reach the plug at all is significant on central plug head motors with the  plug deep in the head. A YZ450F is bad enough - you want to try it in a hurry on a KTM 350SXF! Just lying it on the head ought to ground the plug out enough to spark in my experience. Suggests a not very good spark to me. Low resistance on the ignition windings is usually the cause of problems rather than high. On the weird system old Morinis use people have been known to rewind them with more turns of thinner wire to increase the total resistance and hence the output voltage at low rpm.


Could you hook up a multimeter to the various outputs while it is still running to check resistance and voltage cold, then hot, then stop it and check them whilst kicking over? The output voltage will be lower kicking than idling but  shouldn't vary too much with heat unless there is a winding fault.


Is there a recommended resistance for the coil LT windings you can check cold and hot?

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now