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Won't start when hot, not valves, not coolant temp sensor, no codes.

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2014 kx450f, rekluse core exp, fmf q4. Fresh full rebuild with wiseco, Otherwise stock.

 

I've been having an issue the past 6 weeks that has made my 450 un-rideable. The bike runs great, but if it stalls it will not start back up. I've tried everything I could think of. Borrowed a buddy's 12, tried stator and pickup, throttle body, ecu, fuel tank, installed new coil and wire, new coolant temp sensor.

 

Before this all started, I had got the bike hot. It ran just fine started fine for around an hour of trail riding. Then got crossed up going over some logs and fell over. Stood the bike up and it would not kick start! Turned the clutch pressure plate out and bump started it. Killed it AMD started a few times everything seemed OK. Stalled one more time on the way back to the truck and same thing. Had to bump start it.

 

The next weekend, I was at a race had the bike all nice and warmed up ready on the starting line. Killed it to hear the riders meeting, then it would not start up. Kicked for about 5 minutes then it fired up and I did 5 laps of the scramble, then it stalled. Would not start up for about 20 minutes, then just randomly kicked over. Finished the race just cruising.

 

Got home and pulled the motor apart, found valve stem was leaking and decided at 250 hours it was time for a rebuild.

 

Rebuilt the whole motor with new ss valves. Checked clearances 10+ times.. .005-6 on intake .008-9 on exhaust.

 

Pulled head, oil I'm cyl. Head was slightly warped and intake valve seal was damaged on assembly. Lapped head and cyl, replaced valve seals. All is good now. No oil in cyl anymore.

 

Still the same issue. Bike will run but if I shut it off it will not start back up till it cools off. BUT! If when its hot, I plug in a cold coolant temp sensor it fires right up second kick.

 

I'm at a loss with this. Any ideas would help!

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seems to be a problem and no takers on this post...

since you changed everything around the engine..with same result...i'd assume the problem is inside the engine...cam shaft timing..

check it over, the timing marks are actually different than what it shows in workshop manual.. somewhere is a post with pic , and check it..

cold temp sensor will add more fuel for start up... cam timing out, starts cold, but not hot..seen that a lot..

(that's all I can come up with, read your post over and over...)

 

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did you change/check the air temp sensor ?


Temp sensor has good resistance reading all the way to the ecu.

Timing is done the same way I have always done it with no issues. Line up mark on flywheel exhaust timing mark in line with head and intake mark 29 pins.

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seems to be a problem and no takers on this post...
since you changed everything around the engine..with same result...i'd assume the problem is inside the engine...cam shaft timing..
check it over, the timing marks are actually different than what it shows in workshop manual.. somewhere is a post with pic , and check it..
cold temp sensor will add more fuel for start up... cam timing out, starts cold, but not hot..seen that a lot..
(that's all I can come up with, read your post over and over...)
 

Going to try the fuel pump again. The cold start logic sounds like the motor must be kicked one time to prime, second to start. Puting cold sensor makes it think it is a cold start. Ecu assumes good fuel pressure on hot start? The pump leaked a lot when sitting off the bike.

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Took the pump apart and found 2 different leaks, one at o ring further base of the fuel pump going to the outlet. And one at the fuel pressure regulator. I could blow air with my mouth through the regulator. Blew it out with air compressor, replaced rings and motor, good as new.

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