Yz450f only bump starts

[Michael Dobey]

I picked up an 04 yz450f and have some issues with starting. I cleaned the carb, reset the timing, cleaned the air filter, changed the plug,and reshimed the valves and nothing seems to work. Every now and then it'll start up but die like you hit the kill switch once you idle it down . I unplugged the throttle position sensor and it won't start but I can bump start it that way and it'll run like a top but won't start once you turn it off. Any ideas? 

[FORMA MOTO]

18 minutes ago, Michael Dobey said:

I picked up an 04 yz450f and have some issues with starting. I cleaned the carb, reset the timing, cleaned the air filter, changed the plug,and reshimed the valves and nothing seems to work. Every now and then it'll start up but die like you hit the kill switch once you idle it down . I unplugged the throttle position sensor and it won't start but I can bump start it that way and it'll run like a top but won't start once you turn it off. Any ideas? 

Sounds like a classic case of leaking intake valves or a skipped cam chain timing.

Pull the head and inspect the chain and tensioner. If the chain has even on kink, it's toast. If it's the original, it's toast. Same with the tensioner.

Check you cam timing.

If the timing and chain are good, you probably have a leaking intake valve that only allows the motor to run at a certain rpm and above.

You can do a leak down test, or you can just pull the head, and do a fluid leak test.

[Michael Dobey]

I know it's not the valves or cam timing. I've already gone through all of that with no luck. It's an electrical problem but I can't figure out what's causing it.

[FORMA MOTO]

How can you know it's not the valves if you have not done a leak down or fluid leak test?

Electrical would have to be a dying stator or a dead ECU, assuming you have already tested the plug and coil.

[Michael Dobey]

I don't understand how it could be valves if it idles all day long without the throttle position sensor but as soon as you plug it back in it dies

[Michael Dobey]

And why won't it start with it unplugged but sometimes will with it plugged in

[Jw7054]

Did you set the throttle postion sensor to the right spot? It is adjustable and you can adjust it wrong

[FORMA MOTO]

11 hours ago, Michael Dobey said:

I don't understand how it could be valves if it idles all day long without the throttle position sensor but as soon as you plug it back in it dies

You never mentioned this prior, so how can we help you if you don't list all the issues....

All the throtle position sensor does it change ignition maps, nothing more. It should not matter AT ALL regarding starting.

If you have a SHORT in the TPS or harness connecting to the TPS, then that is different. You need a factory service man

A bike with a slightly leaking intake will be hard to start, usually bump start only. Once started, it runs fine because the metal expands enough to seal the valve.

 

[Michael Dobey]

I believe it is the stator. It's almost like it doesn't make enough spark to start it with the kicker or keep it running at a low rpm. When you plug the tps in it idles down, runs for about 10 seconds at a normal idle, and dies like you hit the kill switch

[FORMA MOTO]

Down load a factory service manual from Yamaha's website and do the stator test

[Michael Dobey]

I have just noticed a misfire at idle and am thinking the kill switch could be bad 

[Michael Dobey]

Unplugged the kill switch and it started right up. Idled for a while with a slight misfire then died and gave a puff of air out by the carb

[OLHILLBILLY]

Throw it off a cliff, report it stolen, collect insurance $$. Move on.. ?

Gotta love electrical problems.