YZ250FX starting and running issues

Tuna
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Tuna
Recently rebuilt top and bottom end after hydro locking it and bending a rod.
Seems hard starting now, bogs and stalls with throttle. Once it’s fully warmed up it is not an issue until it gets too hot in technical terrain and starts acting strange again. Hard to start, backfiring, flaming out and stalling at low speed and throttle openings.
I know lots of folks had MAP or pressure sensor issues on the 14 and 15 models, Yamaha updated the part.
Valves were adjusted and in spec. Replaced the fuel pump last year.
Any suggestions?

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Paw Paw 271
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Paw Paw 271 1/26/2020 2:19pm
1/26/2020 2:19pm
Dirty fuel filter, dirty fuel injector.
You said valves are in spec, but were they tested or replaced when the engine was rebuilt?

Paw Paw
Tuna
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Tuna 1/26/2020 2:23pm
1/26/2020 2:23pm
Paw Paw 271 wrote:
Dirty fuel filter, dirty fuel injector.
You said valves are in spec, but were they tested or replaced when the engine was rebuilt?

Paw Paw
Not replaced, just cleaned up.
Seems like the issue is mostly after the rebuild and drowning it.
Tuna
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Tuna 1/27/2020 4:30am
1/27/2020 4:30am
Any other thoughts on the possibility of a sensor issue?
Paw Paw 271
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Paw Paw 271 1/27/2020 5:52am
1/27/2020 5:52am
Cam timing off?
Was cam chain replaced?

Paw Paw

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Tuna
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Tuna 1/27/2020 7:10am
1/27/2020 7:10am
Paw Paw 271 wrote:
Cam timing off?
Was cam chain replaced?

Paw Paw
Chain was replaced and timing re checked.
Tuna
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Tuna 1/28/2020 12:33pm
1/28/2020 12:33pm
Any other ideas? Just wondering if other folks have had similar issues or symptoms.
Farmer Tom
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Farmer Tom 1/28/2020 6:08pm
1/28/2020 6:08pm
How's your coolant? I had the strangest problem on my '14 YZ450 and it seems similar to your situation. My bike would run fine for quite a while until it got hot, then it would flame out and wouldn't start. You could just kick until you're blue in the face, and then all the sudden fire up and run fine. I found out that I was quite low on coolant and speculate that the coolant sensor was heating up enough to lean the bike way out, making it run poorly. Just a thought...
Tuna
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Tuna 1/29/2020 3:56am
1/29/2020 3:56am
Farmer Tom wrote:
How's your coolant? I had the strangest problem on my '14 YZ450 and it seems similar to your situation. My bike would run fine for quite...
How's your coolant? I had the strangest problem on my '14 YZ450 and it seems similar to your situation. My bike would run fine for quite a while until it got hot, then it would flame out and wouldn't start. You could just kick until you're blue in the face, and then all the sudden fire up and run fine. I found out that I was quite low on coolant and speculate that the coolant sensor was heating up enough to lean the bike way out, making it run poorly. Just a thought...
Coolant is good, temp sensor is working as I have a fan installed. I did need to clean the sensor hosing and pack with grease because it stopped working.
Tuna
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Tuna 1/29/2020 3:56am
1/29/2020 3:56am
kb228
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kb228 1/29/2020 7:31am
1/29/2020 7:31am
Sounds like a bad coolant temp sensor
Tuna
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Tuna 1/29/2020 9:53am
1/29/2020 9:53am
kb228 wrote:
Sounds like a bad coolant temp sensor
I tested it recently as I have a fan kit, I will try it again. Have heard about bad pressure sensors on the side of the TB causing similar symptoms. Issue really shows on cold starts and gets better as it warms up.
kb228
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kb228 1/29/2020 10:22am
1/29/2020 10:22am
kb228 wrote:
Sounds like a bad coolant temp sensor
Tuna wrote:
I tested it recently as I have a fan kit, I will try it again. Have heard about bad pressure sensors on the side of the...
I tested it recently as I have a fan kit, I will try it again. Have heard about bad pressure sensors on the side of the TB causing similar symptoms. Issue really shows on cold starts and gets better as it warms up.
FWIW, The dealer and i both tested the coolant temp sensor on my kx and it tested ok but was indeed the issue
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Tuna
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Tuna 1/29/2020 10:56am
1/29/2020 10:56am
kb228 wrote:
Sounds like a bad coolant temp sensor
Tuna wrote:
I tested it recently as I have a fan kit, I will try it again. Have heard about bad pressure sensors on the side of the...
I tested it recently as I have a fan kit, I will try it again. Have heard about bad pressure sensors on the side of the TB causing similar symptoms. Issue really shows on cold starts and gets better as it warms up.
kb228 wrote:
FWIW, The dealer and i both tested the coolant temp sensor on my kx and it tested ok but was indeed the issue
What was the bike doing that made you look into the issue?
kb228
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kb228 1/29/2020 11:13am
1/29/2020 11:13am
Tuna wrote:
I tested it recently as I have a fan kit, I will try it again. Have heard about bad pressure sensors on the side of the...
I tested it recently as I have a fan kit, I will try it again. Have heard about bad pressure sensors on the side of the TB causing similar symptoms. Issue really shows on cold starts and gets better as it warms up.
kb228 wrote:
FWIW, The dealer and i both tested the coolant temp sensor on my kx and it tested ok but was indeed the issue
Tuna wrote:
What was the bike doing that made you look into the issue?
Hard starting, ran rough or stalled once hot and absolutely wouldnt restart when hot.
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Tuna
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Tuna 1/29/2020 12:14pm
1/29/2020 12:14pm
kb228 wrote:
FWIW, The dealer and i both tested the coolant temp sensor on my kx and it tested ok but was indeed the issue
Tuna wrote:
What was the bike doing that made you look into the issue?
kb228 wrote:
Hard starting, ran rough or stalled once hot and absolutely wouldnt restart when hot.
Thanks for the feedback. Did it show this behaviour on cold starts as well? I will try to borrow a sensor and swap out.
roachslayer
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roachslayer 1/31/2020 8:54am
1/31/2020 8:54am
What year FX you got? There was no '14 model. Never heard of MAP issues, and looking through the parts catalogs I see no change in part number for the MAP sensor from 2015 to 2019.

I have a 2015 250FX, rebuilding the top end and cam as we speak, using updated 2017/19 parts, just as preventative maint, no prior issues.
kb228
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kb228 1/31/2020 10:33am
1/31/2020 10:33am
Tuna wrote:
What was the bike doing that made you look into the issue?
kb228 wrote:
Hard starting, ran rough or stalled once hot and absolutely wouldnt restart when hot.
Tuna wrote:
Thanks for the feedback. Did it show this behaviour on cold starts as well? I will try to borrow a sensor and swap out.
When it was cold it was hard to start. Swapped the sensor and the bike went back to normal.
Tuna
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Tuna 1/31/2020 10:35am
1/31/2020 10:35am
roachslayer wrote:
What year FX you got? There was no '14 model. Never heard of MAP issues, and looking through the parts catalogs I see no change in...
What year FX you got? There was no '14 model. Never heard of MAP issues, and looking through the parts catalogs I see no change in part number for the MAP sensor from 2015 to 2019.

I have a 2015 250FX, rebuilding the top end and cam as we speak, using updated 2017/19 parts, just as preventative maint, no prior issues.
2015. I used a 15 crank but a 16 Piston with updated oil sprayer. The new pressure sensor is supposed to be a different color. Supposedly it was updated in 16.

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Tuna
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Tuna 1/31/2020 10:35am
1/31/2020 10:35am
kb228 wrote:
Hard starting, ran rough or stalled once hot and absolutely wouldnt restart when hot.
Tuna wrote:
Thanks for the feedback. Did it show this behaviour on cold starts as well? I will try to borrow a sensor and swap out.
kb228 wrote:
When it was cold it was hard to start. Swapped the sensor and the bike went back to normal.
Thanks so much for the info. That’s where I will start.
roachslayer
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roachslayer 1/31/2020 11:07am
1/31/2020 11:07am
I think you're on the right track. MAP sensors can be damaged by water getting in it, and given you had a hydrolock issue, replacing it may be the trick. Like I said though, I see no changes at all 2015-2019 on that sensor - no update.

Related side note:
2015 and 2016 FX pistons and oil sprayers are identical, 2016 had a rod update, and 2017 had a major piston/pin/rod and crank and oil sprayer update. If you used a 2016F oil sprayer, that actually is the updated one, identical to the 2017FX update. Yep, I've done a TON of research on all this, as I'm trying to update my 2015. Smile

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