YZ250 Brand new 2021 YZ250 died; won’t start

[MX28]

35 days old. Break in + 0.6 hours. Bike died on track and now will not fire. 
I’ve already contacted the dealer and they think it’s absurd and want to have a look. I assume they’ll go to bat for me because we all agree this is ridiculous. Problem is they’re 5 hours away...

While riding, the bike made a nasty sucking sound while exiting a corner and died abruptly. Go ‘FFFPPFFFFFFFFFPPP’ with your mouth. Yeah, like that. After that there was a strange gritty sound when the bike was cranked (slowly) but this really has gone away and everything is normal feeling.

There is a 9oz steahly FWW on it, the factory provided 180 main jet, and a maxima air filter. Otherwise it is as is from the factory. The incident occurred about 15 -18 minutes of run time after the installation of the 180 main and FWW. 
 

GAS: 32:1 Castor 927 and pump 93

FUELING: the bowl is filling, the jets are clear and unblocked. 

AIR: filter is perfectly fine and it was running perfectly before the incident. The reeds are in perfect condition as well. The inside of the air box looks brand new and is completely free from oil or dirt. 

SPARK: the plug is in great shape and sparks where I’d expect it to in the stroke going off of sound. Using a brand new BR8EG plug makes no difference. 

ENGINE: the factory tooling cross-hatches can still be perfectly seen on the cylinder walls from the intake port. There is no play or discoloration anywhere. It looks (from the intake port) exactly like you’d expect a fresh engine to look. The bike will not start with the kill switch disconnected either. Throttle position just makes it sound deeper, zero effect. I’m not willing to pull the engine apart right this second in case I decide to take it back and have the dealer go through it and document the issues and appeal to Yamaha. Looking through the spark plug hole everything looks clean. Tons of compression. 
 

I’m beyond confused. Anyone have any ideas? 
Thank You!

[GSP2]

Post a video of you making the sound.  That will be helpful ??
not sure I’m doing it right.  
 

Did you pull the side cover and do a visual on the flywheel? Brand new, change points are the FWW and jetting change.  My guess if you look deeper in to those two areas you will find the problem.  

[MX28]

Update since I’m actively wrenching and posting. 
 

pulled the pipe and everything is immaculate there too. Cylinder looks new from this side and the piston is clean

[MX28]

2 minutes ago, GSP2 said:

Post a video of you making the sound.  That will be helpful ??
not sure I’m doing it right.  
 

Did you pull the side cover and do a visual on the flywheel? Brand new, change points are the FWW and jetting change.  My guess if you look deeper in to those two areas you will find the problem.  

Yeah side cover is off. Want a peek? Everything looks normal and timing assembly hasn’t moved.
I’m scratching my head down to my skull I don’t understand this at all. 
when I pulled the carb off earlier I pulled the main and the one next to it to check it for clogging and all clear. I’m not sure what else to do.

[Motox367]

Look threw intake see if skirt is broke

[GSP2]

What is the stock main that was installed from the factory? If you went one up or down, most likely not the issue unless there is still some contamination in the carb. Happens to all of us. Swear the carb is clean, still won’t run.  Clean a second time and viola, it runs. 
 

just thinking out loud.... is the wiring routed correctly around the flywheel? Did the wiring rub on the flywheel and cause a dead short? Don’t ask me how I know that happens but it happens. Did that on my kids 85SX last year ?‍♂️

Edited July 25, 2021 by GSP2

[MX28]

18 minutes ago, Motox367 said:

Look threw intake see if skirt is broke

I did earlier, everything is proper. You mean the piston skirt right? Either way didn’t see anything out of place. 
 

if the skirt broke off the piston wouldn’t it be a lock up situation though?

Edited July 25, 2021 by MX28

[Motox367]

4 minutes ago, MX28 said:

I did earlier, everything is proper. You mean the piston skirt right? Either way didn’t see anything out of place. 
 

if the skirt broke off the piston wouldn’t it be a lock up situation though?

Not always. Sometimes will chew it up spit it out,stall and start back up. But being new hopefully just something simple

Edited July 25, 2021 by Motox367

[MX28]

19 minutes ago, GSP2 said:

What is the stock main that was installed from the factory? If you went one up or down, most likely not the issue unless there is still some contamination in the carb. Happens to all of us. Swear the carb is clean, still won’t run.  Clean a second time and viola, it runs. 
 

just thinking out loud.... is the wiring routed correctly around the flywheel? Did the wiring rub on the flywheel and cause a dead short? Don’t ask me how I know that happens but it happens. Did that on my kids 85SX last year ?‍♂️

I wish it were the case. Only thing I see is the little contact patch where the FWW likes to kiss the cover but that’s it. I wish steahly would tell you this happened and I would have bought the spacer with it...

[MX28]

1 minute ago, Motox367 said:

Not always. Sometimes will chew it up spit it out,stall and start back up. 

That’s why I pulled the pipe off to see if there were reed valves, plug electrodes, engine guts, etc in there. Nothing. I’ll check again and send some pics. I have bad signal in the garage and it’s making it hard to do attachments. If the skirt broke off where would I find it? Wouldn’t it fall down into the case?

[Motox367]

2 minutes ago, MX28 said:

That’s why I pulled the pipe off to see if there were reed valves, plug electrodes, engine guts, etc in there. Nothing. I’ll check again and send some pics. I have bad signal in the garage and it’s making it hard to do attachments. If the skirt broke off where would I find it? Wouldn’t it fall down into the case?

You'd know by looking at it threw intake and exhaust. But yes most times goes in bottom end and gets stuck seizes up. Probably not your issue

[MX28]

1 minute ago, Motox367 said:

You'd know by looking at it threw intake and exhaust. But yes most times goes in bottom end and gets stuck seizes up. Probably not your issue

I can see clear cross hatches all around and everything rotates really smooth. So you wouldnt pull Everything apart to check that? I’m not saying impossible but I do doubt it could be that as well

[Motox367]

8 minutes ago, MX28 said:

I can see clear cross hatches all around and everything rotates really smooth. So you wouldnt pull Everything apart to check that? I’m not saying impossible but I do doubt it could be that as well

Not if piston isn't broke. Have spark? Try new plug stuff like that

[integrationByParts]

Reminds me of an intake issue. Ripped air boot? Some pictures of it all would be great

[GSP2]

Is the flywheel located in the right position? Meaning it didn’t back off the tapered shaft? With a bit this new, I’m convinced it is an issue with one of the two things that were touched.  Anything else beyond the FWW and jetting? When an issue surfaces when it shouldn’t, I always go back to the change points.  

[NYMXer]

8 minutes ago, integrationByParts said:

Reminds me of an intake issue. Ripped air boot? Some pictures of it all would be great

I was thinking loose carb boot (Either side) or loose intake manifold, air filter fell off, etc.... definitely something on the intake side. I doubt it's electrical.

 

[bikerbill2021]

Pull the flywheel off and Check your woodruff key.  Seeing as you replaced the factory FW with a heavier one, there is a possibility the key sheared, knocking off your ignition timing.  Had an 03 YZ that did something similar and everything visually looked fine, drove me nuts why it wouldn't start, but would backfire upon attempting to.  Removed the FW and found the sheared key, and this was on a stock FW

You say tons of compression, but have you actually stuck a gauge on it?  Just to check...

 

Edited July 25, 2021 by bikerbill2021

[GSP2]

Brainstorming...

water in the gas. 

bad plug

timing got bumped 

stator not picking up flywheel signal 

factory harness connector came loose

you checked the reeds so that’s out. Might want to double check if there is a gap between the petals and block 

carb boots on either side

 

 

[Randy300XC]

The Mikuni carb has a center body gasket that when fails does something like this problem. 

[Fearnot]

Sounds like that baby ate her first reed? If not pull the cylinder off.