YZ250 Brand new 2021 YZ250 died; won’t start

[MX28]

58 minutes ago, bikerbill2021 said:

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...

 

No compression check but it feels exactly like it did beforehand. I have a feeling you’re correct about the key too. I’ll check ASAP. 

[MX28]

29 minutes ago, Fearnot said:

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

Checked the reeds bro it’s factory fresh in there. This feels like the twilight zone. 

[MX28]

Could timing do this? I’m perplexed 

[bikerbill2021]

17 minutes ago, MX28 said:

Could timing do this? I’m perplexed 

Anythings possible, the 03 I had would backfire upon attempting to start, which told me something was up with the timing, which led me to the sheered key, new key, some valve lapping/cleaning up of the crank surface, and torqued it to spec and it fired right up

Pop the cover off and remove the flywheel, will take you 5 mins

[Big Ron L]

like others here said check thr woodruff key on the flywheel. i had the same problem when i put a heavier flywheel and didnt tighten it enough. it broke the woodruff key. harbor freight sells the keys.

[kick]

When I bought my 01 yz250 Yamaha was the only company with a 30 day warranty on motocross bikes.  Do they still offer that?  Even though your 35 days out. 

[MX28]

3 hours ago, Big Ron L said:

like others here said check thr woodruff key on the flywheel. i had the same problem when i put a heavier flywheel and didnt tighten it enough. it broke the woodruff key. harbor freight sells the keys.

So something interesting I’ll add. I thought I felt a shift at installation. The noise I mentioned in the OP was definitely a backfire too, I should have just said that in the OP. I’m going to go pull the wheel tomorrow ASAP and take a look. How do I get it off without the flywheel puller?

[MX28]

11 minutes ago, kick said:

When I bought my 01 yz250 Yamaha was the only company with a 30 day warranty on motocross bikes.  Do they still offer that?  Even though your 35 days out. 

The dealer acted like they would aggressively go to bat for me but like I said, I’m determined to diagnose this before making a 5 hour drive one way. 
 

I’ll leave it with them and never make another payment as long as I live if they don’t. My credit score is good enough??

Thanks to everyone for dropping your .02 also. I’ll check the flywheel and report back tomorrow. 

[MX28]

Would a GYTR flywheel be less prone to doing this? Do some of these malfunction? We trust our lives to these machines and I’m not feeling so trusty now hahaha. 

[Shooter88]

1 minute ago, MX28 said:

Would a GYTR flywheel be less prone to doing this? Do some of these malfunction? We trust our lives to these machines and I’m not feeling so trusty now hahaha. 

While I cant really answer that specifically, I've had the GYTR one for about 3 years now and it's been solid. Not really much to go wrong with it as it's just an OEM flywheel with a weight welded onto it. 

[onealmxwilson]

I wouldn’t rule out a bad crank seal. Would explain the sucking sound. Although it would have to be really messed up to keep the bike from running. Maybe installed wrong at the factory? 

Also, if you’re determined to figure it out before going to the dealer, go ahead and pop the cylinder off and be sure everything looks as it should. 

[Tenman]

With 0.6hrs on it. I’d talk to them before I tore into it. Once you tear into it. They probably ain’t gonna do anything

[MX28]

3 hours ago, onealmxwilson said:

I wouldn’t rule out a bad crank seal. Would explain the sucking sound. Although it would have to be really messed up to keep the bike from running. Maybe installed wrong at the factory? 

Also, if you’re determined to figure it out before going to the dealer, go ahead and pop the cylinder off and be sure everything looks as it should. 

 

3 hours ago, onealmxwilson said:

I wouldn’t rule out a bad crank seal. Would explain the sucking sound. Although it would have to be really messed up to keep the bike from running. Maybe installed wrong at the factory? 

Also, if you’re determined to figure it out before going to the dealer, go ahead and pop the cylinder off and be sure everything looks as it should. 

I’m not willing to pull the head for this reason. Carb and FWW is whatever but they’ll be able to tell the heads come off. That nickel coated hardware will show wrench marks. I’m crossing my fingers for the woodruff key and ordering parts tomorrow so I can hopefully make my riding trip still. If I find out it’s not that, it goes to the dealer.

Would a poor crank seal just cause massive amounts of blow by (positive pressure) or oil ingestion (vacuum)?

[ranga97]

1 hour ago, MX28 said:

 

I’m not willing to pull the head for this reason. Carb and FWW is whatever but they’ll be able to tell the heads come off. That nickel coated hardware will show wrench marks. I’m crossing my fingers for the woodruff key and ordering parts tomorrow so I can hopefully make my riding trip still. If I find out it’s not that, it goes to the dealer.

Would a poor crank seal just cause massive amounts of blow by (positive pressure) or oil ingestion (vacuum)?

FW side would cause lean, RHS side would ingest some oil usually. Crankcase is in vacuum most of the cycle. 

So yeah hows that sound go again?

Does it draw air through the reeds when you crank? Air coming out exhaust when you crank? By sucking sound is it similar to holding down the killswitch while youre moving or am I getting that wrong. If there's air moving through the intake, check for fuel on the plug, disconnect the spark plug, give it a heap of kicks and check to see if fuels getting drawn through. 

[Smoking 2's]

Only reading your first post, and a few here and there. Only thing I could think of would be the flywheel keyway broke and possibly the sucking sound is a piece of key cut the crank seal at same time

[Big Ron L]

7 hours ago, MX28 said:

So something interesting I’ll add. I thought I felt a shift at installation. The noise I mentioned in the OP was definitely a backfire too, I should have just said that in the OP. I’m going to go pull the wheel tomorrow ASAP and take a look. How do I get it off without the flywheel puller?

idk, i used  a flywheel puller.  i used a torque wrench when i put it back on so it was tightened properly. how did you get the stock one off?

[MX28]

I just used a torque wrench; 42 ft-lb. 

I’ll drop this here and see what you guys think. I’ve got the FWW off and this is where the timing marks fall for TDC. Interestingly, the rotating assembly is extremely free spinning at the TDC mark. I imagined it would be difficult to move due to the compression action. Here’s the picture I got of everything. It definitely doesn’t seem ‘in time’ seeing/feeling that. 
 

also, if the key breaks, it does not come off. It shears but doesn’t break off. Here’s some pictures. Same position, 2 angles. 

[Motox367]

Take the engine pit and cylinder off. Can reuse base gasket and with engine out don't have to take head off. Or take it back to dealer

[MX28]

Thanks for the help everyone. I got the FW off with a standard gear puller. It wasn’t very tight either. I’m not sure what happened. I don’t think we need to guess anymore.. 

[Kinger317]

1 hour ago, MX28 said:

Thanks for the help everyone. I got the FW off with a standard gear puller. It wasn’t very tight either. I’m not sure what happened. I don’t think we need to guess anymore.. 

 Good deal! I must say I’ve never seen that before. Now forget it ever happened and go enjoy the great motorcycle??