Flywheel weight on a YZ250 problems

[Daniel Hawley]

story time

So i have always been told that a Flywheel weight is the tits on a yz250, so naturally, i installed one within a month of owning my new blue monster. The YZ250 is a genuinely incredible machine and making it as luggable as a diesel was a really great decision. It makes cornering, traction and so many other parts of riding alot easier. Its easy to see why even the top riders of back in the day used them as a mod.

i chose the 11 oz size by Steahly. Today i was riding at AJS motocross in Western Australia when the engine completely went on me. I thought i had blown the bike and i was fairly upset as i had a week of riding planned ahead of me. I spent a couple hours stripping the bike down but found the motor itself in reasonable condition. What had happened instead was the flywheel weight had come a little loose (like there is absolutely no room in that flywheel bay. the 11 oz leaves zero space left before it hits the cover) and wedged itself on the cover. This was what caused the "Seized" engine. I have pretty well destroyed the thread on the flywheel shaft rod thing which ill have to some how fix. This is upsetting as its caused alot of grief and will stop me using that flywheel weight again as its thread is trash and i wont risk that happening again. 

 

Is this a common Problem? i torqued the weight properly and checked it after the first ride so its pretty surprising. This was the flywheel weights maybe 5th ride. 

[KJMac]

Do you have to machine the side cover or run a couple extra gaskets for clearance? Seams like the instructions would have mentioned that. Good luck!

[Tahoe Gator]

Get the GYTR flywheel. It is a direct swap out so no bolting it on. Forget whatever it weighs, but all the weight is on the perimeter so it works more effectively that its weight suggests.

[YZDOC]

6 hours ago, Daniel Hawley said:

story time

So i have always been told that a Flywheel weight is the tits on a yz250, so naturally, i installed one within a month of owning my new blue monster. The YZ250 is a genuinely incredible machine and making it as luggable as a diesel was a really great decision. It makes cornering, traction and so many other parts of riding alot easier. Its easy to see why even the top riders of back in the day used them as a mod.

i chose the 11 oz size by Steahly. Today i was riding at AJS motocross in Western Australia when the engine completely went on me. I thought i had blown the bike and i was fairly upset as i had a week of riding planned ahead of me. I spent a couple hours stripping the bike down but found the motor itself in reasonable condition. What had happened instead was the flywheel weight had come a little loose (like there is absolutely no room in that flywheel bay. the 11 oz leaves zero space left before it hits the cover) and wedged itself on the cover. This was what caused the "Seized" engine. I have pretty well destroyed the thread on the flywheel shaft rod thing which ill have to some how fix. This is upsetting as its caused alot of grief and will stop me using that flywheel weight again as its thread is trash and i wont risk that happening again. 

 

Is this a common Problem? i torqued the weight properly and checked it after the first ride so its pretty surprising. This was the flywheel weights maybe 5th ride. 

Stealthy owners instructions say for you to torque to spec, tighten the 4 grub screws, and check "periodically" to insure it is not coming off. I have ran them without a problems for years as I checked them to insure they are tight. I have 140 hours on one of the engines and it never moved. 

GYTR is the best unit to go as it "is" the flywheel and there are no outer grub screws to maintain. Just install like the OEM flywheel, and ride. 

If you have access to a die, you can re-cut the threads on the crank shaft, if not to damaged, you can clean it up and be lucky to not have to split the cases, and install new crank, bearings, and seals.

[Daniel Hawley]

I had to go out and buy the M12 x 1.25 (super fine thread. kinda hard to find) die nut to re cut the threading on the shaft. worked just fine. Ill look into the GYTR flywheel as that sounds like a good option. 

I do feel like this was just a coincidence. this was maybe 3 rides since the last time i checked the flywheel so i guess it was just unfortunate. ill use a little loctite if i decide to re use the flywheel weight. 

 

thanks guys

[Smoking 2's]

On 4/13/2019 at 6:15 AM, Daniel Hawley said:

story time

So i have always been told that a Flywheel weight is the tits on a yz250, so naturally, i installed one within a month of owning my new blue monster. The YZ250 is a genuinely incredible machine and making it as luggable as a diesel was a really great decision. It makes cornering, traction and so many other parts of riding alot easier. Its easy to see why even the top riders of back in the day used them as a mod.

i chose the 11 oz size by Steahly. Today i was riding at AJS motocross in Western Australia when the engine completely went on me. I thought i had blown the bike and i was fairly upset as i had a week of riding planned ahead of me. I spent a couple hours stripping the bike down but found the motor itself in reasonable condition. What had happened instead was the flywheel weight had come a little loose (like there is absolutely no room in that flywheel bay. the 11 oz leaves zero space left before it hits the cover) and wedged itself on the cover. This was what caused the "Seized" engine. I have pretty well destroyed the thread on the flywheel shaft rod thing which ill have to some how fix. This is upsetting as its caused alot of grief and will stop me using that flywheel weight again as its thread is trash and i wont risk that happening again. 

 

Is this a common Problem? i torqued the weight properly and checked it after the first ride so its pretty surprising. This was the flywheel weights maybe 5th ride. 

That is a surprise to hear, mines(13oz) been on 3 years now, love it! Glad you was able to get your threads cleaned up. Those set screws do barely catch the flywheel but if it's on all the way it's fine. Be sure there's no dirt between flywheel and weight that would cause it to hang out a little to far

[bk29]

This thread reminds me of a really bizarre one in a million crank failure I had on a yz back in 07. The bike was a 06 that I bought new in August 05 , I think it was approaching 200 hours. I’m a woods rider and I don’t think that’s a lot of hours for the low stress riding I’m doing. Anyway I’m cruising a road between trails and the bike starts vibrating and sounding weird to a level that my buddy that’s following me can tell something happened. I coasted off the hill and walked to my truck took it home and took off the cylinder. There was a tiny scratch on the cylinder that made me think that maybe the crank went but probably it was grit that got past the filter. No play or rough bearing feel from the con rod. No where near as much cylinder damage as you see when a rod bearing fails. So I’m stumped, flywheel is tight , flywheel weight is tight and main bearing has no play. Here’s where I effed up. I have a new piston so I put that in and take it down the driveway. Still clearly not right. Sorry this story is way too long. Anyway tore the engine out and now I’ve ruined my new piston and cylinder because metal went through. I was stunned when I split the case and the crank was broken right through the rod journal. Literally the beefiest chunk of metal on a dirt bike. It’s not like the steel was crystallized or something. I think it had to be a harmonic vibration caused failure? I don’t really know. I ran the same flywheel weight on 2 different bikes for probably a 1000 hours so I’m not blaming it on that. My mc club buddies have road so many different 2 strokes and no one has heard of a crank breaking in half. I suppose it I would have really pulled up on the rod with some force I might have felt it problem but I was thinking bad bearing and I didn’t put much force on it.

[sclogger]

By chance do you have an aftermarket ignition cover?  I believe with Steahly you need a spacer if you run an aftermarket cover.  OEM cover does not require a spacer. 

Also, I just happen to be working on a yz250 project bike the other day, there was some rub on the inside of the OEM cover but underneath the Flywheel weight the owner had left the washer in between the weight and flywheel.  Got a feeling this extra spacing was causing the contact.

Edited April 16, 2019 by sclogger

[Daniel Hawley]

5 hours ago, sclogger said:

By chance do you have an aftermarket ignition cover?  I believe with Steahly you need a spacer if you run an aftermarket cover.  OEM cover does not require a spacer. 

Also, I just happen to be working on a yz250 project bike the other day, there was some rub on the inside of the OEM cover but underneath the Flywheel weight the owner had left the washer in between the weight and flywheel.  Got a feeling this extra spacing was causing the contact.

AH! i did leave the washer in there yes. Ill have to remove it. I did finish the rebuild and get here running again today but ill go in and get rid of that washer to save a few mm space in there. Cheers mate