From what I can remember from restoring a DT250mx many moons ago, the tank is supported on a steel hinge at the top, a rubber grommet at the bottom and directly to the rear of this grommet is a lug on the bottom of the tank with a rubber grommet in it. This is where the thumbscrew fits through to secure the tank to the frame.

Remove the thumbscrew and the rear of the tank pulls out just far enough to clear the frame.

Thank you.  Is this the 2-stroke oil tank?  For the Autolube system?  

To be sure after looking at your other post.  The oil tank I am remembering was under the LEFT sidepanel and unlike most yamaha oil tanks, was coloured black.  Under the right sidepanel on the model I am recalling was access to the air filter and below that, a triangular toolbox.  It may be that yours is a different model DT250mx altogether.

BTW most vehicle manufacturers denote left and right as viewed from the rear of the vehicle as I am sure you know.

I know the 125/175 oil tanks swapped sides in 1980.  No idea if the 250 also swapped.  I thought the 250s pretty much stayed the same.  Just because the one I restored was registered in 1980, it does not mean it was a 1980 model.

Edit.  The bottom of the oil tank I speak of can be seen poking out below the left sidepanel, along with the thumbscrew.

Here is a link to the 1979 oil tank diagram.  I doubt it changed much by 1982 as the 250/400 models seemed to be taking their last breath after they were banned for sale in the US.  You can see the wing bolt at the lower right of the tank.  

images.cmsnl.com/img/partslists/yamaha-d...yau1121d-10_408f.gif

P.S.  I merged your new thread with the same exact question into this thread.  It's not a great idea to ask the same question in several places.

Also, you'll need to have about a dozen posts before you can add links.  And the only way to add photos at the moment is to upload them to a third party site like Imgur or Google Photos and link them here

Ah, thanks  that is exactly the oil tank that I see.  Thanks for merging the post - reason I "broke off" was because the original thread was for suggestions for servicing, etc.  But I'm stuck on this specific issue so I thought I start a new thread just for this issue.  With hindsight I can now see how expanding the original post might have been better, rather than start a new one.

And thanks for the tip about uploading images.  I have struggled. I did upload them to Flickr and I'm trying to figure out how to embed the hyperlink or image!

 

Thank you Tinkicker,

I took a few photos, showing the left panel removed which reveals the 2-stroke oil tank.  I cannot see how to access the filler without removing the seat.

I have the photos on Google Images - Once I have figured out how to link them, as you did I will post them!

Regards,
Daren

I have had an idea!  I would bet from your dificulty that someone in the past has refitted the oil tank with the thumscrew lug BEHIND the lug on the frame.  The plastic tank lug should be in front of the steel frame lug.
Remove the thumbscrew and the tank will swing out from the rear.  The seat even has a small cutout in the base for the oil tank cap to clear.

Look to the pic.  oil tank thumbscrew is the bright goldish coloured screw in the pic.  it goes through the tank lug and into a threaded hole in the frame bracket.  Look at the seat base.  You can see the cutout for the cap to clear just in front of the rear fender.

If the tank is behind the frame bracket, it cannot swing out because the frame bracket is in front of it.  If it has the wrong seat without the cutout, it cannot swing out because the oil cap will not clear the seat.

You absolutely do not need to remove the seat if all is as it should be.  It will swing out a couple of inches at the rear.

 

Thank you!

I'm still struggling. I so wish you were standing next to me, in front of the bike! How can I embed a link to Google Images?  So that you can see exactly what I'm looking at.  

I totally agree with you that it should be possible to top up the oil without removing the seat!!

I've tried to link to photos using buttons, image tags (/img) and hyperlinks but the photos don't appear.

DarenMatthews- Again, you need to have a dozen posts here before you can insert links to photos or anything else.