Yellowing gas tank..

Try bleaching it. Some people say it works, some say it doesn't. Scrape a small portion with a blade a few mm deep. If its still yellow then forget it, the staining will be all through the tank. Sometimes you may be lucky and the just the outer surface has some sun damage and a little deeper under that its still white.

have a search on http://www.ozvmx.com/community/ this topic always comes up. Most people live with what they got, buy a new tank or find a better one.

Yes, there is a lot of stuff on Ozvmx about bleaching the tank. Use strong bleach for at least 5 days with all fittings taken off. Scrape and sand the tank and a lot of UV light to help bring it back. You should get it pretty good.
http://forum.ozvmx.com/index.php?topic=2528.msg24388#msg24388
http://forum.ozvmx.com/index.php?topic=4749.0;all

I have an 83 YZ125 coming this week and will be doing the same thing with its white tank that is looking sad. I will post up results, Frame will be going back to black and the dls front brake will be going on my other 83 125.

Iv never done any of this but I do have a question, if the yellowing is down in the plastic could u use say 1500 to 2000 grit wet sand paper to sand it down to the white without reshaping the tank,without leaving sanding marks or leaving flat spots by accident, then buff it out with a sponge buffer to a nice finish?

The problem is that these white tanks turn yellow from old gas, sun damage and even being exposed to the atmosphere; and the off color is all he way through the plastic or deep enough that sanding is impossible without killing the tank shape. I have never seen anybody find a way to change this and make them look good again. I had to find a new oem tank in japan for my 87 and even that and the Clarke tanks don't match Yamaha white.

Weird thing is my KTM and Cagiva tanks are snow white underneath the safety seat and the shrouds

Will the bleach hurt any cast in metal inserts?

You'll need to start with domething rougher than that like 200 grit and work your way up wet sanding

Yes that makes sense, if it's all the way thru than I suppose it is screwed anyhow.

Bleach the inside and outside of tank for a week. Take off all fittings. Leave in sun or UV light to help brighten it. You can get white perforated decals for the tank and period shroud decals for your 86 from MXM... http://mxm.co.uk/?page_id=17&category=21
Your 86 is looking good already.
When you sand and work your way right back to 2000 wet and dry, try using Plastic Renew. People have different opinions but i use it on my old plastics and it brings them up like new... My US edition tank had brown staining and sanding removed it. The front and rear fenders have been sanded and Plastic Renewed. The sidecovers and shrouds are DC.
Here is my US 83....