It's time to do the crank seals on 250. 
I have not changed them since owning the bike and who knows when they were last done. 
It's not running properly in the top end of the rev range and I have eliminated other potential causes. 

Where is the best place to buy seals from? Yamaha Australia no longer keeps them. 
I have been looking at disaster motors in Canada and bike buzzard in the USA. 
I need 62x 30mm for the stator side and 62 X 40mm for the clutch side. 
The bike is already apart and ready to go!

 

If, when you get the seals sorted it still doesn't perform then the first place i'd always look is the exhaust, especially the spark arrestor which is fixed deep in the tail muffler. Looks like a jet engine like vanes on an angle & they get blocked up.

Equally important is the Oring No4 in the carb if it has this original type carb. It seals the jet setter into the bowl so all the fuel passes through the main jet to go up the needle otherwise it just sucks past the Oring & effectively becomes a huge jet & bypassing the main jet.

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www.cmsnl.com/yamaha-dt250-1974-usa_mode.../partslist/B-11.html

Thanks RT325, I will check that O ring regardless. 

Has anyone used disaster motors out of Canada for seals? They seem to know the differences between models and seal sizes. 
www.disastermotors.com/products/yamaha-d...tch-sprocket-250-360
 

My seals arrived today and I'm ready to install them. 
the new seals have Castletons like genuine Yamaha seals, the old do not. 
Do the Castletons face into the bearing or away?

Nice find on the seals!  Much better than the wrong design generic seals some try to sell.

Castle turret bumps are there to prevent seal from being pressed in too far and blocking the oil hole above in the transfer port cutout...  which is a long way of saying bumps face inwards towards the bearing.

Best not to push them in further than flush!! .
Don't rely on the turret bits to stop it
Might be wrong but just visualising the turrets in my mind i 'think' the turrets are slightly relieved so don't sit at full height so probably yamaha allowing for them ending up under the oil hole-oil fuel mix hole so you'll be safe. Happy to be corrected as usual.