Took my recently rebuilt 81 XT 250 out for a test ride but it was very sluggish & did not have the acceleration as it should i do not think i could get it above 40 , did a plug chop test & the plug is a nice tan colour so i dont think its running rich  , i have adjusted the valves & done a vacumm leak around the carb but was ok, Any suggestions ?

Have you checked and double checked the valve timing?

i did adjust them just before i test rode it

He's asking about camshaft timing, not valve adjustment. 

When you rebuilt the engine there should have been instructions on aligning crankshaft and camshaft markings when installing the cam chain to make sure the valve timing is correct.

Valve timing. Good point, never thought of that. There'll be a T mark on the flywheel for valve timing & a notch up top for the cam sprocket to line up with at TDC. There will also be a F mark for firing & an advance mark or two lines close together for advance. Just don't get confused by them. Easy to feel TDC with a screwdriver down the plug hole anyway but you can bet that when you let the flywheel go the magnets drag it oneway or other so need a trick to hold it steady.

Cam sprocket is definitely lined up as i did it when adjusted the valves , need to check the F mark.

Forgive me for saying this, but it doesn't sound like you understand what needs to be done?  The cam mark AND the crank mark need to be perfectly aligned with their respective marks at the exact same time.

I'd align the crank timing markings (likely T, not F, check the manual) perfectly and then check to be sure the cam timing markings are now also perfectly aligned at the same time. 

i get what you mean now & no i never did that so will give it a go . Many thanks

Remember that the cam turns at 1/2 the speed of the crank...  which means both sets of the marks will only line up once every TWO revolutions of the crank. 

So it's probably easier to line up the cam markings and then check the crank markings.  Or if the crank marks are aligned and cam mark is not visible, rotate crank around one more time. 

Yep T mark for valve timing along with circle 'i think it is' on the cam chain top sprocket to align with a notch in the head.