I don't have a manual or much experience with four strokes of that era.  But yes, it should advance smoothly when RPM increases

For my old TW200, the "F" mark was around 10 degrees BTDC at 1300 rpm idle and at about 5,000 rpm the timing was advanced fully to about 30 degrees BTDC. Yours should be similar.

Thanks Mark , will keep you posted .

Generalizing a bit but would be close--if you looks around ANTIclockwise from the F mark you'll see two scribed marks. Those marks at revs. About 2000+ should come up to where you observed the F mark aligning at idle. Also you'll feel the motor free up like you've taken a load off it. Does that make sense, maybe only to me!!. Now I'll go looking for a flywheel photo showing marks. It's been many years but can remember-- but can't remember yesterday lol.

Yep that makes sense lol

If you do dig up any pics that would be great as there are no marks on mine apart from the F T that i can see.

Ok, im out for a while but here's a relevant randon video the was first up & is relevant enough. SR250 if its the early type is realy just an XT250 & looks the same but electric start. & only 228cc or so, that i found out one when i had a cylinder off at work years ago. Anyway--irrelevant. I dragged the video until near the end when he strobes it & is plain to see the advancing. So yours should do similar--long advance movement. Hope i helps.
Drag it to exactly 10 minutes for strobing explanation.
www.google.com/search?client=firefox-b-d...vid:ukOcos56N4w,st:0

There probably is no mark to show "full advance". 

If there were marks on the flywheel, the gap between the FA and the F would be twice as big the gap between the T and the F.

FA - - F - T

Put strobe back on , in idle F mark is spot on but when revved there is no advance at all !!!!. Any suggestions . Regards

Presumably any change in timing is governed by the CDI box, rather than mechanically? I couldn't see any mechanism on the parts list.

I only ask because my 1980 (ie not points model) Suzi GS550E wouldn't rev above 3k due to a stiff mechanical advance/retard mechanism (very similar to the one on the RD200). Once this was freed up it revved out fine.

From what I remember RT325 saying in an earlier post, CDI not advancing timing was a problem he encountered when working at the Yamaha dealer?  (Too lazy to look for it)

I looked at the parts diagrams and it appears to me the advance is what Yamaha called "electrical" which means it's in the magic CDI box.