I dont have a dial gauge but I do have timing disc. Can anybody tell me what 3.2mm BTDC is in degrees please? Failing that anyone know the eye to eye length of a standard conrod so I can work it out myself? 

It's been a while since I did the calculations but off the top of my head I remember the timing coming out right around 21 to 23 degrees BTDC for every model I calculated.  (I assumed no piston pin offset, I'm not sure if Yamaha offset the pin)

The reed valve 250 should have a 125mm rod.  Piston port 250 had 130mm rod.

Thank you MarkT!

Be an interesting experiment to strobe it after, to see how close it 'actually fires' to your degree mark or however you mark it on the flywheel at 3.2btdc.

Gvthnks,  Piston movement is not a linear relationship to crank rotation, con rod angle (determined by con rod length) makes a difference.  For example, with the crank rotated 90 degrees past TDC, the piston will be over half way down the bore due to the con rod angularity...

Near TDC, you need to rotate the crank more degrees to achieve the same vertical piston movement than you do when crank is near 90 past TDC. 

www.torqsoft.net/piston-position.html

Getting too technical for me.
Think i'll just add a lever arm to the stator plate & spring loaded plate screws with a cable to a lever on the bars. Then move it to find its sweet spot of advance--just before it holes a piston!!. Similar to what my BSA B33 had with movable flat points cam plate. Then again Maybe Not!!.

MarkT, thanks for the correction.

Apparently forgetting is a thing. I'm going to delete my post so no one gets confuserated by my math.