I'm with Dave ^^^ far more likely to be a breather issue, than a compression issue unless as Ralph says^^ it has overheated (or similar), you could settle your mind by buying a tester, £9 on eblag .... https://www.ebay.co.uk/itm/AUTOMOTIVE-T ... SwceNZVMoM

Sounds like a valve leaking
If the cam chain was well stretched , could it cause the valves to close late
Anyone had a prob with valves leaking
Was thinking to try lead replacement liquid, add to petrol
Would this do sensors in , mmmm

If tester showed a leak question would be is it head g or valve leaking
Pulled breather tube today makes no diffrence
I supose I could run till it gets a lot worse then I will know what it is

You would hear the cam chain rattling long before it got to the point were valves would
touch it it ever could without snapping the chain.
And a bent valve would show up in the valve clearance being wide, think you checked them.
If the competition was down then at least you would know the head had to come
off I thing the engine as a auto decompressor to aid starting but I have never heard
of one giving problems.

I have in the garage a Kawasaki KLR600, it was left out ONE night some years ago by it's then owner, ( in the winter at -9c no antifreeze ) the water jacket went pop across the cam chain tunnel, this then continued into exhaust port that was not visible. The guy had the chain tunnel tig welded. But the the same thing happened to this..loss of power as the crank case pressurized. If you ran the motor for 5 mins and stopped it, removed the oil filer and got covered in hot oil

Sounds to me like a broken piston ring. These Burgman engines (like many engines) will run ok with a broken ring (usually top compression). I did a top end OH on a 2008 a few years back that had exactly the same issue you are reporting. Only thing, a broken ring, especially the top one will often make a small clicking as the engine runs...but it'll be down on power, crankcase pressure will be up, won't run nice. You could spend £8 on a phone usb endoscope/borescope camera from ebay...something like this:
https://www.ebay.co.uk/p/2m-Endoscope-C ... 2599632400

It will let you take a look at your cylinder bore to see if there's any damage. A broken ring can cause scratching to the cylinder or sometimes no damage if you investigate and strip it early. The 2007-2009 399cc Burgmans had a different piston from the later 399cc engines because the piston use to flutter a bit at high rpm's. This can lead to piston ring breakages at higher mileages. Maybe, just maybe you have this issue.