Mk2 1/4 to 1/2 throttle flat spot

[Bakers250]

I'm struggling to cure this one...
Recently bought this bike and its in pretty good nick, with a recent service. It has 30,000km on the clock and no service history. I believe it has been stored for some time.

The bike idles well, starts okay and will keep up with crf's at WOT.

When using gentle throttle the engine bogs and splutters at 1/4 -1/2 throttle almost choking it to a halt. I can roll off or power through this bog.

To my knowledge, this would be a needle adjustment but it does not seem to cure it, richer makes it worse.

I have a mk2 with a full fmf exhaust (new), new plug br8eg, good oiled filter.
Running premix at 50:1 with motul 710 as previous owner has converted it.

New fuel filter
I have cleaned the carb (twice)
Tried a whole range of new pilot/main jet combos
Checked power valve motion and correct alignment.
Reeds are clean and close fully
Jets and needle are now back to standard. Although It seems better with a 50 pilot and 130 main.

Lights and horn all work as they should...

What should I check next?

[gregorious77]

compression?

[knackeredMk1]

50 pilot and 130 main would sound right as you are premixing so the jetting needs to be richer. Though the FMF would tend to make the need leaning. 48/128 v 50/130 isn't much difference.

Compression would be my next exploration.

[fallenmikethebike]

Additionally, I would check the exhaust packing is it clean-ish.Also check the float height, it's a fiddly job but worth checking.
Are the breather and overflow hoses clear at their outlet points?
Mike