When on full throttle the revs drop and it cuts out it seems to be starving of fuel. i have cleand the filter carb and exhaust, and changed the fuel.

Engine may be too worn or may have run out of oil at some point, compromising the bearings.

Sounds like it might be starved for either air or fuel. Have you made sure there is full gas flow from the tank to the carb? Also recheck the needle valve regulating gas flow into the carb. May be that a speck of trash is partially plugging that flow to carb. Should have in-line fuel filter (add on) in addition to the bowl 'filter

the fuel system is blocked up
perhaps the plug is bad
and the air filter will need replaced

or the fuel tank vent is also blocked up

Pumping the primer bubble while the engine is running should cause the engine to over fuel and stall (if the tuning is correct). Sounds like its not fueling enough, pumping the primer is forcing fuel through the carb causing the engine to pick up rev's, but this is soon exhausted which is why it wont rev on the throttle.
* Check that the fuel line is not damaged and drawing in air. (if this is intact try the next step)
* Strip and clean the carb. Replace any diaphrams if needed.
* Check gaskets seal between carb, air filter and cylinder.

Just out of curiosity what standard carb settings did you use.?

It sounds like your float in your carb is getting stuck at high rpms. Try taking the bowel off and pushing the float up(make sure to have a bucket for fuel to run into). If it sticks that's the issue an easy fix.

Starving for fuel.
Make sure carbs are getting fuel.
Make sure fuel filters are clean,
make sure fuel pump is working.

If carbs are getting fuel but wont rev, you need to clean your carbs because your jets are clogged

strip the carb out and clean the running jets out with carb cleaner i think its starving for fule that why it wont rev up to full throttel,,,

check carb and clean jets

fuel pump could be the cause but also the float in the carby could be not set at the right level and starve the motor at high revs .

when you put on a new air filter you have to rejet the carb in order for the air/fuel mixture to be correct,because with a aftermarket air filter it is allowing more air in than before and not enough fuel. as the piston speeds up upon exceleration the air is not getting burnt off into exhaust because you do not have enough fuel in the mix