Countax tractor runs poorly then picks up?

Countax C800H with Honda 18 hp v twin. 2004 model
Starts okay on full choke, then push choke in but runs poorly for maybe 15 -20 mins (max 2500 rpm). It will cut at this but will struggle then will "belch" smoke and run at full power 2850 rpm and be okay. Pulling choke out again does not help.
Plugs, oil filter and oil were changed at start of this year and have just changed air filter.
Is it the choke sticking on? Any ideas?