Tend to hold the porthole open a smidge with a hacksaw blade until the fire is showing touches of blue and red rather than yellow and orange.Nice! Pot burners are quite sensitive to flue draught. If you’ve got the spec for it I’ve a gauge you can borrow if you want. You adjust it using the counterweight in the stabiliser.
Better still, connect the flue up properly......Oh and make sure you’ve got a CO alarm etc