Hi Kiran,
From your picture, it looks like your heat riser is short. Minimum height should be at least 3x the height of the feed tube, preferably closer to 4x.
Also, with no downdraft or mass to add to the slowing of the exhaust gases, an open test is not going to be
super accurate - only enough to show you that it works and to have a bit of fun experimenting with before going any further.