Explained really well by Nasir bhai. I agree with all his observations and suggestions.
The warm air pre heater pipe goes from the LHS heater box (theres a small pipe connector on the front side) all the way upto the air cleaner. Warm air is drawn via this route when the air cleaner flap is set to the 'winter' position.
Some later model engines had a different pickup for the warm air.
You are probably missing a small metal strip on the carb. A pic from a clearer angle would enable us to point to the exact spot. I cant find a decent picture to post here to let you know what you're missing.
BTW, I see one more thing wrong on your engine. The location of the fuel filter. It must always be *before* the fuel pump which itself is sensitive to dirt, grit and crap ruining it. More importantly, the brass pipe where the rubber fuel pipe fits on the carb is a press fit (not threaded in) which can come loose with the weight of the fuel filter and extra length of pipe.
Locate the fuel filter out of the engine bay ideally, away from hot surfaces. It would be difficult to access but this is a major prevention against engine fires.