Hey there! I have the Yoder 640 and as the pit is initially heating up there's usually quite a bit of burn off smoke that's leaking like crazy through the door (bits of the previous cook on the grates that are sizzling and causing huge amounts of dirty white smoke). This is perfectly normal and in fact is seen on nearly every kind of smoker - stick burner or pellet.
I believe the reason for this initial leakage has to do with good ole' fashion thermodynamics: Your pit, particularly the thick metal, hasn't yet gotten up to temp but it's hot enough to start burning off residue. Heat rises, but in reality it will expand in any direction it can. Since the overall ambient temp of the pit walls/metal is still low - but the outside air is cool - the heat and smoke are in a mad rush to get out any way it can. Then, once that pit really gets settled and toasty, the hole at the top of the stack ends up being the 'path of least resistance' for the heat and smoke and things start working normally.
Now, if you're constantly leaking smoke even well into your cook then you've got a door seal problem, but I suspect - especially with a Yoder - that it's just that initial burn smoke and that all is well!