A couple of suggestions to help maintain constant heat.
Time it such that you are only adding a little fuel at any one time. Adding half new fuel will cool down considerably as it has to catch, burn, etc. Adding 10% fresh at one time is much better, for example.
Conversely, if you're lookin' you ain't cookin'. Open as little as possible. Remember there will be a lag to any specific change. If you change the vent, look at the temp after 5min then after 10min to see if the trend is up or down, and if it is accelerating or slowing. Use that as a guide to your next change.
Thermal mass is your friend. Get yourself some appropriate stones, bricks, etc. If you have any room left over from your fire and meat; fill the space with the bricks. They will help to smooth out the heat from open/closing as they take up space that would otherwise be taken by "cold" air when you open to refuel. Obviously you need to not block actual ventilation.
Overall the kettle is a nice grill; but leaves a little to be desired for smoking. The offset firebox style where you can refuel without disturbing the cook-box has a definite leg-up primarily for the refuelling reason; but with some practice you should still have wonderfully edible results with the kettle.
Happy Eating!
I am projecting here, perhaps, but I think the question you're asking is "why doesn't my salmon come out moist and succulent like what I get at the store?"
A couple reasons.
First: you want to properly cure the salmon for at least 24 hours beforehand. 36 is better. To cure, you will need two whole sides of salmon (or one cut in half), with the skin on. Rub your cure into the flesh--so 2:1 salt:sugar, plus whatever other spices you care to use. The addition of brandy sounds lovely; I like tequila or a nice peaty/smoky Scotch myself, or maple syrup (but obviously nix the sugar if you're doing that). Place the two pieces together, flesh to flesh, optionally including herbs between. Wrap very tightly in plastic, completely sealed, bung into your fridge to let it cure.
Second: when the fish is cured, remove from the wrap, rinse off the cure.
Third: time to smoke. If you want dry and flaky, hot smoke--this seems to be what you're doing and what you don't like. Therefore, it's cold smoke time. The two basic ways to do this are either to add LOTS of trays of ice to the smoker itself, or to route the smoke through a cooler filled with ice and then back into your smoking chamber. A quick Google should provide you with diagrams for doing so.
Do not over-smoke. Fish picks up flavours quite readily, and will dry out if oversmoked, even if cold. Really for salmon I don't think you need much more than 20 minutes of cold smoke.
Best Answer
Don't let the tail wag the dog. When smoking, you want to figure out time and temperature first and worry about smoke second. This can obviously vary widely. That said, there are things you can do to get more smoke going:
If you can smell it, you'll get some smoke flavor. If you can see it, you'll get more. If you're not getting (much) visible smoke then you need some combination of 1) wetter wood 2) lower temperature 3) larger wood. You get less smoke when the wood just goes up in flames. Chips are particularly prone to do this. Larger chunks smolder better. Things also smolder better when they're very wet or the fire isn't so hot. But again, you want to figure out your temperature and then get smoke, not make large temp changes just to get smoke.
If you're doing very low and slow (225F), sometimes that's not enough heat to get your wood smoking well. Smaller chips or higher temperature are the only things I've found that really solve this problem.