If it is bucking and spitting black smoke, you are far too rich. For a cold start, the boost pump is only run for a limited time (full rich, throttle open). The boost pump and individual airplane fuel system will dictate what time is required. For example, the -8 I fly requires the boost pump to run just long enough to start moving fuel. This point is indicated when you can hear it "slow down" and takes about 1 second (yes, I said ONE second). Any more prime than this and it is way too rich at start, no matter how cold. My Hiperbipe on the other hand, is more conventional in that it takes about 5 seconds of boost to prime.
Hot starts are more art than anything, but I have yet to meet a Lycoming that needed boost when hot. If it's hot, the technique that works 99.9% of the time is: boost off, mixture in cutoff, crank engine, advance throttle to full, and wait to fire. When it fires, rapidly go to full rich (it will be spooling down at this point, starved for fuel, so you have to "catch" it before the prop stops), as it catches the second time, retard throttle to fast idle. This all takes about 2 seconds from first catch.