In this video I try to show how to adjust a typical 2 stroke carb. Many carbs will have H and L screws as well as an idle adjust.
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If you have any adjustment screws they may help with the performance. In the case of a single screw it is the minimum butterfly position primarily used for ensuring that a bare minimum of air makes it into the engine during starting. Once the engine starts, the governor and throttle will counter act on one another to adjust the butterfly to maintain air and fuel mix and subsequently engine speed. Depending on the engine and carb this screw may continue to control the minimum idle speed. This single screw also ensures that the butterfly doesn't get jammed shut.
If you have more than 1 screw it is most likely that you have a total of 3. 1 is off by itself and acts as the minimum butterfly position as described above. The other 2 will be found together and act as the min throttle fuel adjust and the max throttle fuel adjust. Min throttle is always flowing fuel so it will affect the idle speed and the max throttle fuel mix. Every time you touch the min throttle fuel screw you will probably have to revisit the idle speed screw and the max throttle screw.
Before I begin making adjustments, I like to document where I am when I start. I slowly count the turns clockwise on all screws until they lightly stop. I write the three numbers down as reference settings. Then I reset all 3 screws to their as found position. If you have absolutely no good idea where the screws should be then start each of them one turn open (counterclockwise) from full shut (clockwise). Most of these engines require a choke when cold and a full throttle for starting. This primes the engine and ensures that the fuel lines, fuel bowl, and carb are full of fuel. The choke ensures a rich A/F mix for cold starting. If a priming bulb exists then pumping the bulb until it is full of fuel for a few pumps primes the engine as well. When pulling the cord, after the first sputter you can take the choke to half (if it has one) or off if it doesn't. Keep full throttle and retry the pull start. If it won't start then verify spark. Since full throttle is being applied, tweaking open the max fuel screw a 1/4 turn at a time will admit more fuel. If it still doesn't start then it suggests a clog in the high throttle fuel path.
Once it starts, run it at full/elevated throttle for a few minutes to warm up the engine. You know it is warm when you can take the choke all the way off and the engine surges up instead of dogging down. This does assume that the fueling is already close. If it is way off then an unchoked operation may not yet be possible. Now you can start the real fuel adjustments. Release the throttle and see if it stays running. If it starts to die then goose the throttle enough to keep it running. If it is slow to respond then open the min fuel screw slightly until the response is no longer sluggish. Once the response is no longer sluggish you can tweak the minimum idle screw such that it just barely keeps the engine going smooth and even. Goose the engine again to ensure no sluggishness. If needed repeat. At this point definitely make sure that the choke is off.
Once the minimum throttle fuel and idle speed screws are adjusted then you can move on to the max throttle fuel screw. Attempt to slowly raise throttle until full (WOT). If the engine dogs down along the way then it may be getting too much fuel. Tweak closed on the max throttle fuel screw and see if it improves. If it doesn't dog down and you can get to full throttle then hold it there and attempt to tweak the max fuel screw open. The RPM should climb. Continue until the point where it begins to die and then turn back closed until max RPM.
Recheck all settings by releasing the throttle and letting it idle, make sure that it doesn't die. Then goose it a few times to make sure that it is not sluggish. Then full throttle to make sure that it doesn't dog down.
This works for all types of small engine equipment. The only thing tricky about marine engines is that these settings need to be finalized in the water with a load. If any of the adjustments don't work then look at that part of the carb for clogs of debris or varnish. Lots of carbs have Welsh plugs with ports, orifices and screens hidden behind.