Atomic 4s have some starting issues, most of which are easily solved.
First and foremost, be sure you are using Marvel Mystery Oil in the fuel - it DOES make a big difference, especially with keeping the valve faces and seats clean, which aids dramatically in starting.
The flame arrestor is held in place by just two screws, and fairly easy to remove with an ordinary screwdriver. After removing the flam arrestor, it should be soaked in a carburetor cleaning solvent, Gumout, etc..., for a couple hours, then thoroughly rinsed in the solution, blown dry and reinstalled. An awful lot of crud will be found clogging the fins of the arrestor, which also messes up the fuel mixture.
The carburetor float must be set exactly, which is somewhat difficult to do without the aid of a float gauge. Sometimes you can eyeball the float level and hit it right, but it's a 50/50 shot at best. Of course, while you have the carb off the engine, why not give it a thorough cleaning, too. Can't hurt.
The spark plugs should be replaced every spring after burning off the fogging oil from inside the cylinders. Be sure to replace those old plugs with the hotter burning plugs, which keeps them a lot cleaner and makes the engine much easier to start.
You can find a lot more information on which plugs are best suited for your engine at Moyermarine.com .
Hope this helps,