B & S engine won't start all the time

I have a Husqvarna YTH2348 with a Briggs & Stratton engine 445577 2187 G5 that I've been working on and its driving me crazy. I have cleaned the carburetor twice, put in all new gaskets, changed the fuel filter, adjusted the valves,new battery, oil and filter change, new air filter, drained the fuel and put in new. The mower runs great with a small shot of carburetor cleaner and will continue to start afterwards as long as the engine is warm. After it sits for a while it won't start again without another shot of carburetor cleaner. It starts right up and will continue to run and start until it sits again for a while. Any ideas? The flywheel is rusty, would that make a difference? Key is ok. I'm totally lost at this point.