My 600 series MTD tractor (has 18.5 horse B&S) has been giving me fits for some time now. Occasionally it will not start, but cranks fine. This can happen when it's cold or hot, it can be the first time I have started it (or not) that day or the 10th time it has been started that day. It is getting spark so I believe it must be fuel related, is there a float in the carb that could be sticking... when it does not start I always smell fuel almost immediately after cranking begins? Thanks

Yes, the float could be sticking open and flooding the engine. Do you notice an increased amount of black smoke?