whats wrong with my engine?

i have a briggs engine it won't start.i have replaced spark plug rebuilt carb checked valves for sticking the engine has good spark gas coming to carb.i also replaced gas line and filter the engine seems to have good compresson. it wont start using starting fluid either.the engine is on a snapper mower.its old i don't know the h.p. of this engine. any suggestions would be appreciated.