Start with simple word searches in the "Lookup by key phrase" section such as "carburetor" or "gasket".  If this produces too many results, add another word to your search phrase until you get a phrase that produces a manageable set of results.
In "Lookup by key phrase", leave out common words such as "of", "or", "and", etc.  These words may produce huge sets of results that have nothing to do with what you're actually looking for.
Look up parts by Make and Model when possible; this produces a much more reliable set of results.