When I was in GT in '06 I got 90 days for me and 40 for the bike at the La Mesilla border, don't know why you got less. According to the rules then, you should have got 90 day tourist visa good for a cumulative 90 days in the CA4 (Guat, El Salvador, Honduras, Nicaragua).
The expat Americans I talked to in Antigua said they go to a border, exit and come back, to get a fresh 90 days on their tourist visas. They used to go to El Salvador as it is closest, but now with the CA4, they have to go to Mexico or Belize. Of course if you have a bike, you would have to do a border run every 40, or in your case 30, days.