For the past year I am having issues with both engines, starting, staying running and idling. I have changed the fuel filters and I noticed that they are never full ( I am using just a replacement filter should i be using a certain type?) they are usually half empty. Engine is performing like crap, sputtering and running rough even with new plugs etc..

just wanted to know where to start from, and if there is a structured approach to fixing the problem, rather than randomly start replacing stuff. I read quite a few posts here (which are excellent) and I am trying to develop a plan of attack.