1994 GMC S15 starting

Engine Mechanical problem
1994 GMC S15 6 cyl Four Wheel Drive Automatic 110k miles

When I start my 1994 GMC Jimmy S-15 It sounds as tho it is going to turn over. However It wont turn over unless I pump the gas once or twice. I have a worn out catalytic converter and or muffler and just recently had problems with the exhaust making my jimmy shake and idle very ruff when I would come to a stop. This however has stoppep a little after I used a bottle of seafoam. So heres my main? What could be causing my truck not to turn over without pumping the gas peddle once or twice?