The unloader valve is either stuck open or is defective. The most common problem is the valve gets gunk in it and it gets stuck open not allowing the pressure to build. If you have an owners manual look at the parts section and you will see where the unloader valve is located. If you don't have book go to jackssmallenginerepair.com and go to parts look up and type in your model #. Also check hose to gun and nozzle for obstruction.