GMC Repair: 87 gmc 1500, maximum vacuum, vacuum test

Question
I have an 87 gmc 1500 with a 350 engine. We have replaced the fuel pump and the relay. The truck will roll over and crank for about 5 or 6 seconds but it will shut back off. My husband says it is as thou the fuel lines are not right because its not getting fuel to the carburator. We hecked the lines and there right. What else could it possibly be.

Answer
Try this crystal this is a inspection chart hopefully this helps.
INSPECTION


-  Make certain that there is fuel in the tank.
-  Check for leaks at all fuel connections from the fuel tank to the carburetor.
-  The engine should be running.
-  Tighten any loose connections.
-  Check all hoses for flattening or kinks that would restrict the flow of fuel.
-  Check for air leaks or restrictions on the suction side of the fuel pump. Leaks on the suction side will seriously affect pump output.

FUEL PUMP FLOW TEST


-  Remove the fuel supply line from the carburetor and insert it into a suitable container.
-  Crank the engine.
-  The fuel pump should supply 237 ml (0.5 pints) or more in 15 seconds.
-  If the flow is insufficient, check for a restriction.
-  If there are no restrictions, check fuel pump vacuum and/or pressure.

FUEL PUMP PRESSURE TEST


-  Disconnect the fuel inlet line at the carburetor.
-  Install a low pressure gage to the line.
-  Start the engine.
-  Fuel pressure should be 27.5 to 44.8 kPa (4 to 6.5 psi).  
-  If the pressure is low, check for restrictions in the fuel tank sender unit filter, lines and hoses.

CAUTION: Hold the carburetor fuel inlet nut while tightening the fuel line fitting to prevent carburetor damage.



-  Connect the fuel inlet to the carburetor.

FUEL PUMP VACUUM TEST


-  Disconnect the inlet hose at the fuel pump and connect a vacuum gage.
-  Crank or run the engine until the maximum vacuum is reached.
-  If the vacuum is less than 50.6 kPa (15 inches Hg) , replace the fuel pump.
-  If the vacuum is 50.6 kPa (15 inches Hg), check the fuel lines and hoses for leaks, kinks, or splits by disconnecting each section of line and connecting a vacuum gage. Crank or run the engine until the maximum vacuum is reached. The vacuum should be at least 50.6 kPa (15 inches Hg).