jstone4646 Posted March 24, 2019 Share Posted March 24, 2019 My new project is an 84 351, it cranks over fine but I cannot get any fuel pressure from the mechanical pump. When I disconnect the line off the carburetor and try to crank its just empty. I have noticed though if I try to siphon fuel from that hose I get the feeling like there is a lot of air in the system, like when you were a kid and blew bubbles in a drink through a straw. I also noticed a gas leak under the tank area after manually pumping in about 10 gallons there was maybe 1/4 gallon on the ground. I do not know where it is leaking from and it could have just been from the small pump I was using not being far enough into the tank when pumping it in...no idea. Was hoping for some suggestions on where to look....also do these 351s rely only on the mechanical fuel pump or is there another small electric pump either in the tank or along the way? Thank you in advance. Link to comment Share on other sites More sharing options...
Gary Lewis Posted March 24, 2019 Share Posted March 24, 2019 Jeff - There are hoses from the tanks to the hard lines, and if you have two tanks there are hoses from the hard lines to the switching valve and then again back to the hard lines. I've seen those hoses so porous that they'll leak gas though the sides of the hose, and air into the system when the truck is running - or trying to run. That air leakage will kill the vacuum the pump uses to pull fuel. So I'd check those hoses for leakage. And, I see two gas cans in the back of the truck. If you run a hose from the inlet of the fuel pump to one of those sitting on the ground the pump should pull the gas and prove the pump is good. Link to comment Share on other sites More sharing options...
jstone4646 Posted March 25, 2019 Author Share Posted March 25, 2019 Jeff - There are hoses from the tanks to the hard lines, and if you have two tanks there are hoses from the hard lines to the switching valve and then again back to the hard lines. I've seen those hoses so porous that they'll leak gas though the sides of the hose, and air into the system when the truck is running - or trying to run. That air leakage will kill the vacuum the pump uses to pull fuel. So I'd check those hoses for leakage. And, I see two gas cans in the back of the truck. If you run a hose from the inlet of the fuel pump to one of those sitting on the ground the pump should pull the gas and prove the pump is good. Thank you Gary, I followed your advice and ran the mechanical pump intake straight into the fuel can....worked like a charm. Once I connected everything back up the truck ran beautifully....I guess it was just struggling to 'prime the lines' so to speak. The previous owner replaced all the fuel lines from the tank up to the carb, so it was nothing but air and I never realized (my first 351) that these motors have only a mechanical fuel pump. Once I had it primed to the carb, it stayed alive a lot longer and (just a guess here) allowed the engines mechanical pump to spin faster/create more suction, and pull fuel into the motor. No issue now, she cranks every time (and I put in a new battery). Runs decent but she is idling pretty high and also runs a bit rough so I will need to read up on how to adjust a holley 600, along with a fresh set of wires/plugs....oil change, etc. Oh and I also found out my hose connection between the metal fill tube, and rubber portion leading into the gas tank was loose, which explains the bit of fuel on the ground after the manual fill. Link to comment Share on other sites More sharing options...
Gary Lewis Posted March 25, 2019 Share Posted March 25, 2019 Thank you Gary, I followed your advice and ran the mechanical pump intake straight into the fuel can....worked like a charm. Once I connected everything back up the truck ran beautifully....I guess it was just struggling to 'prime the lines' so to speak. The previous owner replaced all the fuel lines from the tank up to the carb, so it was nothing but air and I never realized (my first 351) that these motors have only a mechanical fuel pump. Once I had it primed to the carb, it stayed alive a lot longer and (just a guess here) allowed the engines mechanical pump to spin faster/create more suction, and pull fuel into the motor. No issue now, she cranks every time (and I put in a new battery). Runs decent but she is idling pretty high and also runs a bit rough so I will need to read up on how to adjust a holley 600, along with a fresh set of wires/plugs....oil change, etc. Oh and I also found out my hose connection between the metal fill tube, and rubber portion leading into the gas tank was loose, which explains the bit of fuel on the ground after the manual fill. Welcome. Glad it worked out so easily. And, is seems like you have turned a corner. Well done! Link to comment Share on other sites More sharing options...
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