Hi guys; Have an issue with the mechanical fuel pump on my 1985 F-150 4x4 351HO. Was not getting fuel to carb, so tried a new fuel pump; no go. Had never worked on a carb before, but had to try. Took me a few months to find problem and rebuild. During this time carb was installed a few times to work out problems, but had no issue with fuel at the carb. Finally got carb right, installed an inline filter, and put back on. Truck had been sitting again; Turned it over and no fuel at the line going into the carb, so I thought either pump is bad, clogged or has lost prime. Bought a Mr. Gasket mini electric pump and installed on frame between tanks and engine. Still no fuel. Bypassed the mech pump and got fuel and truck started, ran smooth for a few minutes and began to stumble. My neighbor came over and after scratching our heads thought the small elec pump may no be producing enough volume for the large V8. So I bought another mech pump.
Question: This will be the third pump in a year. Before I use another new pump, why wouldn't elec pump push the fuel through the mech pump? The first time I changed the mech pump I found out that was not the problem.
Does anyone have any thoughts?
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Okay, found out there was a kink in the short piece bypassing the mech pump. It is running smooth now. Question remains, why won't the elec pump push fuel through the mech pump? Do they stop up that easily? Would like to either reconnect mech pump or install new one and remove elec pump for another use if possible.

 

The mechanical pump is a diaphragm with two check valves, kinda like a resuscitator the EMT's use to start a person breathing again.

I wouldn't expect an electric pump to be able to pump through that.

I personally suspect one or more other factors are needing attention and contributing to your problem; I wonder if one or more of the short pieces of rubber hose between steel lines are dry rotted and leaking (happened to me) or something similar....

 

Thanks for the reply! I have inspected all the lines from the tank switch to the carb and not found any leaks. To bypass the mech pump I just used a metal tube of the correct diameter and put the two lines going in and out of the mech pump together, so the lines are clear. Engine is running now, just drove it up and down the road. If the mech pump was stopped up, the lines weren't. Still can't figure why I couldn't get gas just using the elec pump as a back up between the tanks and engine if the mech pump is okay. The mech pump actuation lever was not loose when I pulled it and inspected it. Feels like it is still tight and should work. Put it back in and kept the bypass around it using the original lines and the elec pump works fine now...Weird, but then I'm not a great mechanic.