To start, I don't really have a baseline to judge this from, because my truck has an electric fuel pump.

86 300 I6 with a carb

I had a buddy over this weekend to help him work on his truck. He said on the way to my house he hit some traffic and his fuel tank got a little lower than he normally let's it get before he could hit a gas station. Ever since it nearly ran out of gas it's been stumbling and sort of hunting for a happy place to idle. The first thing we did was pull the old fuel filter and swap it out. The old one was filthy and we were thinking it sucked up some sediment.

We replaced the fuel filter with a clear inline type that sits maybe 6 inches before the mechanical fuel pump. I had him start the truck and I watched the fuel filter fill up with gas and then it looked like all the gas got sucked into the motor and the fuel filter was no longer filling with fuel. At this time it started to idle rough and dropped a few rpms. Should the mechanical fuel pump have enough suction to keep the fuel filter full of gas? Or is it acting normally? I've never had a chance to watch a mechanical fuel pump in action so I have nothing to base my judgement from. We're thinking that the mechanical pump might be bad but before we replace it we want to know if this is an abnormal action. Thoughts?

 

Take off the fuel filter and spray carb cleaner into the fuel inlet of the carb.

 

that won't really do much if anything.
Get a fresh can of gas and a long hose to fit the pump inlet and 0lace it in the gas can. 1 see if it runs smoothly 2 if you have the filter on it see if it has fuel when running. Most all filters are after the pump not before so can't say if what you see is normal or not?

Now do know there is a filter sock in side the tank on the end of the pickup line. Could be that is dirty / plugged?
Could try blowing air back thru the fuel line to clear it just don't use too high of a psi or you can blow the sock off or apart.
Dave - - - -

 

The sock on the sending unit did come to mind. I had one in a truck that was eating itself and pumping rust through the lines. Unfortunately he has the long midship tank in his truck and it'll have to be dropped if that sending unit needs replaced.

 

It will clean out the needle and seat.

 

The clear inline filter can have a lot of air in it and still work fine. I have used one for years in the same position before the fuel pump and it often appears to only be partially full of gas.

 

No one said it would be easy
A floor jack to support the tank if the tank has some fuel in it if not a lot then jack just to hold it up till the straps are undone and lower it to the ground. Works the same for rear mounted tanks too.


The sending unit would not need to be replaced just the sock on the end of the pick up tube. You can get them at the dealer and come in 2 different sizes to fit 5/16 or 3/8 tube IIRC.


It would be a good time to dump out any crud, flush out the tank before putting it back in place.
Dave ----


PS: DO NOT USE A NORMAL DROP LIGHT when working with gas. If gas hits the bulb it can break because of the temperature change or the bulb breaks it could light off the fumes.
I saw a gas station burn to the ground because of this.