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Half tank fuel issues

Hi guys first post so please be gentle.

I’ve got a 1983 Mazda GLC hatchback, and I’m having a hell of a time with the fuel tank I think…
Car runs fantastic right up until it hits 1/2 tank and then it’s like the tank falls off. No fuel flow what’s so ever fuel filter bone dry.

Could that be a clogged evap can?
It’s not a vacuum issue I think (there’s no fuel cap but I have a non vented on order)
I’ve blew clean the tank to evap can line so that’s clear.
I just drained most of the tank to see if it would cause the same issue but it ran fine.
I know it’s 1/2 tank because it only takes ~20L and it’s a 42L tank. And once it’s topped up it runs fine.

I’m at wits end I want my other 20L of fuel

for how long/far ?

My first guess is the fuel pickup tube in your gas tank has a hole or crack in it at about the half tank mark.

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It ran for a good 30 min at various RPMs
The fuel pickup has a toilet trap bend in it, it’s from factory could that be a issue? I’ll take it back apart and see about doing a light pressure test on it

Another possibility is a weak fuel pump…

Have you tested fuel pressure and flow rate ?

Brand new 4psi electric pump and still tied in to the manual fuel pump.

I thought that was the issue at first so that’s why I added the electric one. Turns out the electric one wasn’t strong enough to push the fuel to the carb so I tied it back in to the manual one to provide a boost and back pressure.

When it stops running and you take off the gas cap is there a vacuum sound? Have you tried restarting after you just remove and replace the gas cap?

He’s not running a gas cap at this time.

That is suspicious, an electric pump should be more than adequate. The fuel line must be restricted or there is a small leak allowing air in. Also check the power supply to the electric pump.

Oops, missed that. Time to check fuel pressure at the carb.

Brand new rubber line from tank to carb temporary untill it’s not -fyou degrees outside Haha. But it was doing it as well before I added the electric pump in making me think it was the mechanical going bad but running 2 pumps should give me more then enough flow. As indicated by the full clear fuel filters. When it does die it dies hard and drains the entire system. Electric pump hammers away because it’s empty fuel filter bone dry and carb site glass empty

Hayns says it’s a 3-4 psi requirements so with a 4 PSI electric and still running the factory mechanical I should have enough pressure, it’s only at 1/2 or below that it craps out.

It sounds like you’re not getting gas at the carburetor- have you taken off the air cleaner and looked in the carb while pumping the gas when this happens? If no squirts of gas something is blocking the fuel flow.

But why would it run fine till 1/2 tank? Tank was cleaned and flushed and brand new fuel line

I think you need a new fuel pump and gas cap.

Look, we aren’t there, we can’t see the car, we’re just giving you ideas based on what you’re telling us. Do with them as you will.

Take the electric pump out of the line circuit and restore it back the way it was. Run the engine until the mech pump is starved. Remove the inlet line at the pump and rig in your electric one with a switch for power. Fire it up. Does gas flow? No? The inlet is not submerged in the gas or there is a hole in the inlet tube. Two approaches; vacuum or pressure. You can verify the leak exists by capping off the inlet and using a small hand vacuum to see if it will hold. If it leaks, you could find it by capping off the inlet and back pressurizing the line at the pump. Just a few PSI. Listen for the air leaking out…

I verified that it has the required PSI to function (measured at carb inlit) removed the dip tube and checked for a leaks with a vacuum hand pump. It held 20hg for 10 min so there’s no leaks in the piping. Measured top of tank to bottom it gave me 4.5 inch measured pick up tube and it was 4.75 inches so I think it’s at the low spot of the tank.

Im 90%sure my charcoal can is clogged or malfunctioning I have unhooked the fuel tank vent line from it and I’m letting it vent to atmosphere. Would a bad can cause my issues?

When you fill the tank, you claim you can only get 20L into it.

Are you positive there was still 20L in the tank and that it only runs on the top half?

Or is the tank empty and stops filling when you get 20L in it?

It’s full now and I pulled out 2 20L pails and there was still some in the tank so I’m sure that it’s only drawing ~20L and everytime I fill it when it does die it’s anywhere from 18-21L that will go in