‘76 FJ40 Fuel System Conundrum

[CaliFJ]

2F with OEM fuel system (but added the smog crap for California). I have been using a Dakota Digital for the past 5 years.

Symptoms:
1) With full tank, fires up and runs smooth. When the gauge shows 3/4’s I get back firing and then she’ll quit. Add fuel to the tank from spare can and starter fluid in the carb. After some doing, fires up and runs great.
2) Carb refurbished about 5 years ago, added smog gear (purchased in Colorado in 2015, and then registered in Cali). Passed initial smog, but hasn’t since 2019.
3) somewhat fresh fuel filter
4) no fuel leaks

And yes, I searched. The engine quitting at 3/4 full has never been brought up.

 

[MUDZLLA]

Sucking air/vapour in from suction line inside tank is my guess. Do I get IH8MUD Silver status for guessing right?

 

[CaliFJ]

Sucking air/vapour in from suction line inside tank is my guess. Do I get IH8MUD Silver status for guessing right?

How would you go about troubleshooting such a theory?

 

[B y r o n]

The only thing I can think of is disconnect your fuel line before the fuel pump and let it siphon the gas from the tank. If there is a leak/hole in the metal line in the tank the gas will stop flowing. Based on your numbers there would only be 1/4 of a tank in your bucket. If you get more, like 1/2 a tank, then the problem is likely somewhere else.

 

[pjohnson]

Is the sight glass full when it happens?
Do you have all the vapor return smog parts properly connected?
Vented or non-vented gas cap?

 

[GA Architect]

Throwing this out there....did you happen to switch up the fuel supply and return lines by chance?

 

[CaliFJ]

Is the sight glass full when it happens?
Do you have all the vapor return smog parts properly connected?
Vented or non-vented gas cap?

ASFAIK, the smog gear is all still hooked up properly. Sight glass is a bit less than half full. The carb jet spray is strong so I don’t think it’s the fuel pump or filter. Changed fuel cap but same issue quitting at 3/4 full.

 

[CaliFJ]

Throwing this out there....did you happen to switch up the fuel supply and return lines by chance?

I haven’t touched the hoses yet.

 

[CaliFJ]

The only thing I can think of is disconnect your fuel line before the fuel pump and let it siphon the gas from the tank. If there is a leak/hole in the metal line in the tank the gas will stop flowing. Based on your numbers there would only be 1/4 of a tank in your bucket. If you get more, like 1/2 a tank, then the problem is likely somewhere else.

I think the issue is to much pressure. I popped the fuel cap, reinstalled and she fired right about at 3/4 full. Running great. Could it be something in here?

 

[pjohnson]

Tighten the gas cap and see if it happens again.
Try a vented cap if it repeats.

 

[wimberosa]

deja vu.... I had this exact thing happen on my sears lawn mower Gas cap was too tight a seal and eventually suction was incapable of drawing
the fuel out of the tank as there was no air displacement possible. Ran great with a full or near full tank

 

[sggoat]

Seems that too much pressure would not cause what you are seeing(unless the carb float valve is somehow involved)-and the engine is flooding.(your fuel pump should bypass any high pressure if it gets to above ~ 3 psi)--but if the float valve is sticking open-----but that prob , if present, would occur regardless of the fuel tank level.
Think the backfiring symptom should give you some avenues to explore--But-that issue is usu connected to timing/vac--nothing to do with fuel level--
You may be on to something with the fuel tank vent system(cap, etc.) Maybe the tank is actually under a vacuum??-- remove the fuel tank cap and tie-wrap -several folds of an old tee shirt on the fuel fill port(as a filter) see if the issue is still there--if so, the vent system is plugged up

 

[CaliFJ]

Seems that too much pressure would not cause what you are seeing(unless the carb float valve is somehow involved)-and the engine is flooding.(your fuel pump should bypass any high pressure if it gets to above ~ 3 psi)--but if the float valve is sticking open-----but that prob , if present, would occur regardless of the fuel tank level.
Think the backfiring symptom should give you some avenues to explore--But-that issue is usu connected to timing/vac--nothing to do with fuel level--
You may be on to something with the fuel tank vent system(cap, etc.) Maybe the tank is actually under a vacuum??-- remove the fuel tank cap and tie-wrap -several folds of an old tee shirt on the fuel fill port(as a filter) see if the issue is still there--if so, the vent system is plugged up

Will definitely try the rag approach. I’ll remove and clean all the hoses near the fuel fill. re your comment about backfires. It only happens right before it quits. Usually two quick shots. So I’m betting the pressure issue is suddenly making it run rich just before it sputters and dies.

 

[pb4ugo]

Back fire thru the carb would be a lean condition(running out of gas)or the timing could be way off, which I doubt. I think it has to do with the gas cap or the fuel venting system.

 

[MUDZLLA]

How would you go about troubleshooting such a theory?

Put a clear filter inline on the gas line and look for air bubbles