M880 Fuel Problems: Welp - I'm Out of Ideas

[Potomac Valley Organics]

This old 77 has had a persistent fuel problem for several years. It just chokes out of fuel and stops running. Here's what does NOT appear to be the cause:

1. Dirty fuel tank - removed, cleaned, reattached fuel tank
2. Bad fuel lines - all lines have been replaced
3. Bad rollover valve - removed rollover valve from the system altogether
4. Fuel sending unit - replaced it and made sure its vending properly
5. Bad fuel pump - replaced mechanical fuel pump TWICE and even just put an electric pump on to no avail
6. Bad cam/eccentric - the thing pumps fuel like mad when its not connected to the carb
7. Bad fuel filter - replaced fuel filter twice
8. Vapor lock - insulated all fuel lines and even experimented with a three-port valve

Sometimes she fires up fine. You can watch the fuel almost fill the clear filter. There's always a little bubble in there though. Then, over the course of an hour or so idling - or faster if you're driving - you can watch the fuel filter empty out of fuel and see the bubble get bigger until the carb sucks air and the truck stalls out.

Any ideas lads? Cuz I'm out of them.

 

[foxtrk2]

you said you replaced fuel lines does that also mean you replaced all flexible or rubber lines in the system ? . if not that's where id look sounds like a line colapsing in on itself or a crack some place in the line letting air in

 

[NDT]

What about the in tank fuel pickup? They get rusty and develop holes.

 

[73m819]

Tank vent, line. or what ever blocked causing a vacuum, had a 63 chevy station wagon do the same thing due to this issue, took forever to find the cause of the issue.

 

[Potomac Valley Organics]

Fuel lines are all clean and straight. Replaced the pickup/sending unit in the fuel tank. Vent line to the fueling throat is open.

To be clear, you can disconnect the fuel line at the carb and watch the fuel pump out of there consistently with quite a bit of force - the whole fuel delivery system up to the carburetor seems fine.

The only thing I'm wondering now is the prior owner put a Holley 4 barrel on it. Could that be causing my issue? I'm gonna swap it out with a stock type and see if that doesn't help.

 

[AMGeneral]

I am going to throw this out there, I had a similar situation with a 1973 W100 I owned, same symptoms, it ended up being the fiber(plastic) cam gears that were used in the 318s of that era.

I replaced it with a double roller setup for a 360 engine and it cured the problem.

Just a possible suggestion.

 

[porkysplace]

Fuel lines are all clean and straight. Replaced the pickup/sending unit in the fuel tank. Vent line to the fueling throat is open.

To be clear, you can disconnect the fuel line at the carb and watch the fuel pump out of there consistently with quite a bit of force - the whole fuel delivery system up to the carburetor seems fine.

The only thing I'm wondering now is the prior owner put a Holley 4 barrel on it. Could that be causing my issue? I'm gonna swap it out with a stock type and see if that doesn't help.

Your floats probably need adjusting , there is a screw on the side of the float bowl on Hollys and you want the float set so the fuel just barely trickles out the screw hole on the side.

https://www.youtube.com/watch?v=LKEonVeOXco

 

[Tow4]

Are you sure it's fuel related? Pour a little gas in the carb while cranking to see if it fires.

 

[Potomac Valley Organics]

[FONT=Tahoma,Calibri,Verdana,Geneva,sans-serif][/FONT]Tow4 - Yeah it'll fire. Sometimes it runs for hours before shutting off.

porkysplace - Yeah, I'm gonna give this a shot, thanks.

 

[bikeman]

I'd say it's the carb as well. Not sure what, but if you've fixed or replaced everything up to that point, then it's gotta be what's next.

 

[rchalmers3]

I'll pile on with the carby float adjustment, and add an extremely long shot: If the fuel tank is not venting, that could be a source of fuel starvation. You might try simply removing the fuel cap and running the engine to test this possible issue.

 

[Potomac Valley Organics]

I'll pile on with the carby float adjustment, and add an extremely long shot: If the fuel tank is not venting, that could be a source of fuel starvation. You might try simply removing the fuel cap and running the engine to test this possible issue.

Did that. I usually run it with an open gas cap for diagnostics, and just made sure the vent line was clear yesterday.

I'll report back about the carb, thanks.