Fuel lines on 1973 (why different hose size?

[Wolverine]

I have a fuel line problem on my rear tank. After 5-10 miles gas not getting to engine (sputtering). If I switch to aux tank, engine comes back. Once I switch back to rear tank 5-10 miles same thing happens. Sending unit is only 2 years old, reads accurate, and I am fairly certain I don't have a dead cat in the tank blocking the fuel. Fuel filters new/clean... I am thinking it may be a block in the fuel line from rear tank to switch valve. So, I plan on replacing fuel line but here are my questions... The current (both) fuel lines are 3/8" rubber hose with cloth sheath at the fuel sending units and at switch valve (standard right?). However, after 5", both the fuel lines switch from 3/8 rubber/cloth hose to 1/4 plastic hose. Is there a reason (safety, pressure) for this? Can I simply use 3/8 fuel hose from point to point and not complicate the fuel line with multiple hose connection, reducers?

 

[Lawndart]

Have you tried removing the fuel cap when this issue occurs or just driving without one?
This would point to a venting issue is all is OK without the cap.

 

[thegreatjustino]

A 1973 Bronco would have come from the factory with a return line fuel system. Your original sending units would have had two ports on them and the switching valve under the seat would have had six ports instead of the more common three.

The return line was a smaller diameter than the feed line.

Additionally, a '73 would have had the vapor recovery system which would have smaller lines going from the fuel tanks to the recovery bottle in the quarter panel behind the driver's seat.

Are you sure you're not getting the three different fuel lines mixed up when trying to trace where they go? After 47 years, who knows what previous owners have done.

 

[Wolverine]

Six ports ? Wow. Not my set up at all. The switch valve has 3 ports, not 6. 1 from aux fuel line, one from main (rear) fuel line. The switch lever under the seat has the large 'fin' side and small arrow pointer side (which looks like a picture I saw of an original switch)(not after market). The fuel sender units I replaced 2 years ago, were single ports (not 2 port) but perhaps the previous owner used the return line system (tube) as the feeder line. This was a 73' but has 50% 76' replacement parts (brakes, front end, etc...)

 

[Wolverine]

p.s. to Justino, I can't locate the recovery bottle (probably removed), so again maybe the previous owner just reused the smaller return fuel lines as the fuel delivery. Anyway, my plan is to bypass the existing fuel line (get rid of the 3/8 reduced to 1/4 back to 3/8 line) with just a 3/8 rubber fuel line (heat shielded). Anyone see any issues?

 

[gnpenning]

I would try lawndarts suggestion first. Free, easy and reasonable possibility.

 

[DirtDonk]

Only a few Broncos actually came with the return lines. Those were delivered originally to high-altitude, or very hot (think AZ) areas where vapor lock was a thing.

Your plastic line should be 5/16" and that's what the fitting on your stock sending unit should be. The EB fuel lines should be 5/16 from tank to carburetor. Aftermarket carburetors and aftermarket tanks use 3/8" though.
Is yours still a stock rear tank?

Paul

 

[DirtDonk]

You said fuel "filters" plural. Where are they and how many do you have? Are you carbureted or fuel injected? Return line or no?
And again, stock tank or aftermarket?

Fuel filters can get blocked up, and if more than one, it does not always take much blockage to get the engine to stumble.
If your plastic line is indeed 1/4" diameter, then it should be replaced with at least 5/16 if not 3/8. Whatever works best for your setup really, because even 5/16" line can support 250hp and even more if the engine is decently tuned. On a modern vehicle probably better than 300hp.
A 3/8" diameter line would cover all bases for now and the future, but if you are still running a stock fuel pump and carburetor you have to remember you'll have to adapt the larger 3/8 to the smaller 5/16 at those components.
Not impossible of course, but can be a bit of a pain depending on how you decide to do it.

Paul

 

[TheLimeRanger]

i just put new tanks, sending units, tank switching valve in my 74. My lines were 3/8. i know that some were 5/16th as the switching valve i bought from Toms had 5/16 ends on it. seemed silly to me to reduce it from 3/8 to 5/16th, so i returned their valve, and bought another one with 3/8 fittings. im 3/8 from tank to valve to fuel pump, so you can definitely change it all if you want, but i. feel like you might have a clogged vent though

 

[DirtDonk]

I wonder if the manufacturer changed the sending unit, or if another was substituted?
The EB senders that I have seen were 5/16 fittings as part of the unit itself, while the ones from most of the pickups I've seen were 3/8" diameter fittings.

The same for the sock filter went to Ford and bought two sock filters for my pickup and they gave me 5/16" versions that did not fit. Went back and got the correct one, but I don't remember if we looked up a different application or the guy just knew what was up and got the right ones.

Paul

 

[Wolverine]

Thanks to everyone that responded. Here is my long winded update... I ran a 3/8 fuel line bypass from rear tank sending unit to valve switch. It fixed the problem. I did this before I read the note from DirtDonk about needing 3/8 for > 250 HP as my engine is a 306 carburated (rated @ 365hp). Good thing I used 3/8!! So then I tore out the old fuel line. As I described 3/8 cloth shielded @ fuel sender (TBP replacement in 2018) then reduced to 1/4" plastic 5' run, back to 3/8 at switch valve. After all this, a friend of mine said 3/8 to 1/4 to 3/8 was is a traditional fuel line european set up in BMWs? Again to DirtDonk, I was running fuel filter at tank and at engine, so I just yanked the fuel filter at the sender. Thanks to all of you for helping me.