Fuel line size question

[dave6672]

I have a 67 C10 that originally had a 283, 2 barrel, so it has a 5/16” fuel line. I have swapped in a stock 72 350, 2 barrel that was in a Monte Carlo, and it had a 3/8” line originally. Both carbs are Rochester 2 barrels, but the 72 version on the 350 is a larger diameter carb at the air horn. So I have a 5/16” sending unit and original 5/16” line running all the way to the fuel pump. At the pump inlet it goes to 3/8” and is 3/8” to the carb.

The truck idles fine and will take throttle at idle, but stumbles badly under load, when trying to drive it. Could that amount of 5/16” line on an engine that originally ran a 3/8” line, cause that kind of issue?

The plugs are new so they are hard to read, but maybe a bit lean looking?

[Steeveedee]

A stumble from idle will not be caused by low flow. You already have a carburetor full(?) and it takes several seconds to a minute to drain the float bowl low enough that the accelerator pump won't squirt. You may see fuel starvation at extended high speed or towing up a hill, something like that. I'd check base timing and then accelerator pump squirt.

ETA- I'd also check for vacuum leaks.

[RustyPile]

The fuel line size causing this problem is the last place I'd look.. My '71 came stock with a 250 cu. in. I6.. The truck now has a '75 454 truck engine and the 5/16" line supplies the fuel just fine.. New spark plugs are only the first things in a tune up.. Plug wires, distributor cap, ignition rotor, fuel filter, timing, compression, and gasoline quality are among a few of the items to look at before tackling the fuel line (re)size.

[Steeveedee]

Now that I think about it, I pulled a 6k pound travel trailer up steep hills with my truck. The aux tanks have a 1/4" nipple on them, and even the 400 engine pulling all that weight didn't starve for fuel.

[dave6672]

Thank you all for the replies. I’ve ruled out vacuum leaks and plugs, points, etc. all new. Sounds like my fuel lines aren’t an issue, so I’ll look elsewhere.

[geezer#99]

Check the fuel pump output.
Put the end of the fuel line in a jar and crank it over. It should have full strong shots. If it sputters you’ll know what to change.

FWIW I fed these two guys with a 5/16 inch line. Pulled to 120 more than once.

[dave6672]

The pump is on my list to check. It is new but the brand is “Sparta”. Made in China and was only something like $22.

[mr48chev]

Back around 1975 I had a 57 Chevy panel with a 327 with 12.5 Jahns Pistons, Engle Cam 202 heads and an early Corvette 2x4 intake on it. That thing would just about pull the wheels when I got it but about 2/3 of the way through first gear it would run out of gas, let up on the throttle and shift and hit it and it would pull hard until it was time to go to third. Still it would cruise down the road at well over the 55 mph speed limit without a hassle. I decided one Saturday morning to figure it out and it ended up having a Natural gas fitting in the connection to the carbs with a small orifice in it. I went to town and got the proper fitting at Lee Hardware and put it in and that bugger pulled hard thorough all gears then. Bad part it got about 4 mpg. As others said a restriction in your fuel line or not enough size to the fuel line will cause the float bowl to go dry during a hard pull or a hard run though the gears but unless you do that you may never notice it.

First I would see if letting off the gas then getting on the gas again helped it pull good for a bit before it "stumbled" a bit again. That would be restriction in the fuel line somewhere. That could be smashed line, kinked line at a bend, twisted line at a fitting I don't even remember how many GM rigs I have seen with a twisted fuel line right at the fitting at the carb where someone installed the filter in the carb and twisted the line because they didn't take the flare fitting loose before they took the big fitting out of the carb.

Still without driving it I can't tell as it could be accelerator pump or it could be a plugged circuit inside the carb. A weak accelerator pump will cause a stumble immediately on acceleration while a sticking power valve will cause issues. Throw in how long the engine sat between the time it sat in your donor and the time you had it running in the truck may be reason for a carb cleaning and rebuild .

Mikes has some pretty solid info in this pfd on the two barrel Rochester https://carburetor-parts.com/assets/2%20Jet.pdf

[Getter-Done]

Is the mechanical advance working freely?

[dave6672]

Good thought. I know the vacuum advance is working ok, but I’ll check the mechanical. I blew out all fuel lines today and they were clear. I also disconnected the fuel line at the carb and it pumped strong with plenty of volume. The carb had been rebuilt by the guy I bought the engine from but I took it apart yesterday anyway and blew out all small orifices and checked it out, etc. but it was all clean already and the float adjustments were spot on. The accelerator pump is giving a good shot, I guess I need to check whether it’s coming in soon enough to keep it from stumbling. Also switched the coil, since it’s easy, but that didn’t help. So still chasing it. Thanks to all.

[Sicklajoie]

I know this might sound like a "no **** sherlock" statement, but make sure the fuel filter is installed correctly?

[dave6672]

Originally Posted by Sicklajoie

I know this might sound like a "no **** sherlock" statement, but make sure the fuel filter is installed correctly?

No problem. I have checked it and it’s installed correctly, although it’s a plastic screen type, which I’ve never seen before. I’ve only seen the paper and bronze ones before.