What is the correct size of the hole in the bypass port on a fuel filter?

MJCJ · 2025-09-21T23:04:56-0400

Matt1981CJ7 said:
A 30 second Google search turned up this thread about this exact issue.

The alternatives to the incorrectly China-made Wix filter #33040 are AC Delco GF423 and Baldwin BF886. They both have the proper restriction in the return port.

That said, I'm going out on a limb to say I don't think the fuel filter's return orifice size is the OP's problem.

Thanks, I'd actually already seen that post, I was digging for quite a while. Problem is that post was over two years ago and the AC Delco filter from NAPA had no date on it, so no way to know if it was a new run or old stuff that NAPA sent them. I already have a newer filter than that from NAPA that isn't right. A lot has changed in two years, and besides it didn't answer the question I originally asked, and appears that nobody on the site knows the answer to it either. No judging on my part, I don't know myself. I'll just go start pulling out boxes at the parts store until I find one with a restriction in the supply line.

MJCJ · 2025-09-21T23:27:11-0400

80cj said:
I've been using NAPA/Wix fuel filters for quite a while and although I had starting problems, the plumbing (restricted fuel line) and fuel pumps were to blame. Since I went to a Facet Dura Lift fuel pump, things have been working as I had expected. In his other thread, I asked the OP what Facet fuel pump he was using but I guess he forgot to indicate.

Sorry about that, honestly I've been bombarded with so many questions that have nothing whatsoever to do with the question I asked on this post that I just kind of stopped replying to most of it. I really just asked what I thought was a simple question, but nobody knows the answer, which is no big deal I'm no worse off than I was when I started...FWIW to you, I have a Facet cube pump, can't remember the model off the top of my head it's a 4-7 PSI pump. Operating pressure is generally around 3-4PSI which won't work well with an unrestricted fuel filter return line, and frankly it doesn't. I thought I'd take the easy way, and get a proper filter first, then start working through other issues if they still present. Everything in my fuel system is brand new, properly sized and set up correctly. This filter is the weak link more than likely as it's the only thing in the whole system that's out of spec.

People are trying to be helpful, I can't get down on them for that, but it just seemed like such a simple question. Kinda went sideways...

MJCJ · 2025-09-21T23:31:36-0400

Fourtrail said:
What size is your return line? Smaller port/orfice in the return filter does matter but ihmo the smaller return line also makes a difference in pressure to the carb. Worked on a jeep with a low pressure electric pump mounted at the tank, then the 3 port right after the pump and short return line to the tank. No fuel to the carb, would only cycle fuel right back into the tank. Restriction due to the smaller return line is what keeps pressure at the carb.

It's brand new 1/4" steel line from Inline Tube. Problem is, and the question I'm trying to figure out, is what size restriction is supposed to be in the fuel filter? Currently I'm essentially running unrestricted 1/4" from the filter which I know is wrong.

MJCJ · 2025-09-21T23:33:36-0400

jeepersid said:
put a regulator on that has a by pass and good fuel filter before the reg and be done......

Those do work well, but it should be unnecessary with the correct fuel filter. The point of this post was to try to figure out what the correct fuel filter is supposed to look like...I'll figure it out.

MJCJ · 2025-09-21T23:34:25-0400

jeepersid said:
buy 4 or 6 filters and cut them up to see whats really in there. IF ya really want to know ..............J/S...

I'm trying to avoid that...

Fourtrail · 2025-09-22T00:31:43-0400

The filter 3rd port/return line is a calculated leak and it sounds like your facet pump is supplying less volume than the stock mechanical pump and therefore is allowed to return a larger percentage volume of fuel supply than what would be stock and you end up with less pressure at the carb inlet. The 3 port that works fine with a mechanical pump with the 'factory' size bleed may not work for you, in your instance you may be best off to use, and it pains me to say this, jeepfellers diagram with the pressure regulator in the return line to tune in proper pressure at the carb inlet. You could also solve your problem by putting a restriction in the return line just off the filter and fine tune the orfice size until you get proper pressure at the carb inlet. with an external return restriction you won't have to worry about replacement filters in tbe future.

gehlsurf · 2025-09-22T05:55:24-0400

I have a Facet pump 4-7, too, but the duralift style. Here....

Facet Dura-Lift 12v Fuel Pump, 1/8 NPT, 4-7 psi, 120" lift | Pegasus Auto Racing Supplies

Facet Dura-Lift pumps are able to draw fuel from farther away than standard Facet pumps. This pump can lift up to 120 inches and can deliver up to 33 GPH.

www.pegasusautoracing.com

It is really close to the carb, so definitely working to spec. You can see the top of it on the fender.

Again, I've never had issues with any filter, large return barb years ago, smaller barb, no idea what brands, a few different types. I've used this pump on a Weber and now Holley.

Are you sure your pump is in a good spot to work?

MJCJ · 2025-09-22T08:58:42-0400

Fourtrail said:
The filter 3rd port/return line is a calculated leak and it sounds like your facet pump is supplying less volume than the stock mechanical pump and therefore is allowed to return a larger percentage volume of fuel supply than what would be stock and you end up with less pressure at the carb inlet. The 3 port that works fine with a mechanical pump with the 'factory' size bleed may not work for you, in your instance you may be best off to use, and it pains me to say this, jeepfellers diagram with the pressure regulator in the return line to tune in proper pressure at the carb inlet. You could also solve your problem by putting a restriction in the return line just off the filter and fine tune the orfice size until you get proper pressure at the carb inlet. with an external return restriction you won't have to worry about replacement filters in tbe future.

The problem is that my filter is unrestricted, so it's not regulating the fuel flow at all. Honestly, all I wanted to know is if anyone knew what the size of the restriction hole is in the filter, and it appears that they don't know either so I'm moving on. I'm trying to keep things as simple as possible if I can. Thanks for the input!

MJCJ · 2025-09-22T09:00:35-0400

gehlsurf said:
I have a Facet pump 4-7, too, but the duralift style. Here....

Facet Dura-Lift 12v Fuel Pump, 1/8 NPT, 4-7 psi, 120" lift | Pegasus Auto Racing Supplies

Facet Dura-Lift pumps are able to draw fuel from farther away than standard Facet pumps. This pump can lift up to 120 inches and can deliver up to 33 GPH.

www.pegasusautoracing.com

It is really close to the carb, so definitely working to spec. You can see the top of it on the fender.

View attachment 4335671

Again, I've never had issues with any filter, large return barb years ago, smaller barb, no idea what brands, a few different types. I've used this pump on a Weber and now Holley.

Are you sure your pump is in a good spot to work?

I have a push style pump, it's back at the tank where it needs to be. I'm good, I'll figure out the filter thing and move on from there. Thanks!

Matt1981CJ7 · 2025-09-22T09:15:45-0400

MJCJ said:
People are trying to be helpful, I can't get down on them for that, but it just seemed like such a simple question. Kinda went sideways...

It might seem like a simple question to you, but it's an obscure one. In 14 years on this site, this is the first time I recall anyone being concerned about it.

It sounds like your pump isn't supplying enough operating pressure to me. My mechanical pump supplies 7-8 psi to the carb with no return line. 4-5 psi with the return line.

As Fourtrail said, it would very easy to create the restrictions in the return line that you think is your problem. Or, keep chasing this rabbit down the hole.

MJCJ · 2025-09-22T09:53:28-0400

Matt, the forum thread you linked to in this thread is specifically about this problem, it's a known issue...if you take your current setup and remove the restriction in the 1/4" fuel filter return your carb will likely have fueling issues.

Let me put it this way,I'll answer all your questions... I have a brand new bb2-barrel carb, brand new fuel tank, brand new sending unit, brand new fuel filler neck and vent hoses, brand new pre-formed 5/16" supply lines from the tank, brand new pre-formed 1/4" return lines, brand new hoses on the correctly repaired and functioning tank vent/charcoal canister system. New fuel pump pushing 4-7 PSI to the carb with the return line blocked off. Not blocked off, the engine will idle but run out of fuel when driving. Runs perfect with the return line blocked off. Sounds like a problem with the return line to me. It could be the carb having float issues, but more likely it's the fuel return system, which is much more easy to fix first. If you had the same situation as me, and you knew that the fuel filter didn't have a restrictor built into it, would you run down all these rabbit holes folks are suggesting, or just replace the filter with a correctly manufactured one and then take it from there?

All I asked, and it was a simple question, is what is a correct filter supposed to look like before I buy another one that's manufactured incorrectly. I'm trying to have some things set exactly right so I can eliminate certain problems from consideration when working on it. It's okay that nobody else knows either, and it appears that as long as it has some sort of restriction it should be okay. I think we get jaded to the idea that if someone is asking a question it's because they don't know what they're doing, which is sometimes the case. I do know what I'm doing here, just looking to save a little time when getting a new filter. I'm good, I'll run to a few stores to see what they have and try to find one that looks right. If I still have starvation issues with the right filter in, then I can look at the pump/floats, etc. Trying to start with the most obvious basics first then moving up from there.

I was going to put a fuel pressure gauge in the cabin of the truck so I can monitor the pressure under driving conditions, got blown up for wanting to do that in another thread where I asked a simple question about the hose tee that the sending unit is screwed into. Seems like these days some folks on this site have to have a "take" on what a guy is doing instead of focusing on what he actually asked. Then they get frustrated when I don't provide them with all kinds of information that they need to do a diagnosis I didn't ask for...things tend to go sideways.

In any case, I truly appreciate the time you put into trying to help, you're one of the "good guys" on this site and I appreciate you.

Fourtrail · 2025-09-22T10:14:51-0400

One more comment to annoy you... I still think that your electric pump is not supplying enough volume for the calculated leak to function correctly and no return filter, factory size bleed or not is going to give you the correct volume at the carb. With the aftermarket pump, you are going to need an aftermarket solution to get enough volume at the carb to continue to run when you put your foot into it.

MJCJ · 2025-09-22T11:45:11-0400

Fourtrail said:
One more comment to annoy you... I still think that your electric pump is not supplying enough volume for the calculated leak to function correctly and no return filter, factory size bleed or not is going to give you the correct volume at the carb. With the aftermarket pump, you are going to need an aftermarket solution to get enough volume at the carb to continue to run when you put your foot into it.

I actually think you might be right, subject of a few other threads I've started and kind of abandoned. I just need to fix the thing that I know for 100% sure is wrong, then I'll move on to supply, etc. You're comments don't annoy me, I appreciate the help...

JEEPFELLER · 2025-09-22T11:52:40-0400

In Post 21 you say

"No judging on my part, I don't know myself. I'll just go start pulling out boxes at the parts store until I find one with a restriction in the supply line."

Either you wrote this wrong or you are looking at the wrong end of the filters! .... and in the wrong hole!

The restriction is gonna be inside the little port (the return line port of the filter) as if it were mimicking the last scenario in my diagram

IN A PERFECT WORLD (which it is not----and so the need for a gauge right at the carburetor to verify the setting)

I could turn my regulator to (edit) "WIDE OPEN" and basically it would act like it wasn't even there

I could turn my regulator to "Seven" and it partially dams up the return line and would provide "7 PSI" to everything

before the return line --- still allowing the excess to run on thru the return to the tank.

And 4, 5, 6, 7.5 (7.5 is what my Holley requires) or what ever you set it too and your gauge agrees with the setting.

-----JEEPFELLER

HandsOn · 2025-09-22T15:17:37-0400

Interesting conversation for me for sure. It sounds like many of you are already aware of the restriction that the OP is referring to. I know you’ve all said you haven’t pulled one apart, but have you ever looked into the port and seen an actual orifice?
It was always my understanding, correct or not, that the whole purpose of using a smaller return port, was to BE the restriction. In theory at least.
No need for an orifice when the return line was already smaller.
Unless a specific application, such as the CJ with this engine, needed it to be restricted even further? Never heard of it, but that certainly doesn’t mean it didn’t happen.
So if there was, I’m learning something new today. Which I like.

And what we’re probably seeing with the inconsistently manufactured units, is that somewhere, someone decided it would be cheaper not to build it that way and should work just fine.
Or the original designs called for a smaller opening that the hose barb was attached to. Later manufacturing methods had the hole as the same size as the hose fitting. Perhaps?
We see these kinds of changes all the time in replacement and reproduction parts.
Can’t wait until someone chimes in and says they have an old original that they never threw away. Would love to see what’s on the inside of a factory original filter.

Even the return-type filter on the old 60s Corvair turbo models with 5/16 input and 1/4 return, didn’t have an actual restriction/orifice that I’m aware of.
I should go pull one apart to find out. Got plenty of old ones, but finding them in stacks and boxes won’t be fun.
And I don’t think that one of those would be in the decent price range for just buying one as an experiment. Probably cost five times what a Jeep filter costs. Hmm, now I’m going to have to look into that as well. 🙄

Good luck MJCJ. hope you’re able to find one in the modern supply chain that has the specified orifice and are able to measure it.

MJCJ · 2025-09-22T15:56:40-0400

JEEPFELLER said:
In Post 21 you say

"No judging on my part, I don't know myself. I'll just go start pulling out boxes at the parts store until I find one with a restriction in the supply line."

Either you wrote this wrong or you are looking at the wrong end of the filters! .... and in the wrong hole!

The restriction is gonna be inside the little port (the return line port of the filter) as if it were mimicking the last scenario in my diagram

IN A PERFECT WORLD (which it is not----and so the need for a gauge right at the carburetor to verify the setting)

I could turn my regulator to (edit) "WIDE OPEN" and basically it would act like it wasn't even there

I could turn my regulator to "Seven" and it partially dams up the return line and would provide "7 PSI" to everything

before the return line --- still allowing the excess to run on thru the return to the tank.

And 4, 5, 6, 7.5 (7.5 is what my Holley requires) or what ever you set it too and your gauge agrees with the setting.

-----JEEPFELLER

View attachment 4335718

Yeah, I typed it wrong, I was talking about the return line. I can use a regulator, I have another thread where I was talking about doing that with a more powerful pump. First, though, I need to get the right filter which I'll do first. I'm trying to just set it up the way it came from the factory and worked fine for 40 years. Hopefully I don't need all the other stuff, though I might need a pump that puts out more volume. Honestly I'm not sure it's a pressure issue as much as a volume one. There are pumps that put out 5 psi at 20 GPH and others that put out 5 PSI at 70 GPH. I think I just need a little more capacity, but I don't want to go overboard because I have a 1/4" "leak" in my filter. I'll get it set up, thanks for the help.

MJCJ · 2025-09-22T16:00:00-0400

HandsOn said:
Interesting conversation for me for sure. It sounds like many of you are already aware of the restriction that the OP is referring to. I know you’ve all said you haven’t pulled one apart, but have you ever looked into the port and seen an actual orifice?
It was always my understanding, correct or not, that the whole purpose of using a smaller return port, was to BE the restriction. In theory at least.
No need for an orifice when the return line was already smaller.
Unless a specific application, such as the CJ with this engine, needed it to be restricted even further? Never heard of it, but that certainly doesn’t mean it didn’t happen.
So if there was, I’m learning something new today. Which I like.

And what we’re probably seeing with the inconsistently manufactured units, is that somewhere, someone decided it would be cheaper not to build it that way and should work just fine.
Or the original designs called for a smaller opening that the hose barb was attached to. Later manufacturing methods had the hole as the same size as the hose fitting. Perhaps?
We see these kinds of changes all the time in replacement and reproduction parts.
Can’t wait until someone chimes in and says they have an old original that they never threw away. Would love to see what’s on the inside of a factory original filter.

Even the return-type filter on the old 60s Corvair turbo models with 5/16 input and 1/4 return, didn’t have an actual restriction/orifice that I’m aware of.
I should go pull one apart to find out. Got plenty of old ones, but finding them in stacks and boxes won’t be fun.
And I don’t think that one of those would be in the decent price range for just buying one as an experiment. Probably cost five times what a Jeep filter costs. Hmm, now I’m going to have to look into that as well. 🙄

Good luck MJCJ. hope you’re able to find one in the modern supply chain that has the specified orifice and are able to measure it.

i expect those old filters had a restriction in them too. 1/4" isn't that much smaller than 5/16", at the low pressures we're talking about for a carburetor I don't think the difference in size would provide much resistance. Well, from experience, actually I can tell you it doesn't. I think the hole is probably around 1/8" or so. This simple question kind of stirred up a lot!

Matt1981CJ7 · 2025-09-22T16:16:50-0400

HandsOn said:
Interesting conversation for me for sure. It sounds like many of you are already aware of the restriction that the OP is referring to. I know you’ve all said you haven’t pulled one apart, but have you ever looked into the port and seen an actual orifice?
It was always my understanding, correct or not, that the whole purpose of using a smaller return port, was to BE the restriction. In theory at least.
No need for an orifice when the return line was already smaller.
Unless a specific application, such as the CJ with this engine, needed it to be restricted even further? Never heard of it, but that certainly doesn’t mean it didn’t happen.
So if there was, I’m learning something new today. Which I like.

And what we’re probably seeing with the inconsistently manufactured units, is that somewhere, someone decided it would be cheaper not to build it that way and should work just fine.
Or the original designs called for a smaller opening that the hose barb was attached to. Later manufacturing methods had the hole as the same size as the hose fitting. Perhaps?
We see these kinds of changes all the time in replacement and reproduction parts.
Can’t wait until someone chimes in and says they have an old original that they never threw away. Would love to see what’s on the inside of a factory original filter.

Even the return-type filter on the old 60s Corvair turbo models with 5/16 input and 1/4 return, didn’t have an actual restriction/orifice that I’m aware of.
I should go pull one apart to find out. Got plenty of old ones, but finding them in stacks and boxes won’t be fun.
And I don’t think that one of those would be in the decent price range for just buying one as an experiment. Probably cost five times what a Jeep filter costs. Hmm, now I’m going to have to look into that as well. 🙄

Good luck MJCJ. hope you’re able to find one in the modern supply chain that has the specified orifice and are able to measure it.

This is from the link I posted on page #1 of this thread. If you look closely, the USA made Wix does have a restriction in the return port that is actually much smaller. The China made version doesn't have the restriction.

Later in that thread, another member posted 2 alternatives that are restricted. Or, at least were at the time.

80cj · 2025-09-22T18:24:51-0400

I have an extra Chinese made Wix filter #33040. The orifice in the 1/4" return tube will accept a #50 wire gauge drill (.070) but will not accept a #49 (.073) FWIW. .

HandsOn · 2025-09-22T19:13:55-0400

Now there you go! Some nice hard, Data.
I was thinking a drill bit might do the job of measuring, if someone had the right type of filter.

Hopefully that’s just what MJ was looking for.
Well, I suppose that, even more than the actual data, you were looking for a filter that would actually work!

Still wishing good luck on that!😉

What is the correct size of the hole in the bypass port on a fuel filter?

MJCJ

MJCJ

MJCJ

MJCJ

MJCJ

Fourtrail

gehlsurf

Facet Dura-Lift 12v Fuel Pump, 1/8 NPT, 4-7 psi, 120" lift | Pegasus Auto Racing Supplies

MJCJ

MJCJ

Facet Dura-Lift 12v Fuel Pump, 1/8 NPT, 4-7 psi, 120" lift | Pegasus Auto Racing Supplies

Matt1981CJ7

MJCJ

Fourtrail

MJCJ

JEEPFELLER

HandsOn

MJCJ

MJCJ

Matt1981CJ7

80cj

HandsOn

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