The Stovebolt Site Forums The Shop Area Engine Shop Rochester B fuel line inlet leak

I have a problem with the fuel inlet connection. My fitting bottoms out and fuel leaks from here. See the attached picture. Is there a solution to this? I’m wondering if I can tap it for a slightly larger fitting? I was also thinking about a copper crush washer? I look forward to your comments.

I would use the copper washer or fiber washer and Permatex Seal Lock. Make sure the fitting you're installing is like new and not distorted.

As Otto says, but also take time to clean and true up the face of the inlet or no gasket is going to develop a seal….
Just adding a gasket or crush washer may keep the fitting from bottoming out which will go a long way to not leaking…
I would start with a gasket and Teflon tape on the threads, if that leaks then I would go with the seal lock type products …
I’m a methodical person… try simple first……. then get more extreme…
Chuck

"My fitting"............now that's the problem. What fitting? Where did you get it? Have you tried properly installed teflon tape? Can we see the fitting. Is it for direct flare tubing? or pipe? straight threads? or what? Please post good clear pics of it and it's nose. Some fittings are supposed to bottom out. Some folks call bottom out as the outside nut is hitting, some mean the inside nose is hitting. Which are you?
We can't guess a solution before we see the parts.
If the soft Zink/Aluminum cast carb needs tapped or worked, time to get a new carb or section.

I will say this as delicately as I can........
1. If it's leaking it means you are planning on using it on a motor. Yipes!
2. That carb needs to be cleaned, examined for cracks, then thrown away.

Fitting pictures. 5/16 inverted flare x 1/8 Male.

Let me add a little more context to this discussion thread. The picture at the beginning of this post is from a donor carburetor. When I thread in the fitting in the donor it only goes about half way in before my calibrated elbow says we are snug. The picture in this photo shows the same fitting bottoming out as I have described above in the carburetor I am looking save. I’ve done all the work I can to ensure both halves of the carburetor are flush with each other before fitting in the gasket. What I have noticed is gasoline on the top lip of the air horn. Hope this helps clarify the input I am looking for. Thank you.

1) I have been advised, by Carbking himself (at least that’s how I remember it), that modern fuel tends to “wick” its way up carb castings….up and out of vent holes. You will see the tiniest drop of fuel on my YF long after the truck has been shut down. Not much I can do about that.

I share that to indicate what you are seeing may not NECESSARILY be a leak at the fitting.


2) That said, it is NOT normal for that particular fitting to bottom out. Pipe thread seals at the taper and bottoming out prevents that. I suspect you know that.

I’d suggest finding another fitting, of any kind, that is known to be 1/8 MNPT. Try it in the same hole and see what happens. That particular hole should accept a 1/8 pipe fitting. But it’s hard to say what has happened to that carb in the last many decades. Perhaps someone tapped it for something different? Perhaps it was a factory defect?

I am unfamiliar if the process of re-tapping or helicoiling a carb body. Pretty soft material. I’d assume it can be done but I can’t speak from any sort of experience.

OK Darren, let me try to understand. The most recent pic shows an adapter fitting screwed into the carb, then a fuel line with a flare nut screwed into the adapter with teflon on it's threads. Answer each question in order. Still not exactly sure of the situation.

1. The original pic is your carb you are using that has a leak?
2. The most recent pic is from the internet but is exactly how you plumbed your carb when you had a leak?
3. You had a leak when motor was running and fuel pump was pumping pressure?
4. When you say "bottom out" you are saying the adapter nut snugs tight up against the carb?
5. So using the most recent pic, where is the leak?
6. Do you have a second leak thru a gasket?
The adapter fitting you are using has straight threads. The threads are not doing the sealing. A copper crush washer as others have said or a teflon washer may solve depending on where it is actually leaking.

Be SURE there is no crack at the carb inlet nozzle, adapter or flare. We will watch for your answers to be sure we give the correct suggestions.

1. The original picture is a donor carburetor
2. The most recent picture is how the carburetor is plumbed today.
3. When I start the truck there is no sign of a leak, but once I drive it a couple miles the leak is aparent.
4.When the adapter nut is snugged up to the carburetor it "bottoms" out. This is to say, I don't believe it can be tightened any further without stripping.
5. I can't exactly see any leak, just gasoline on the upper rim of the air horn. If I run for long enough it will run down the bowl. I originally thought this was a leak between the two halves, but I now think it might be coming from the adapter fitting.

There is no crack at the carb inlet nozzle.

Picture without the fitting is my donor.

Picture with the fitting is the carburetor installed on the truck. Shown on the bench.

If the nut face of the adapter is touching the face of the carb inlet, you can't strip the threads.
My opinion is that if it does not leak at the fitting pretty quick, it is coming from elsewhere... and it would not be on the upper air horn. If wind blew it there from a fitting leak you would also see lots of gas on top of motor.

If you are using the proper pressure fuel pump, it's a throttle bushing, gasket leak. If you are over pressurizing with electric fuel pump or ? then you are forcing gas out somewhere. Wait for Carb King to comment. He is the expert here.

It is pretty much necessary for you to discover exactly where the leak is coming from. Drive-inspect, drive-inspect....or let it run in the driveway and watch. Gotta know that. Show a pic of where on the upper section it is pooling/leaking/wet. How much of a leak are we talking after a few miles? On your drive, do you stop a leave motor running, wipe all the gas off and watch?

Can't see the inlet face of the carb you are using. That face has to be flat and perpendicular to threads or needs washered. Maybe the seal is done at the nose of the adapter fitting. I'm not sure.

Bartikomonos is correct that your visible leak may, or may not, be at that fitting.

But I’ve also never seen a B with particular fitting bottomed out. I can’t imagine that’s ok. Perhaps some variants have a different inlet thread style that depends on a washer or something. I’ve never seen one that isn’t pipe thread. And pipe thread doesn’t work like that.

Every one of these I've ever seen (please go back to 1964 as a starting point) is a pipe thread situation...meaning the further you screw it in, the tighter it gets. Now for the disclaimer: I have seen a handful of these (maybe 6 or so) where the fitting screwed all the way in. On those, leaks were common. The pipe thread will stop leaks by design. Why did these half dozen or so screw in all the way? I can't unscramble that egg because I didn't mess them up but every one of them leaked in some fashion...meaning some leaked very notably and others dribbled so that after a while the whole side of the carburetor would be covered in gas. How to know if yours is leaking here or elsewhere? Detective work 101...cut a strip of old t-shirt or other fabric about 1/4 inch wide by around 8 or so inches long, wrap it around the fitting where it attaches to the carburetor air horn (but not where it will touch the dividing line between the air horn and the float bowl), tie it in a knot and idle or drive for a while. If it is notably wet, your leak is there. If it remains dry but the gasket/dividing line between the air horn and the float bowl is wet then your leak is there and you have another problem. If that dividing line gets wet while you're idling it is very likely your float/needle/seat isn't working correctly or (maybe and/or) you have way too much pressure from your fuel pump and you're pushing so hard against the needle/seat that it can't hold back the input. Something like that. Let's first find out where the leak is...

Over time, I have seen a LOT of carbs with either aluminum or zinc air horns where the fuel inlet fitting bottoms out. This is caused by "its tight", "well give it another 1/4 turn". Each time the fitting is tightened beyond the necessary torque, a bit of thread in the casting will be removed, allowing the fitting to screw further into the air horn.

The use of teflon tape can also cause this issue, if the tape uncovers the brass thread, the overall diameter of the fitting and thread will cause the bare thread to cut into the housing.

There at least two "fixes".

(1) If the carburetor in question is from a supercharged Duesenberg or a V-12 Packard, or, or, or, where the value of the carburetor exceeds the value of a totally restored 3100 Chevy, you fix the issue. This is best done by machining a new fuel inlet fitting on your lathe with the male thread a few thousandths oversize. The oversize thread will prevent the fitting from bottoming and allow it to seal; until "give it another 1/4 turn" a few times.

I do not know if tooling is currently available to cut the female inverted flare portion or not. 40 years ago when we were restoring factory multiple carb setups, and special fittings were not expensive, simply because they were not available to be expensive. I paid a tool and die maker a pretty good chunk of cash to machine special tools to allow me to do the female inverted flares. If I can do it, so can others; it just costs money!

(2) If the carburetor in question is not appreciably valuable, pitch it, and start with another carburetor.

I would concur with others about determining the source of the leak. I like Jon G's comment about the old T-shirt. My approach is to dry the area as much as possible with paper towels, they use a blow hair dryer (not mine, I don't need one for half a dozen hairs ) to thoroughly dry the area. Once dry, one should be able to start the engine, then watch it run at an idle until the leak appears. Once the source of the leak is known, it is much easier to determine a fix or replace.

The other Jon

Im running the same mechanical fuel pump I’ve been running since the late 1970’s. Did not see this problem till recently.

Guys I appreciate all your experience and expertise is trying to help me solve this mystery. I know the easy solution is to just start new. But starting new is expensive and I loving fixing old things folks thought could not be rehabilitated.

I appreciate all the input!

Does your brass piece have tapered threads or straight threads? I simple dial caliper can be used to determine that.
You need a taper thread for that application. A straight thread will never stop turning until it bottoms out.

It is tapered.

All info here, and internet, points now to Rochester inlet threads/adapter male threads are 1/8 NPT. The line side of adapter is 1/2-20 inverted flare. NPT Should not bottom.....but doing it on both carbs? Hard to think about this now. Maybe the leak is elsewhere and this is all moot. We will stay tuned.

The fitting does not bottom out or run all the way in on the donor carburetor. Only on the carburetor I’m trying to save. See the picture of the donor carburetor. This is what started me down this path. If your wondering why not use the donor carburetor it because there is a crack on the accelerator side of the airborne.

That's actually good news. It at least confirms several things. 1/8 NPT is correct and the non donor threads are "screwed". So find a good top from Carb King or Mike's carburetor or Ebay or wherever......to start with. Then drive and observe.

"I loving fixing old things folks thought could not be rehabilitated.".... we all feel that way here. This is not a case for that. Carbs are too important. Leaks are too dangerous. Carbs are too soft to fix important treads. JB weld and all that jazz is no good. Donor carbs or pieces are cheap. It's too easy to replace it right and too hard to fix it right. Put an ad here in "wanted". Lot's of guys have good parts. This is AD Land.


THIS is a "Make Offer", free ship.

Not telling you what to do, just one avenue.

This confirms that the fitting, or a fitting, has been overtightened in the carb you are trying to save. If you really want to save it, read my earlier post.

Jon

I did not realize until looking just now that one could buy NPT helicoils.

https://www.mcmaster.com/product/91733A401

If this was to work, I suspect you’d have to use some sort of fuel resistant epoxy or sealant and make the (1/8 MNPT x 5/16 female inv flare” fitting a permanent part is the carb.

The investment even in that scaled-down project would far outweigh the replacement value of a Rochester B.

I am not terribly familiar with the types of epoxy’s that folks use to plug fuel tank holes and what not. You could probably go even more cave man and just drill a little oversize, goop up your fitting of choice, and jam it in there.

It is slightly possible there can be a galvanic corrosion problem between the stainless coil and the Zinc/Al body. Also the stainless coil and the brass fitting..... and all three in close proximity. Gas always has some water in it which will act as an electrolyte. Not to mention the cost of the tools, drill and tap for one helicoil.
As far as I can tell, CarbKings advice is to find a new air horn.

JW Cave Man 51 is trying to goop you to success. He means well. He will be a good scapegoat on this one.

By the way Darren, your motor an under hood work is excellent looking. Good job.

I think the emphasis here would be on SLIGHT POSSIBILITY of galvanic corrosion.
But I would have to agree on the cost. That and the fact the end of the tap may have to be ground off to get the threads correctly cut for a helicoil without bottoming out in the hole.
Like Jon (carbking) said, unless that was an extremely rare carburetor, it wouldn't be worth repairing.
I agree with JW51, that a real caveman fix would be to epoxy the fitting in place. I have actually done that on a diesel fuel return line fitting and it's been working for years now. It was a fitting where the tube was pressed into place, but had worked loose. That fix is still in place after many years. I recall that I used JBweld on it.

A soft copper washer between the inverted flare adapter and the carburetor housing like a brake line banjo fitting uses will solve the problem permanently. Just carefully draw file the carburetor housing to give the gasket a smooth surface to seal against. No it's not "original"- - - -yes, it works! Problem solved!
Jerry

I would suggest spotfacing 1/16" from the face and then using a 1/8" NPT short projection tap to chase the stretched threads.

https://www.mcmaster.com/products/short-projection-taps/

I have done unspeakable things with JB Weld and a thread tap.

Not on my Stovebolt (yet), but in other mechanical devices in my life where the cost of a proper fix exceeds the value of the entire device.

I just wonder that if a male pipe fitting that goes all the way in can get tight enough for a washer to "crush seal" 4-6 PSI. Worth a try. Darren indicates he feels that it is tight.

Bartamos, thank you for your comments on my engine. I agree it looks pretty clean. Thought I would share a couple other pictures with you. This is why I have tried so hard to save this old carburetor.

I'm going to give the copper crush washer a try. I'll be sure to let you know if it works out!

With a drop of Loctite, all it needs to be is "snug". Use a heat gun to break the bond if it ever needs to come apart again.
Jerry

The copper crush washer fix may work, although they usually need to be fairly snug. If the threads bottom out in the hole, cutting a little off of the end of the fitting would fix that.

The engine bay is super sano.

A fuel compatible fiber washer may work better than a copper washer since you won't have to apply as much torque on the already compromised threads to get a good seal. Since you are dealing with tapered threads, your washer (copper or fiber washer) will need to be thin so that the tapered threads of the fitting will firmly engage with the tapered threads of the carb.

Tried the fiber washer first. I’m seeing an improvement but we are not there yet. There is still a very slight amount of weeping. And this is making me weep. So close. Next I’m going to try a copper crushable washer. I had to do this one step at a time. If I had gone to the copper washer first and it leaked there would still be questions. No easy approach.

Permatex Super 300 was designed specifically to seal aviation gasoline. A TINY bit of it will seal a gas leak, and it never hardens. DO NOT use the poop-brown 3D Permatex brush-on liquid- - - -it's not the same stuff. Super 300 is almost black. Lots of it on Ebay.
Jerry

O’Reilly will order it for you, as well. I think maybe I got my last container from the big jungle site.

Good stuff. It’s pretty easy for me to make a mess with the brush applicator while coating a gasket

Recently learned that I can be a little more precise dipping my finger (nitrile glove on) and spread it that way.

NPT repair inserts. The 1/8 one uses a w drill. The kit is over a hundred bucks.

I just can’t get the fuel line inlet fitting to stop weeping. I dramatically decreased it from where it was using a copper crush washer and thread sealant. But there was still fuel leaking so I have thrown in the towel and purchased a rebuilt. Straightening the two halves was fun and rewarding. The process does work! Probably spent more than I should have doing all this. I’ll have to write it off as entertainment. I got the rebuilt installed and the truck is running better than ever. I want to thank everyone for their help and input. This I a great community. Cheers!

Great to hear and thanks for the followup!