Pickup and uniflow line positions

[Matt Brown]

I need to do a bit of maintenance on my tank today. The vent and uniflow lines came loose where they enter the tank. While I have it out, I also wanted to fix the pickup line as the engine has always gradually leaned out for the last 10 laps or so and hiccups a bit before it finally dies. I wanted to get a concensus on best way to position the pickup line and the uniflow line.
It’s also been 40 years since I last pulled a tank apart. Is it best to use a torch to pop the back end cap off or will an iron do the job?

Thanks, Matt

[Brett Buck]

I need to do a bit of maintenance on my tank today. The vent and uniflow lines came loose where they enter the tank. While I have it out, I also wanted to fix the pickup line as the engine has always gradually leaned out for the last 10 laps or so and hiccups a bit before it finally dies. I wanted to get a concensus on best way to position the pickup line and the uniflow line.
It’s also been 40 years since I last pulled a tank apart. Is it best to use a torch to pop the back end cap off or will an iron do the job?

  DON'T use a torch, unless you are *extremely careful* not to overheat it. Get it just a tiny bit too hot (which takes an extra second of heating) and you will never get it soldered back. I recommend the burner on a electric stove, or a soldering iron/gun, pick out the corners.

       Brett

[Matt Brown]

  DON'T use a torch, unless you are *extremely careful* not to overheat it. Get it just a tiny bit too hot (which takes an extra second of heating) and you will never get it soldered back. I recommend the burner on a electric stove, or a soldering iron/gun, pick out the corners.

       Brett

Just curious, why will it not solder back if it’s overheated?

[Dan McEntee]

   Hi Matt;
    I do use a torch to heat the bac end cap for removal, but do so gently and it doesn't take a huge flame. You don't want to over heat and oxidize the metal, which is easy to do. Fill the tank with water first to expel all the fuel, then empty the tank, otherwise you will get a small "whooooop" and mini explosion when you heat it. Check the pick up tube for cracks, and in fact, replace it anyway and the uniflow tube. If it was working OK for the most part and only recently started to give you troubles, put the uniflow back where it was. The tubes being loose was probably the source of your problem. Do a search on here for building uiniflow tanks and you will find a days worth of reading.. Use a good flux and keep it to a minimum, .030" rosin core solder and a soldering iron with as broad a tip as you can get. When assembling the tank, flush out all the flux before replacing the end cap, and when putting the end cap back on, only apply solder to the outside of the joints and again keep it to a minimum. When finished with air checking for leaks, flush the tank again with white vinegar, and then fuel again rght away, and you are good to go.
   Type at you later,
    Dan McEntee

[Dan McEntee]

Just curious, why will it not solder back if it’s overheated?

    You can easily burn the tin off the tank material and then will be left with oxidized steel. It takes very little heat to get the cap loose, it just takes a bigger volume. The stove top burner trick works well also if mama will let you near it!

   Take it slow and easy and keep things clean,
  Dan McEntee

[Matt Brown]

Rosin core is the proper choice? I have always used sta-brite silver solder on tanks.


Matt

[Paul Smith]

Actually a good strong heat gun, intended for paint removal, will melt most solder.  You can get a good one from Home Depot or a cheap one from Harbor Frieght.

This saves the danger of fire or a family feud.

[Dan McEntee]

Rosin core is the proper choice? I have always used sta-brite silver solder on tanks.


Matt

    No need for StayBrite on a tank. It's a silver solder, and the flux for that is especially aggressive and toxic and you need to get things a bit hotter for it also. I use Ruby Fluid if you can find it. It's the "cleanest" soldering flux I have found. NoKrode,  or how ever it is spelled, is not the best, in my opinion, but will work. See what the local shops have around. Since the demise of Radio Shack, small diameter solder is the hardest thing to find around here, but I have a good supply. That is one of the secrets to easy soldering, using the small diameter solder, and it does NOT have to be acid core. I only keep rosin core around and use it for everything. Soldering is a skill that has to be learned and practice makes perfect. Keep the joints as tight as you can and with no gaps also. Part of what makes soldering a tank work is the capillary action of the solder flowing along the joint and you lose that with any gaps. Neatness counts!
    Type at you later,
    Dan McEntee

     PS to add:  One of the reasons to use common lead solder is that the joint then can be flexible to a certain extent. Silver solder like Stay-Brite can get quite a bit harder even when done correctly. You may have heard the term "hard soldering" somewhere along the line and it can be used a a term to describe silver soldering. The vibration that the tanks we use can be quite excessive and violent. A silver soldered joint could cause the tank material to crack around the joint in extreme conditions. Silver soldering can be quite strong in structural uses and is quite light. A lot of older competition racing bicycles used silver brazed frames because of the strength and light weight of the joint. Silver brazing is a similar process but done at higher temperatures. When at welding school a couple of centuries ago we had sivler soldering and brazing demonstrated to us and the joints tested to destruction and you would be amazed at what a joint would take! But we don't need that for model fuel tanks. Everything has it's place, there is a correct tool and process for every job.

[Matt Brown]

I have quite a lot of experience with soldering. From the real silver solder to sta-brite to copper pipe plumbing and a lot of electronics work.  I used to convert old regular profile tanks to chicken hopper style back before GRW tanks were available. I never needed to remove an end cap so all I ever used was an old Weller gun.
I’d never considered the stove to pop the end cap off but I’m gonna try it!
This particular tank has always ran good until the last 10 laps or so. Usually it starts leaning out right after the hourglass and gradually gets leaner through the clover. Then I get a couple laps clean running but lean. The next 5-7 laps it starts coughing and trying to die before finally cutting off. I figure the pickup and uniflow vents are not in ideal locations and I want to fix that!

Matt

[Dan McEntee]

  Matt;
   It may have just been how it was installed, and only needs to have the back end shimmed out a bit. Even if the tank is installed dead flat, and the nose is yawed out in flight,  the front of the tank is out farther than the back, and centrifugal force will make the left over fuel run to the "highest" point. I would just fix what you have and restore it to original position and try it with the back end shimmed out a bit, at least 1/8". If you haven't taken it apart, just try patching the loose ends, and test fly it as is with the back end shimmed out 1/8" to prove the point. If it does the same thing, then you do have a problem.
   Type at you later,
    Dan McEntee

[Dan McEntee]

I have quite a lot of experience with soldering. From the real silver solder to sta-brite to copper pipe plumbing and a lot of electronics work.  I used to convert old regular profile tanks to chicken hopper style back before GRW tanks were available. I never needed to remove an end cap so all I ever used was an old Weller gun.
I’d never considered the stove to pop the end cap off but I’m gonna try it!
This particular tank has always ran good until the last 10 laps or so. Usually it starts leaning out right after the hourglass and gradually gets leaner through the clover. Then I get a couple laps clean running but lean. The next 5-7 laps it starts coughing and trying to die before finally cutting off. I figure the pickup and uniflow vents are not in ideal locations and I want to fix that!

Matt

   Again, it was running this way because it was running out of fuel.The angle of the tank made the fuell flow spotty until the final drops went through. Angle the back end out and it will use it all and then cut off clean. This is an issue caused by the nose being yawed out. Might also be an indication that you could move the leadouts forward a bit.
   Type at you later,
    Dan McEntee

[Matt Brown]

I went ahead and pulled the rear cap off. The uniflow vent was flopping in the breeze and it’s original position where it was very poorly soldered was just on top of the pickup line and both were exact same length. I shortened  the uniflow line back about 3/16” and soldered it to the pickup line and just above it. Soldered on the cap and only had one leak along the lap seam. Got that sealed up and it passed pressure testing. While the cap was off, I fixed the cracked joint where the two vents enter the tank which was the problem I pulled the tank for in the first place.
I put in about 2oz water and shook it about 5 minutes like a paint mixer. Drew that out best I could then pumped in 2oz rubbing alcohol and shook it for another 5 minutes. I figure the water should wash away most of the insides and then the isopropyl will remove the water. I drew the alcohol in and out of each tube several times to clean them. After drawing off the alcohol I used my syringe to blow air in and out of the tank to dry it. I wondered if there was anything better I could use to clean the inside like I did with the water and alcohol.
Now I have a few other issues to figure out on this plane.

Matt

[944_Jim]

Find a small 6" cast iron frying pan or a cheap throw-away frying pan to set the tank's end-cap on. That way it heats evenly all the way around and releases the entire cap at about the same time. The pan catches the solder so you don't have to pick it out of the stove eye.

[Matt Brown]

Find a small 6" cast iron frying pan or a cheap throw-away frying pan to set the tank's end-cap on. That way it heats evenly all the way around and releases the entire cap at about the same time. The pan catches the solder so you don't have to pick it out of the stove eye.

We are currently in a rental with one of those glass cooktop stoves. I just set it on the glass and it worked fine. Once I saw the solder liquify I grabbed the tank with one gloved hand and popped off the cap with my other hand protected with several layers of paper towels.


Matt

[Dave_Trible]

Sounds like you have it ready to go now but to be honest I'd have pitched the thing and made a new tank.  It had a lot of solder problems from the beginning or the tubes would have stayed in place.  An old tank can have oil, corrosion, rust or any number of things working against being re-soldered with great success.  I'd agree most of the run problems should be fixed when you cant the back of the tank outboard some.  The tube arrangement you have is pretty close to what I use.  I try to keep the uniflow line submerged as long as possible, holding the pressure head on the fuel until the last lap or two.  When the pressure finally escapes the engine goes lean for just a half lap or so then cuts cleanly.  For what it's worth I've never cared for engine-manufactured pressure.  It's normally less than just pointing the uniflow line directly into the airflow off the prop and therefore gives me more tank pressure on the fuel, keeping the engine run about the same through the whole run.  When you have the uniflow inside the tank pop open part way through the run it's not really doing anything after that- it's basically back to a suction tank.  I've long just used resin core electrical solder and not had to worry with or about flux corrosion.  Once the tank is built other than the back plate I wash it out with acetone then solder on the back plate and pressure test.

Dave

[Matt Brown]

I kept forgetting to say that the tank was canted as far as it could be in the fuse. This wasn’t an old tank, as least not to me. I bought it from Brodak back in May or June. It always had cut off like this from the time it was installed. I think the reason why it cut off so poorly was the uniflow vent was never attached to the pickup line as long as I had it. And the position it was in when I popped the rear cap off pretty much explained it. The vent was floating inboard about 3/8” and a 1/8” high.
I think I will stop at Lowe’s tomorrow after work and pick up a can of acetone to help rinse it out better.


Matt

[Peter in Fairfax, VA]

Matt,

Nice job.  Way to jump in there and see what is going on.

Acetone from a drug store could work, though I'm not sure you need to do that step.  Maybe some fuel and attention to your fuel filter might just get you back in the air.

And I've had good luck re-soldering tanks that have seen use.  Had a loose tank fill vent and attached it with a Weller 140/110, no problem. Made no special effort to clean that tank.

best,

Peter 

[Dave Harmon]

I kept forgetting to say that the tank was canted as far as it could be in the fuse. This wasn’t an old tank, as least not to me. I bought it from Brodak back in May or June. It always had cut off like this from the time it was installed. I think the reason why it cut off so poorly was the uniflow vent was never attached to the pickup line as long as I had it. And the position it was in when I popped the rear cap off pretty much explained it. The vent was floating inboard about 3/8” and a 1/8” high.
I think I will stop at Lowe’s tomorrow after work and pick up a can of acetone to help rinse it out better.


Matt

I learned to never fly a new fuel tank without removing the back cover first and fixing the crappy cold solder joints on the uniflow and pickup tube on a factory tank.
I too had/have a tank where the uniflow line had popped loose and was waving around in the center of the tank.
I can solder a LOT better than they can anyway.

The copper tubes in the assembled tanks are corroded slightly preventing a good solder joint so I replace the tubes and spin the new copper tubes in a drill with 600 grit.....gets em' nice and clean....then bend em' and assemble.
But.....I stopped doing that too....now I just get the kits and put them together properly the first time with a little GC liquid rosin flux and they go together easily.
This is a little more work on the front end but saves hours of messing around on the back end.

I use lacquer thinner or acetone to clean any remaining rosin out of factory assembled tanks....I fill it all the way and plug it up then let it sit for a couple of days, then blow it all out. Then partially fill, shake and blow out again through the pickup tube.
Of course scrubbing the inside of a factory tank first is always a good idea.
I also use Puretronics 'contact cleaner' inside the tank....this stuff will instantly dissolve the rosin so you can pour it out before the back cover goes on....cleans up the outside of the tank too.
Anything that is dirty or slightly corroded or has some oil on it....finger oil...etc etc...cannot be properly soldered.
This cleaner does the trick instantly.....don't depend on the flux cleaning the surface of the parts you are trying to solder.
Available at electronic joints.

[Brett Buck]

We are currently in a rental with one of those glass cooktop stoves. I just set it on the glass and it worked fine. Once I saw the solder liquify I grabbed the tank with one gloved hand and popped off the cap with my other hand protected with several layers of paper towels.

   There you go.

    Brett

[Brett Buck]

This particular tank has always ran good until the last 10 laps or so. Usually it starts leaning out right after the hourglass and gradually gets leaner through the clover. Then I get a couple laps clean running but lean. The next 5-7 laps it starts coughing and trying to die before finally cutting off. I figure the pickup and uniflow vents are not in ideal locations and I want to fix that!

  Maybe, but another possibility is that the yaw angle of the airplane is excessively nose-out, or that, and some combination of the tank being canted slightly the wrong way. Just canting the rear of the tank outboard 1/8" or so can have a dramatic effect on this sort of behavior.

   Brett

[Dan McEntee]

   Again, it was running this way because it was running out of fuel.The angle of the tank made the fuell flow spotty until the final drops went through. Angle the back end out and it will use it all and then cut off clean. This is an issue caused by the nose being yawed out. Might also be an indication that you could move the leadouts forward a bit.
   Type at you later,
    Dan McEntee