Sealing vintage motorcycle gas tank leak?

99sport

Well-known member
Short version: I have a leak in my gas tank, It appears that it is NOT leaking at the soldered seam along the bottom of the tank, but is leaking from under the attached (spot welded?) mounting tab. I do not see any cracks / damage to the tank on the inside with an inspection camera. I’m not sure how to fix this as I can’t identify the source of the leak. I am considering:
Kreem tank liner: https://www.revzilla.com/product/kreem-fuel-tank-liner
Any suggestions?

Long version: I have a 1972 Kawasaki 2 stroke motorcycle that I recently moved from California to Illinois. I left the bike with the gas tank full and added Stabil in early June. When I returned to Illinois yesterday, I found the tank sealer / liner that I put in the gas tank 20+ years ago had separated. (I don't remember what I used.) After removing the failed liner, I soaked the tank in Evaporust for a couple of days and found that it was leaking at the back. It seems the tank has been leaking from that area for a long time as the paint is gone there, but it was such a slow leak that I never noticed. (That area was coverd in black grime before I cleaned it and it is hidden under the seat.) It is now leaking fast enough that I cannot use the tank. The rust is removed from the tank and I need to fix the leak so I can use the bike.

Pictures of the inside and outside of the leaking area are below (note that there appears to be a seam on the inside of the tank along the tank centerline that is not visible on the outside). You can just make out the outline of the bottom of the mounting tab on the inside of the tank. (By the way, the dates on the inspection cam pictures are wrong.)

Any suggestions on how to repair this leak?
 

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HighPockets

Active member
Properly cleaned, Kreem would probably do it if not at a stress point. Since yours seemed to be under stress I would think it needs to be brazed. Have an expert do it. You don't want it to explode on you.

Gene
 

flman

Roadrunner, Genius of Birds ALWAYS WINS! NO FAILS!
My uncle used to solder gas tanks with a large torch heated soldering iron. No flame involved.



If I were to braze, I would rinse it out with water, put my torch to the tank opening to burn off any fumes, then start brazing away.
 

220629

Well-known member
...
If I were to braze, I would rinse it out with water, put my torch to the tank opening to burn off any fumes, then start brazing away.
Um... maybe rinse with some dishwasher soap and water many times?

Back in my younger days I used water to triple rinse a type 1 VW fuel tank before brazing. Triple rinsing is a laboratory standard after all. I was young, but smart enough to think, "Maybe there's some residual fumes left in that tank?". I directed my torch to the tank fuel fill neck. There was a very loud noise as the tank took off like a rocket. It was a learning experience. That was my one and only fuel tank brazing experience. I will add that it was successful.

Since that experience I've noticed that one method some professionals use is to flood the vessel with inert gas before applying the torch.

Carry on.

:2cents: vic
 

SD26

2012 NCV3 3500
Somewhere, I've got pictures of an old G5 I used to have for a while...

Similar to Kreem, I think POR15 makes a motorcycle tank liner product. I think I remember some people telling me that it was much better than Kreem.
 

lindenengineering

Well-known member
Um... maybe rinse with some dishwasher soap and water many times?

Back in my younger days I used water to triple rinse a type 1 VW fuel tank before brazing. Triple rinsing is a laboratory standard after all. I was young, but smart enough to think, "Maybe there's some residual fumes left in that tank?". I directed my torch to the tank fuel fill neck. There was a very loud noise as the tank took off like a rocket. It was a learning experience. That was my one and only fuel tank brazing experience. I will add that it was successful.

Since that experience I've noticed that one method some professionals use is to flood the vessel with inert gas before applying the torch.

Carry on.

:2cents: vic
That's my old dad's'wartime method with a fag (a woodbine--coffin nails) hanging out the corner of his mouth !
Flood it with nitrogen or fill it with water.
Dennis
 

NBB

Well-known member
Water conducts heat - a problem. Nitrogen purge is normal for brazing. It prevents oxidation of the metal as well. The paint and whatever that inner gray coating is will all vaporize, though, such that you’d want to repaint the area, which might not be in the budget.
 

99sport

Well-known member
I have gotten some excellent advice here. I think that HighPockets is probably right that the strain at the mounting tab will likely jeopardize a tank lining repair, however I am going to try Kreeme anyway.

My rationalle is:
-I dont want to attempt the repair myself - I have reworked soldered copper fittings when I didn't want to replace them, and while it usually goes well, once in a while the repair takes too long and the joining parts get too hot and all the solder runs out and everything falls apart. If the solder / braze flowed out of the seam in my 50 year old tank I would have a real problem as new parts are hard to get.
- I am out in the middle of nowhere and I don't know where to take it - it took me 4 years to get someone to show up to trim my trees.
- I'm not sure where the actual leak is. It is leaking from under the tab that appears to be spot welded on, so the tab would probably have to come off to make a successful repair.
- It has the original paint on it and I want to keep it (and the "patina")
- It is only ridden a couple of times a year, and the slow leak, while unfortunate, isn't so much to prevent me from riding it - it is however a problem for storage.

So I am going to try Kreem. I looked at the POR 15 product, but Kreem was cheaper and seems to have good reviews. I do plan to leave the rubber band off that hooks over the mounting tab so the tab is no longer under strain. Even if it still leaks slightly, with the liner I can store the tank empty, or below the level of the leak, and not worry about rust forming. My girlfriend rides the bike around the yard once in a while, and a few minutes of leaking a couple of times a year won't be the end of the world. After all, it looks like it has been leaking unnoticed for quite a while. Finally, if Kreem doesn't work, I can always do a more permanent repair at a later time.

Thank again for all the great responses
 

rollerbearing

Well-known member
I propane torch soldered a gas tank out of a Ford Falcon multiple times before I finally got it sealed - it had many pinholes from a crumple some time in it's history.

Washed it well with soap and water. Took out the sending unit/float assembly and stuck a shop vac hose in the fill hole way deep into the tank. Had the vac pulling full air flow constanly through the tank to suck out any remaining gas fumes as I soldered. The air didn't cool the tank metal so much that it prevented the solder from flowing well.

I've used the same trick on plumbing when a shut off didn't quite shut off or when upper story water keeps draining. Just open a couple of valves up the line somewhere and let the vac suck the water out up there - works lots better than the white bread plug trick.
 
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Larry M

Well-known member
I worked on storage tanks in the 80s and 90s. We had to have an inert tank before we could do any hot work. We always used dry ice. For this tank, 1/4 pound would be more than enough of solid or pellet form dry ice. Don’t use a C02 fire extinguisher. You may create static electricity.

Filling the tank with water didn’t work since it acted as a sink on the inside and you couldn’t get the metal hot enough to weld or cut.
 

99sport

Well-known member
I finally found time to coat the tank. I did a couple of rounds of phosphoric acid and MEK before coating. Interestingly, while the tank leaked gasoline through a fine crack as shown in the earlier pictures, it did NOT leak water / acid at that spot. I assume this has something to do with the difference in viscosity / surface tension between water and gasoline. I am therefore hopeful that the Kreem will fix the leak.

As the tank drain is slightly raised above the bottom of the tank you cannot get all of the Kreme out of the tank through the tank drain and there will be a small pool at the lowest point of the tank while the Kreem is drying - I put the pool over the leaking area in the hopes of minimizing the chance of a leak there. I will also leave the rubber strap off of the bracket at the site of the leak.

The Kreem is in, but probably needs another day to dry before I add gasoline. Final (not very good) pictures of the tank liner attached - the inspection camera picture is the same view as in the last post.
 

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