Kohler 12hp gas tank leaks
 

So, when I bought this old Cub 123, it had a leaky gas tank. Seemed to be the weld between the bung and the tank body, and it appeared the PO had tried fixing it a few times with epoxy, to no avail. I picked up a used/vintage steel tank at a flea market, and all was good... for about 10 months. Now my new tank seems to be leaking, and although I haven't pulled it off the machine for a full inspection yet, it appears it might be from the same area.

I'm festidious about the fitting of the tank straps, and ensuring the soft webbing is properly placed between the tank mount and the tank, so I don't think it rubbed thru there. Seems to me, there may be a flaw in these tanks. Has anyone else had similar experience?

Mine's got the original glass (clear plastic?) fuel bowl on the aluminum casting hanging directly from the tank bung, and a steel line running from that to the carb. The tank is all steel, with a square flange and four spot welds on the bung.

Fred

They make some stuff called Kream for motorcycle gas tanks. Awesome stuff. I used it on an old Kawasaki gas tank that was rotted to death and no more leaks.

The bung is soldered in, not welded, and there is no flaw- these things are 40+ years old; you can't expect things to last forever. This is probably one of three things:

-The solder joint for the bung cracked and is leaking
-The PO left off some of the padding and the mount bracket wore it thin, which then cracked
-Water in the gas over time rusted pinholes in the tank that may be invisible (or nearly so) to the naked eye

Cool. I'll have ot check that out as a back-up option. Thanks!

Thanks, Matt. Both tanks appeared to be clean / no signs of rust. I'd be surprised if the rust is so bad as to have gone thru, while being completely invisible to the naked eye, but I suppose anything is possible.

First tank failed at the bung, without question. Not sure about the second, because I haven't removed it yet, but it appears to be the same. Pretty sure it's not an issue of rub-thru, as this tank was pristine when I installed it last year, and I've checked on the webbing at the mount many times.

I've brazed new bungs into fuel tanks before, but it's something I hate doing. Always scares the piss out of me, no matter how well I rinse and clean the tank first. Something about putting fire to a metal tank once containing gasoline...

Anyone expert in soldering these steel tanks, please speak up! Interested in what flux and solder you used. I just moved, so I don't have the full shop set back up yet, but I do have acetylene, mapp, and oxy-acetylene set up already. I also have a MIG welder, but no gas bottle at the moment, rigged with flux core for outdoor welding. If the old bung is truly soldered, it should not be too much trouble to remove, clean, and re-solder.

Thanks!

I have found sometimes bungs are spot welded into a tank,and also soldered so they are leakproof.
They are spot welded so when over tightning a fitting, the softer solder joint is not twisted allowing the threaded fitting to fall off,
and/or vibration does not fracture the solder, also for assembly purposes.
I wash out tanks well with hot tide laundry soap. then solder or braze/silver solder as needed.
if a fitting is soldered do not try to weld it or braze it,-- resolder, using a large soldering copper, is the best procedure.
If it is brazed you can silver solder or rebraze it.
An once of Muratic acid diluted with snipped pieces of a zinc jar lid is the best cleaning solution for soldering.
Heat the fitting enough to cause it to soften/bubble the solder and brush in the acid to allow a good sweat joint, not a "mudded" glob of solder
Done right, the solder is sucked right into the joint like a sponge taking water.

Thanks, George! Since I don't have muriatic acid handy, can I use good old fashioned plumber's paste flux? I'm talking about the old Rosin stuff, not the modern water soluable, although I do have both.

Thanks again.

Yes you can use that.
I have a old can of soldering paste for clean jobs,
but if things are dirty, cut muratic acid is the only thing that will clean them up to allow a good sweat joint.
You must cut it with zinc, a jar lid works well,do not dilute it with water.
Small snippings of the zinc are added till the acid quits eating them, then it is diluted properly to do a great job of cleaning material for soldering
This past winter I repaired a 1915 honey comb radiator, @ that age, it needed a good cleaning acid.
A soldering paste was just not up to the job.

I hope you're passing this knowledge onto someone, ol'George! You definitely know your way around a torch and solder better than me.

My grandfather was a similar wealth of obscure knowledge, as an old-school plumber, who also apprenticed as a blacksmith. I picked up what I could from him as a kid, but too much of it was still beyond my years when he passed away.

Do yourself a favor and wash the tank with soap and water 3 or 4 times before putting the fire too it. I had to do mine and took a long stick with fire on the end to flush out gas fumes, took three times before they were escaped, the tank literally "popped" four feet each time with what looked like flame from a jet engine out of both the in and out holes. Just a little safety percaution...

I used silver solder to fix my leaky tank. It leaking at the same spot. I rinsed it with soapy water a few times. I also used a trick recommended form a mechanic friend. I piped the exhaust from my truck into the tank to help displace oxygen and gas fumes with good old CO2 and CO. Not sure if that helped or not, but I didn't blow up and the tank stopped leaking.