SF tank leak near the left knee pad rubber - how to fix?

jko_78

jko_78

Full member
Location
The ever windy Northwest of Europe
Hi,

Behind the rubber knee pads, there is a small leak just underneath the knee pad rubber on the left side of my SF tank (the "humped" one). I've tried using the search function, but can't really find if it is a common problem, or if there is a common fix.

It has been soldered earlier, worked for a few years, then it was epoxy coated, worked for a few years, but it seems there is some rust underneath the area where the rubber pad normally sits that has me a bit worried.

Is this a common problem with a common fix? If it is not I can post some photo's when I get to the bike again.

Br Jeroen
 
Unless there's a leak coming from the neck filler and dribbling downward, it sounds like the previous soldering and tank coating has failed, so you'll have to remove the tank and try re-soldering it. The internal coating will most likely have to be removed or patched since it's probably also failed- what type of coating was used? Have you been using ethanol free gas?
 
You have successfully demonstrated that solder and epoxy are just temporary solutions. I suspect the only permanent fix is to weld up the hole properly with steel. Yes, it's a pain in the arse to do, especially if the tank has a lining. You have to make the tank safe to weld by filling it with inert gas or by some other method. If the repair can't be hidden under the rubber pad, you may also need to re-paint the whole tank.
 
jko_78 said:
Hi,

Behind the rubber knee pads, there is a small leak just underneath the knee pad rubber on the left side of my SF tank (the "humped" one). I've tried using the search function, but can't really find if it is a common problem, or if there is a common fix.

It has been soldered earlier, worked for a few years, then it was epoxy coated, worked for a few years, but it seems there is some rust underneath the area where the rubber pad normally sits that has me a bit worried.

Is this a common problem with a common fix? If it is not I can post some photo's when I get to the bike again.

Br Jeroen
Click to expand...
Hi Jeroen,

These tanks weren’t the best that Laverda (had) made. The lower seam has a tendency to leak, you often see repairs there. The same counts where the plate that holds the knee rubbers is welded on the tank. This can rust through completely, especially when a tank has suffered a period of neglect in our damp climate. I think it is useful to thoroughly check the condition of the tank in that area to avoid more woes to occur in the future.

Marnix
 
start by de-gassing the tank, empty all fuel and then leave it outside for a week with the filler cap the lowest point, (petrol/fumes is heavier than air) next get a hair dryer and blow warm air into the tank until it is so hot that you cannot even touch it, fuel evaporates at quite a low temperature (I have used the exhaust pipe outlet on a petrol car at a race track for this) then take the tank to a grit blaster and get him to blast the outside and any of the inside that he can, now you will see where your tank is damaged and you can go from there.
CLEM
 
Hi all,

Thanks all! So it seems it's a more common problem. I cleaned up the area as best as I could, it seems to stem from where the knee pad plate attaches or somewhere below. The seam at the very bottom that was soldered seems to held up. Pretty dirty mess; the tank does have an epoxy based liner in there.

As I posted in the other thread about my 750, upon closer inspection it's more rusty than I expected (or hoped, perhaps :D). Behind the emblems, some areas are touched up, etc. Paint ranges from acceptable to gone depending on where you look.
Might try the UHU trick for very very short term but looks like the root cause will need addressing.
Somebody put some gooyey glue underneath the crusty old rubber (my dad still has a good set of NOS, but the tank needs to be leakfree first)
You can see where it's moisty with fuel:
fueltank1.png
here's the right side:
fueltank2.png
Out of curiositiy, does the GT tank with luggage rack suffer this as well? And can you find them....

Jeroen
 
The plates that hold the tank rubbers on are spot welded on, you can drill out the welds and remove the plates allowing you to clean the area hidden by the plates and getting any holes welded or brazed. Rubbers can then be stuck back on with mastic instead of replacing the plates.
 
Looks like your tank could do with a re-spray anyway, so you may as well do a proper job on it following the recipe in Clem's post #7.

PS: Get rid of that rubbish tank liner while you're at it. The fact that the tank is leaking is testimony to its effectiveness. Tank liners are worse than useless in my opinion. If they work at all, it's only a short-term solution. They all fail eventually and end up causing more of a headache than you had in the first place. You still have to do a proper fix on the leaky tank, but now it's got a bloody failed liner in it that has to be dealt with.

I've never tried this myself, but I've been told that the best way to get rid of a tank liner in a steel tank is to chuck the tank on a bonfire for a few hours. That'll also strip the paint off, get rid of any old solder or epoxy repairs, and burn away any lingering fuel so it'll be safe to weld. When the fire has gone out, you'll just have a bit of loose ash inside the tank to clean out.
 
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Dellortoman said:
Looks like your tank could do with a re-spray anyway, so you may as well do a proper job on it following the recipe in Clem's post #7.

PS: Get rid of that rubbish tank liner while you're at it. The fact that the tank is leaking is testimony to its effectiveness. Tank liners are worse than useless in my opinion. If they work at all, it's only a short-term solution. They all fail eventually and end up causing more of a headache than you had in the first place. You still have to do a proper fix on the leaky tank, but now it's got a bloody failed liner in it that has to be dealt with.

I've never tried this myself, but I've been told that the best way to get rid of a tank liner in a steel tank is to chuck the tank on a bonfire for a few hours. That'll also strip the paint off, get rid of any old solder or epoxy repairs, and burn away any lingering fuel so it'll be safe to weld. When the fire has gone out, you'll just have a bit of loose ash inside the tank to clean out.
Click to expand...
It’ll definitely “ burn away any lingering fuel..”- this in turn will highlight the areas weakened by rust!😂
 
Exmoor beast said:
It’ll definitely “ burn away any lingering fuel..”- this in turn will highlight the areas weakened by rust!😂
Click to expand...
Exmoor, it is clear that you have no faith in the method Clem and myself mentioned and see it as a fatal mistake. It was the advised method given to my workshop manager by a guy whose job it was to cut up used 44 gallon fuel drums with a gas axe.
 
Tippie said:
Exmoor, it is clear that you have no faith in the method Clem and myself mentioned and see it as a fatal mistake. It was the advised method given to my workshop manager by a guy whose job it was to cut up used 44 gallon fuel drums with a gas axe.
Click to expand...
It was the “ chucking it on a bonfire to burn away any lingering fuel “ bit I thought comical. How a welder prepares a tank prior to welding is up to his own satisfaction- I never take anyone’s word for it and wash them out with a hose, but you know that. I’ve not had one go bang.
I’ve bollocked a mechanic for angle grinding a BMW with the tank on & I’ve seen a K100 fuel pump spark in a half empty tank because the ignition was on when changing the filter. I have put out a Z1300 that burst into flames ( mechanic was turning motor over with tank off and plugs out- fuel injection spat fuel over sparking leads!) and I C90 that had no spark -produced a spark with work bench puddled with fuel.
My Montjuic burst into flames whilst trying to start it in my shed..next to a Vincent Comet,Gt750 and a Harley that I had at the time.
Feel free to chuck your tank on a bonfire- don’t worry about me, just thought it funny, that’s all.
Funny thing about petrol is you can drain 35 year old brown watery sludge out of a non running Greek moped- put half a pint on some old pallets to get them going and it’s like the opening sequence of the “A” Team…….dont ask!
 
I think Lee is referring to the 'leave the tank inverted' technique for draining fumes from a tank, Exmoor. At the end of the day we each have our own methods, tried and tested - but I personally can vouch for the inversion method. i'd reckon Clem has had done 'a weld or two' in his short years :cool: so I'll take his word as confirmation of its effectiveness.

The bonfire is just a suggestion for burning away fukt tank linings ... with a humorous side benefit of ensuring no fumes remain in there!

I've also seen first hand how easily a fire can start ... a Zane with the injectors off but fuel pump still operative!! Don't touch the starter button! (No - not me, but i was too close for comfort.) You have to be quick-thinking to quell it before it takes out the bike, the building and who knows what else!!
 
I have had a Lawn Mower and an XT500 burn, once they get going a bit you have buckleys of putting them out. The XTs steel fuel tank boiled in the heat and the fuel cap turned into a Bunson Burner. It happened on a soft soil dirt road, 10 blokes tried smothering it in dirt. Waste of effort. The cause, discovered later was it had a fuel tap that came out vertically and took a core sample of dirt if laid on the ground. That dirt jammed the needle and seat, carb overflowed and bingo. Most taps on later bikes came out horizontal, I wonder why.
 
Yes, the "burn away lingering fuel" was meant as a tongue in cheek comment.

There used to be a motorcycle painter who had a bodyworks shop not far from me. He did very nice work, but he showed me one of his mistakes, a tank from a fuel-injected Ducati that he blew up. He left the tank upside down for a few days to "drain" the fuel vapour away, but forgot that there was a fuel pump inside the tank that evidently had a bit of fuel trapped in it. A bit must have dribbled out into the main body of the tank when he moved it to his workbench to do a bit of welding on it. He reckons he almost shat himself when it went bang and jumped 10 feet in the air. It was pretty impressive how it had split open and the steel had wrapped right around the other way so it was still roughly tank shaped, but with the inside now on the outside. He had to buy the customer a new tank.

That bloke doesn't do bodyworks any more. He and his wife opened a bakery. He could probably blow something up there if he tries hard enough.
 
Well the spark was also down to me, I did the wiring. It was points and batteryless and my guess is I missed some exposed copper, anyway, it was completely flogged to death, it stated every 10,000 kicks and less if stalled or hot, had the worst handling and non-existent suspension, and had massive low-end toque that would spin the rear and wheelstand at the same time. Other than that it was a good bike NOT. I acquired a later model Yamaha 250 MX bike without a motor and graphed its long-travel suspension including its single shock alloy swingarm that I converted to twin shock and mounted a couple of Ohlines piggyback shocks onto it. That required the steering head to be pulled back on a Motolinner to reduce the trail to less than the 150mm it had. After all that I could finally ride the thing at speed, it surprisingly handled and slid really well. Still wouldn't start most of the time. Then thankfully it caught fire and put me out of my misery. I then bought a 1992 Husky 350 four stoke, it was brilliant. Started really easy, handled like a charm, and had brilliant Showa suspension that got dialled in and would tractor up a cliff and also run stupid fast. The only thing it missed was more modern weight saving, it was slightly heavy and an electric start. I was by then riding with some very serious ex-racers and that required my upgrading to a brand new 2005 TE 250. That bike was by far better than my ability, it required way more aggression than I could apply. You had to charge hard toran or it would fall off its cam. Anton Meo won a couple of world titles racing one. I should have fitted the optional 300cc kit that added torque and tamed its hit. My cousin now rides it. I am well past all that these days, chugging about on the Atlas suits my old bones fine.
 
The lawn mower story was just as dumb, dont check for spark if you spill fuel just after filling the tank. Dry fuel-soaked lawn clippings catch fire very easily. It's good and expensive to learn shit when you're young. Best to not repeat this stuff. The bike fire happened while riding the bike, I felt something, looked down and the bike was fully engulfed, I stepped off at probably walking pace and the bike dropped to the ground. I tried picking it up but stopped trying as the fire was fast getting bigger. 10 minutes of everybody trying to bury it on that soft dirt road. In the end, we had to just step back before someone got burned. Both were unstoppable without pro firefighting gear.
 
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