Specjaliści BMW R75, Zündapp KS 600 i 750


Tadeusz Pawlak

All 'veteran' (old bikes) owners know this state of frustration when they notice that inside of their fuel tanks is covered with thick rust. Usually we take a look inside only because our bike suddenly refuses to operate. First we adjust the carburettor (or carburettors). We discover that some openings of the jets are blocked. We clear them and at the same time we spot that something's wrong with fuel level.

Level is high enough, but it's like the fuel itself is not there. The engine starts, and when we're racing it suddenly stops. It takes time to notice that fuel barely runs instead of flowing through the pipe leading form gas tap to carburettor.

First we check if the clearance of the pipe had not decreased its diameter. Usually at this point everything appears to be fine. Then we disassemble gas taps' setter and observe something really shocking – setters' strainer is almost completely blocked with brown grease.

Some of us would just clear the strainer, assemble the setter back, mount the pipe and start the engine. They're happy. Until the strainer gets blocked again. Then they're wise enough to diagnose the problem instantly and are confident that everything is perfectly fine.

More inquisitive people would remove the fuel cap and take a look inside. By the way, please, doing that under no circumstances use anything like candle, lighter or a match. Well, that would solve our problems pretty quickly, but I assume you'd like to have a few more rides in your life, won't you? So, use a flashlight, and leave cigarettes at home.

What do we see inside of the tank? Not much, really, because thick layer of rust absorbs the light. What we can notice are traces of corrosion protection (it shows as lacquer or galvanic coating). We make a decision to clean our tank and make it rust-proof. We disassemble the tank and here's where the problems start.

How do we clean the tank?

The most efficient way would be sandblasting – but will Mr. Frank, who sandblast elements for me from time to time, listen to my request and perform the operation truly carefully? After all, he could set the highest pressure and make holes in the corroded tank. Once he made this way a mincemeat of a pretty good wing. And his apology didn't do any good once the damage was done.

No, I won't give it to Mr. Frank. I have a compressor, I can borrow spray gun and do the sandblasting myself. But hey, wait a minute... I've read somewhere that even in tanks that haven't been used for a very long time, there still can be remains of fumes which can explode in case of a spark. During the sandblasting sand electrifies on outlets of the jets. I've seen sparks myself. Mr. Frank may be a daredevil, but I still have many kilometers to ride ahead. No, no experiments.

Chemical means of removing the rust are usually based on caustic soda. They work fine but I remain careful. Too concentrated solution, too long period of cleaning... and the most important issue: am I able to sluice chemical down to the very last particle? If not – I don't even dear to think of consequences.

Sharp gravel, as I hear, can be useful when it comes to cleaning. I can buy some and put it inside the tank.

Add about quarter liter of water and seal off the filler. All that's left is a daily portion of physical exercise, let's do some swinging. Three or four hours of fun.

Then I throw gravel out, rinse and drain the tank. It isn't cleaned perfectly, but according to the producer of special lacquer which is used to protect insides of tanks, it should be clean enough.

Described version was directed to hard-working people. Here's the tip for lazy ones:

Sharp gravel, same as in the first version, same amount of water. We seal off the filler and wrap the tank with foil, rags etc. thick enough so it's completely immovable when we put it in a concrete mixer of average size. We turn the mixer on and from time to time change horizontal position of its basket. Two hours and our tank should be satisfactorily cleaned.

Having two hours off, we can get acquaint with the instruction of the lacquer with witch we are going to paint the inside of our tank for protection.

Tip: employ the lacquer only in well-vented rooms.

Binary lacquer forms elastic, smooth layer, resistant to all known liquid fuels, oils, water, diluted alkalies, acids and hydraulic fluids containing alcohols. Lacquer should be applied in the range of ambient temperatures from +10 to +35°C (50 to 95°F). The tank should be dried, dedusted and cooled/heated to the temperature mentioned above.


The proportion of lacquer to activator should amount to 4:1.

(The best option is to mix the content of both cans at one stroke.)

How to use it:

- dismount gas tap.

- very carefully seal off mounting hole of a gas tap.

You can't just shut it with your finger.

- to protect glossed outside surfaces, wrap the tank in a foil.

First layer.

Pour a mixture of lacquer and activator into a tank. Seal off mounting hole of a gas tap, with duct tape or unnecessary filler plug. If you want to use an original plug, put between it and the hole some protective foil. Otherwise it would later require painstaking cleaning.

For a few minutes swing the tank in all directions, so lacquer can reach every nook and cranny and form an even layer. Then for at least ten minutes put the tank aside in the position which will allow the lacquer to flow down to the lowest part (usually it's where the tap is located).

Remove the sealing and empty the tank.

After about 3 hours repeat the operation to form a second layer.

To avoid runs and stains (although they're not much of a problem in this case), as well as to form even layer, from time to time slew the tank in all planes. Remember that lacquer remains liquid for approximately 8-12 hours.

Lacquer desiccation.

After about 8 hours lacquer stiffens. Remove the seals from all holes and set the tank so the filler hole is at the bottom.

Time of stiffening depends on environmental temperature and thickness of formed layers.

It is not advised to desiccate in temperature higher than environmental (about 20°C, 68°F).

Fill the tank with fuel only after at least 8 days.

In my Zündapp KS 750 I cleaned the tank and lacquered its inside five years ago. Until today it's surprising how does it look after I remove fuel filler lid. Light grey inside is invariably clean and looking at it I think that couple hours of work were not too high price for this effect.