Rusted gas tanks can be a big problem as others have explained, but for someone who enjoys mechanical work, they can be a bargain. I was given an old Hyosung gt250 with a badly rusted gas tank for free and got it running. If it was a better bike like a Ninjette, I'd be willing to pay for it. But I would not try to start or run the bike after buying it before doing several things first.
Regarding signs of a rusted tank, peeling paint below the tank can be a sign of leaking gas, brake fluid, or coolant. Any of these can be a problem, but may not indicate a rusted tank. Gasoline can leak from petcocks, fuel pumps (on bikes that have them,) fuel lines, over filling gas tanks, or a leak in a gas tank that isn't caused by rust. I bought my Ninjette seeing peeling paint on the gas tank below one of the screw holes for securing the upper fairing. This is a common problem with early ninjettes after someone forces a bolt that was too long, too far, puncturing the gas tank right behind the welded nut or screw hole. This can often be fixed easily with a little tank or leak repair epoxy.
A another way of detecting a rusted tank is to disconnect a fuel line to drain, collect, and inspect some of the gasoline coming from the tank. If the tank is rusty and the gas in the tank is old, you can see the rust color in the drained gas. Fresh gas is clear. If the tank is rusty but was recently flushed and filled with fresh gas, the drained gas will still be clear.
If you do have a rusted tank, if the drained gas is really dirty, colored, colored or thick, or if you know that the bike has been sitting for years with fuel that was not stabilized in the tank I wouldn't try to run the engine after you buy it before doing several steps. Trying to run it can bring rust, gelled gasoline, water, and crud from the tank into your petcock, fuel lines, fuel pump, and carbs or injectors. My steps below are for a carbureted bike.
- Drain the float bowls of the carburetors. Try to catch whatever comes out so you can examine it in a clear container for water, rust, and other crud.
- Improvise a temporary external fuel tank. I use an old plastic lawn mower fuel tank. Others use funnels, glass containers, or gas cans. Use anything gasoline can't melt that you can attach fuel line to and suspend or secure above the carburetors.
- After you've closed the drains for the carburetor float bowls, connect your temporary external fuel tank directly to the carburetors using clean fuel line. Bypass any petcocks or fuel pumps.
- Put some fresh gasoline in the external fuel tank. You may add a tiny bit of fuel system cleaner. Just remember that if you put 12 ounces of gasoline in the temporary fuel tank, you don't want to use much more that a hundredth of the fuel system cleaner that you'd use in a car's 12 gallon gas tank.
- Securely suspend the temporary fuel tank above the carburetors. This is like an IV line in a hospital except it doesn't need to be more than a few inches higher that the carburetors.
- Try to start the engine, jump starting if necessary.
If you are lucky, the bike will start but will probably run rough. You can then do all the stuff needed with the carbs, tuning, oil, plugs, air filter, choke, throttle, battery etc. If the bike won't run try some basic things to check for spark and compression before concluding that the problem is the carbs. To confirm that the problem is the carbs or fuel delivery you could briefly try a tiny bit of starter fluid. Starter fluid can be dangerous to the engine though, especially when the engine is cold and not well lubricated. Do not try to keep the engine running with starter fluid or use much of it. It won't fix it help the engine. It could only help you diagnose with the cost of potentially damaging the engine. If the starter fluid helps the engine run, it just tells you that you have a problem with your carbs and need to fix them.
Running the bike more with fuel system cleaner may clean carbs that are only a little dirty. I mean carbs that have restricted flow through some internal passages or jets, but not so dirty that any passages or jets are completely obstructed. Seafoam or chemtool B12 are strong but aggressive. Techron or Gumout Regane with ptfe are safer. I like Marvel's Mystery Oil (mineral spirits and a little oil) for maintenance and prevention with carbureted engines. Try to run the engine at varying throttle levels and RPMs to use different jets and passages and get the cleaner to all passages and jets. If the bike absolutely won't run at certain throttle levels or RPMs, stop; your carbs are too dirty and need to be disassembled and manually cleaned properly.
If the engine runs, but roughly, some will even fill the carburetor bowls and passages (those that still have some flow) with concentrated cleaner by doing the following. Stop the engine, pour the gasoline out of the temporary fuel tank, and replace it with fuel system cleaner. You won't need much. A couple ounces should suffice. Restart the bike and run the engine until it stops. You can try to restart it, but it won't run well when the carburetors are full of too much cleaner and not enough gasoline, which is what you want. If you crank the engine too long after it stops running, concentrated cleaner is going into the engine which you probably don't want. Let the bike rest for a few hours or a day to let the concentrated fuel system cleaner dissolve the gel and varnish in the carburetor as well as it can. Drain all remaining fuel system cleaner from the temporary tank, fuel line, and carburetor float bowls. Try to collect and examine what drains from the carburetor float bowls. Hopefully this will start to looked clearer and cleaner. Try again to run the engine with pure fresh clean gasoline and note any change from before. If it's much better but not as good as you'd like, you can rinse, lather, and repeat. After you are done, change the oil which will be contaminated with some of those cleaning solvents.
If your engine runs just a little rough but we'll enough to ride safely, you might find that riding a few hundred miles may help a little or a lot. If course you need to repair it replace the rusted gas tank, petcock, fuel line, (fuel pump, and fuel filter) first. Riding the bike is the only way most of us can run the engine with load, most effective getting fuel and cleaners flowing through partially obstructed passages. I like to start with 4 fluid ounces of techron and one gallon of gasoline. If there's a marked improvement, as I ride more I use less techron until I'm down to a half ounce of techron and then switch to marvel mystery oil. If there's not marked improvement, back to disassembling and manually cleaning the carbs.
These methods can be easier, least costly, and less time consuming than removing and fully disassembling the carbs to clean them properly, but will never be as effective. Nowhere near as effective or safe as bringing or sending your carbs to someone experienced and skilled like Ducatiman.
Rusted gas tanks are difficult to repair effectively or costly to replace. Alot a few hundred dollars for that. You should also rebuild the petcock and replace all fuel line and clamps, replace a fuel pump if it has one, and add a good decent sized clear inline fuel filter. This should be maybe $50 and a half hour or so on a Ninjette. The gas tank and carburetors are the hard parts.
On the Hyosung I cleaned the badly rusted tank (which wasn't leaking) by shaking acid and other solvents (one at a time but together with small nuts, screws, and BBs to scrape and dislodge rust) in the rusty tank.
Lots of turning and shaking and a few solvent changes later, the solvent came out clear. After rinsing it with rust arrestor, I reinstalled the tank, together with two very good clear inline fuel filters, one on each side of the fuel pump. Some will use a fuel tank liner, but they can peel or dissolve and be as bad as rust. If it was a much better bike I would have replaced the tank.
Clearly a rusted tank is a problem. If you enjoy working on bikes and the seller discounts the bike appropriately (or more) buying a bike with a rusted gas tank can be a problem.