CB500 Fuel tank

I got a tank to fit my second honda. The tank over flow is clear. But behind the overflow there is another smaller outlet that is blocked. Can anyone throw any light on its use and should it be free or blocked of. And how the devil can it be cleaned out. I have tried a length of wire but to no avail. Any suggestions welcome.
                   Joe

try to spray in this outlet some carbs cleaning agent, let it be there for a while and counter blow it with compressed air if available. 
i'm not sure, but should be the fuel tank venting to the atmosphere, 
since the tap closes hermetically, somehow the fuel has to move by gravitation, this should be a "connection" of the tank's internal volume with the atmosphere (i suppose) 

or, put some fuel in the tank, close the tap and check if the fuel flows, if it doesn't, then for sure this is the tank's venting. suck through the hose which goes to the left manifold to keep the fuel's cock valve open.

Bigger one is overflow, smaller is vent. When blocked, the fuel will not flow properly due to underpressure creating in the tank.
I mean, at first it will flow normally, but the more fuel out, the slower will it flow. This may cause engine running good at first, but dying after some time riding.

Thanks chaps. yes what i find is the tank is pressurising and when i open it it blows the fuel cap open. I have tried the carb cleaner through it via an old syringe it goes eventually but not enough to do the job it is supposed to do. Looks like yet another tank needed.
                             Joe

You can try to unblock it wit steel cable. This resembles the method plumbers use when they deal with blocked piping with a drain/plumbing snake. 

Get some twisted cable (not sure if braided will work) approx half the diameter of the tube, about two feet long (should be enough for any twisties inside). Clamp one end in drill chuck and slowly screw the cable in the tube (mind the rotation direction - you want the cable strands to tighten when revolving). This may be tricky at first when you have a lot of cable hanging loosely, it will try to fold and tangle on itself. Maybe start with rotating by hand, and later use the drill.
The cable will guide itself even in twisty tube due to it's flexibility, and thanks to rotation the cable will scrub anything it encounters like a wirebrush.

P.S.
On some afterthought - if you have old speedo cable (inner part) it might work even better, as it is designed to work in this manner (rotating while bent).

yes, it could be a wire of a bicycle's brake line cable, for example

Thanks for that. I will give it a go. For now i am going to use the black tank i have with the overflow pipe broken inside the tank. I have just blocked the overflow pipe and will just have to fill it carefully till i find a fix or replace the tank with another. Have to keep an eye on here hoping someone comes up with a breaker. Thanks again.
                                    Joe.

Mig welding wire would fit and work quite well

Rob the dog wrote:Mig welding wire would fit and work quite well

Rob
Thanks i will get round to trying all these idea's. At present i have just acquired a wire cable used for up and over doors so that will be tested soon. But i am trying to find a good black tank or even wrecked bike local ish  to make this bike good condition again. Joe.