Hello l have a suzki gs250 82model leaking fuel from rear of the carby (pilot air screw)+the overflow. have checked the float hight needle and seat seems to be cutting the fuel off ok. bike runs but very rich any help would be great thank you darren

The service manual says 27mm +/- .5mm to answer the question.

try screwing the bolt of the fuuel holderr in the carborator, whhichh is generally used for overflow, screw it tight ,

First, confirm the fuel pressure going into the carb is 1.5 to 3 PSI.
Unplug the fuel line at the carb and plug in a fuel pressure gauge:

http://www.harborfreight.com/fuel-pump-and-vacuum-tester-93547.html

If the pressure is too high, it could force fuel into the float bowl by forcing the float needle off it's seat.

When you replaced the float needle and seat, did you verify/readjust the float level?
The stop at the back of the floats can break if they are installed improperly, and the brass pin that holds them in place can wear an oval hole in the float pivot. New floats are not a bad idea if you're doing a rebuild.
Also check the float to see if it is punctured and fuel is getting inside it, as this weighs it down so it won't "float" as it should.

I had to read this TWICE.. your Needle / Seat are not seating properly and although you believe since it has been replaced it could not be the reason, it probably is.

This could be almost anything, probably minor. Good premium unleaded fuel, fresh/clean and gapped plugs, fuel filters clean,. Carbies clean and jets good, correct float levels, no air leaks in intakes, air filter clean, Electrical connections 100%, ignition coils OK, no sign of moisture behind magneto/stator cover or on source/pick-up coils. Engine earth wire secure from head to frame?
So....
New plugs, 0.7mm gap to start with and set air* mixture screw 1.5 turns out.
To flush carby with cleaning solvent (spray can from auto access.) Take float bowls off (check for sediment) and fuel lines off and spray till it pours out of float needle seat, Loosen carby clamps and swivel carbies outwards 90 degrees and unscrew pilot jets and spray thru also. Check operation and smooth return of choke.
* Air mixture screw controls fuel-air mixture at idle speed and is a trial and error thing. Pilot and Main jets regulate cruising and acceleration mixtures and ranges according to rider style and riding and climate conditions.
If there is any undue noise comming from the top of the engine with revs slowly rising from idle to 2000 revs, you may need to check camchain tension and valve clearances, easy done with some simple tools and a manual. Downloadable free with an engine search!
Goodluck and Cheers.

the spring is just to hold the needle in place, it just hangs there, I would suggest you check the float for seepage, if its one of the older metal ones its likely leaging gas into it and sinking, causing your flooding situation. Something else to sheck is that you have theseat installed correctly, Remember the ridges on the seat piont away from the needle. and remeber to set the float hight level..

sounds like your float in the carb is stuck or the needle valve is bad. remove the bowl and push the float up to level. this should stop the fuel flow. be aware that only slight pressure should be needed.if it does not ck the needle valve. also mak sure you float is not damaged.

Carburetors have a Float Bowl on the bottom. This float bowl has a small amount of fuel, that the carburetor uses. (The float bowl's amount of fuel, is replenished from the gas tank. The fuel is gravity fed from the gas tank, through the Fuel Line)

The float bowl has a Float Needle inside. The float needle is attached to the Float. The Float resembles a thick doughnut. On one end of this doughnut, is a flat piece of metal shaped like a U on the end. Looking at this -> U shape, there are two holes at the top. There is a metal pin that goes through these holes, and the pin is attached on it's end's, by the main body of the carburetor.

The flat metal U shape, and the metal pin, works together to form a type of hinge, allowing the float to pivot on it.
Where the U shaped hinge is attached, the float needle, clips to it.

The float needle has a tapered neoprene tip, and sits in a Float Needle Seat. This seat is a barrel shaped piece of brass, and has a tapered hole at the bottom. Fuel comes up through this seat, and the float needle regulates how much fuel can come in.

As the float drops down, it draws the float needle out of the float needle seat, and fuel comes in. As the float bowl fills with fuel, the float rises, and pushes the float back up, also pushing the float needle back into the float needle seat, cutting off the flow of fuel.

There is a specified height of the float, in relation to where the float sits in the float bowl. If the float sits too high, the float bowl fills up with fuel, and overflows. This overflow goes through an Overflow Tube, on the carburetor.
(If the float sits too low, the engine starves for gas)

A LOT of times, crud builds up in the gas tank, makes it's way through the fuel line, and gets into the float bowl. It can get in-between the float needle, and the float seat, thereby flooding the carburetor out, and fuel comes out of the overflow tube.

The float bowl is also attached to the carburetor with a brass main jet nut. At the bottom of the float bowl, this will look like a brass head of a bolt. Under the brass head is a small O-ring. This can leak. First step is too insure this is tight. (Do Not Over tighten! This is part of the main jet, and is expensive! It's Brass)

Where the outer part of the float bowl itself, attaches to the carburetor, there is another O-ring.
A large one. This O-ring, like the main jet O-ring, can deteriorate over time. Additives that gasoline has in it, heat from the engine, and heat from the sun does this. Plus just age. Neoprene breaks down after time.

It could also be that the neoprene tip of the float needle has broken down.

You indicated you adjusted screws on the bottom.
These are the Main Jet, and Idle Jet -> Air Mixture screws. All they adjust is how much air goes through these jets.
The amount of fuel going through, is predetermined by specific passageways, that are drilled through the main body of the carburetor.

This only comes into play when the engine is running. With the engine off, and fuel leaking out, you have crud under the float needle, or one or more of those O-rings has deteriorated.