I have a O.S 25 engine that when priming won't suck the fuel from the tank. I check it and there aren't any leaks in the fuel lines, and the engine is compressing. I am putting a finger over the carborator and have the needle-valve at 1.5-2 turns from closed. What could be causing this?

The only thing you didn't mention you checked was the pressure line from the tank. Check that and see if that is ok then get back to us.

CGr.

Possibly fuel line clogged. Possibly you have wrong line (vent) coming to carb. Possibly fuel line in tank has gotten stuck up in front of tank.Remove fuel line from carb, blow air into tank through vent. Fuel should flow out of fuel line. If it does and you still have the problem put fuel line back on carb. Open needle 3 or more turns blow through vent line , carb should become full of fuel. If not then you have a blockage in your carb.

ok so the tubing is fine, in fact i took it all out of the plane to run the tests. no blockage in the fuel lines as i could easily blow fuel through them, however when attached to the carb with the needle valve at 3-4 turns and throttle open there was still trouble blowing fuel through.

Check if there is an adjustment screw with a small spring around it on the carb. If there is look on the outside of the carb where the screw is, there may be a small hole for air and if the screw is set open enough you will have to cover both the carb opening and this hole for it to suck fuel while priming. I noticed this on my o.s. 46la the screw is on the front of mine so the hole is directly in front of that screw on the outside of the carb. I almost thought I was going to have to remove my carb and replace the oring because mine wouldnt prime without an electric starter 'till I noticed this(should this screw be open that far?I never messed with it.) .</p>

The air bleed screw should be around two turns, make sure hole is clear. Take a fuel bulb and back flush from Carb to inlet side of needle valve, make sure to catch fuel in small bottle. If that doesn't do it check the plactic piece where it goes into carb. Checl bottle for forign objects then you can blow through carb with crank in closed position. I had a same problem with 46LA and one of thosed fixed it. Jim
PS make sure needle valve is four or more turns open before back flushing.

Is the tank plug in line with the carb? If the fuel tank plug is lower than the carb? readjust if needed so that it is even or slightly above the carb throat. I have also seen the carb throttle lever come loose. It will still move as if it's working as it should, but the carb guts will move only slightly when the lever becomes loose. So it will seem that you should be pumping fuel when in fact, just the lever is moving and nothing else.

I had a similar problem recently and it turned out to be the brass fuel tubing within the tank. All externat plumbing was good and pressure tested ok, but the tubes inside the tank were eaten away allowing air into the fuel line.
Check inside the tank.

Amen.
I quite regularly get cracks in the clunk tube.
crbhawking
Instead of sucking the fuel, try putting your finger over the end of the silencer and allowing exhaust pressure to blow it through.
If that doesn't work then there's a hole or blockage (fuel filter?).

Do you filter your fuel as you're filling too?