There's a couple of points not mentioned by others that I think would be helpful to you at this stage of the game. Just running the engine tells you nothing.

First, there are a couple of way's to verify that your idle mixture is correct.

While the engine is running at idle, hold the nose up. If it's too lean it'll die. If it's too rich, it'll speed up and run better. Now, do the opposite by holding the nose down. If it's too rich, it'll die. If it's too lean, it'll speed up and run better.

Final check. Run the engine up to full throttle to clear it out, then back to idle. Let it sit at idle for 10-15 seconds, then snap the throttle to full power.

If it dies or hesitates, the idle mixture is too lean. Richen it by turning the air bleed screw IN, not out. The air bleed controls air, not fuel so it works the opposite of the main needle valve. The main needle valve only controls full throttle. The air bleed controls everything from idle to 7/8 throttle.

Check to make sure that you've got at least 1/2" between the back of the fuel tank and the clunk. Any less than this and the engine will run lean.

Also, if the fuel line runs through a firewall, make sure the fuel line slides FREELY through it. If there's any resistance, the fuel flow will be restricted and the motor will die.

Make sure you're running tank pressure from the muffler. O.S. engines will become unreliable without it.

Lastly, change to a different glow plug. You can spend the big bucks and buy O.S. plugs, but I've had excellent results with Fox R/C longs and K&B R/C longs.

Glow plugs, like spark plugs come in different heat ranges. Maybe the one you're running is too hot and is overheating the motor. The coldest plug is the McCoy MC-9, but it's pretty pricey. Next is the Fox R/C, followed by the K&B R/C long. 4-stroke plugs are the hottest because they need to keep the fires lit every other turn, so don't use a 4-stroke plug.

Dan