Small Engines (Lawn Mowers, etc.): Rough Running B+S, walbro carb, poulan lawn tractor

Question
I have a Poulan lawn tractor with an 18hp B+S Intek enging 31H777 type. It surges, i.e. seems to fire erratically at all speeds. Seems worse at low speeds. Best when under load. Thinking it was ignition related, I changed the plug and magneto armature. No change.
Thinking it was fuel related, I made sure the fuel cap was venting, changed the fuel and air filters and cleaned out all (Walbro) carb jets and orifices.
No change.
I can improve running by manually holding the choke plate closed about 1/2 way. Makes me think it's running lean. No obvious gasket or hose leaks. The idle mixture screw has no effect. I can even remove it with the engine running.

Any ideas?

Answer
 Hello Don:

 Spray WD40 Aroung the Gaskets and Hoses of the Carburator to Check for Vaccum Leaks. Have you Checked for a Blown/Leaking Headgasket? Send me the Model Numbers Off of the Engine, Please. With the Model and Type Numbers I can Locate a Breakdown of the Engine and Better Assist you. Hope this Helps. I am here if you Require more Assistance. Let me know what Happens, Please. I enjoy knowing the Outcome. Thanks.

 Good Luck

 Respectfully

 John