Small Engines (Lawn Mowers, etc.): Briggs and Stratton Engines, briggs and stratton, holley carb

Question
My 1993 B&S 18hp Model #422707-1214-01 engine Seizes up and down upon load. Ex: Put it in gear and drive away w/o deck engaged. Runs well at idle. Having problem w/ idle speed mixture screw. Engine smooths  and runs at its best when screw all the way tight. Engine dies at this setting upon just increasing rpms sitting still. Will accelerate better as I loosen the screw. Wierd. Should be for idle only. When just driving, the engine rpms fall and then increase to normal.Have rebuild carb,replaced fuel filter, fuel line, air filter, cleaned tank, new plugs, cleaned flywheel, set armature gap .010-.014. Checked all hoses for defects. Smells a litlle rich at idle and does spit a little gray smoke out exhaust upon initial increase of rpms with throttle control lever. sitting still can increase throttle and engines runs smooth. My old 340 Duster with a Holley carb was easier to diagnose than lawnmowers. Thanks for any help in this matter.

Answer
I'll bet when you were working on that duster it was more than a couple years ago, now our gasoline has so many additives in it it loses octane faster and messes with these small engine carburetors. I believe you have something trapped in one of the small passages of your carburetor. I suggest getting a can of Deep Creep, or Sea Foam and drenching the carburetor in it, make a point of getting it in all the small holes and passages.