Small Engines (Lawn Mowers, etc.): old 3.5 hp Tecumseh, air mixture, adjustment screws

Question
Hi - A friend has given me an old Mountfield mower with a Tecumseh 3.5hp 4 stroke horizontal engine. I have cleaned and tinkered with it and the more I do, the more I am determined to get it to run well. It starts OK, but runs a little erratic, has a mind of it's own, it seems to like fast so I close the throttle down but then it can be too slow. I should add that there is no model number on it so getting parts and help has so far proved difficult. The spark plus is also very black (dry), also with black deposits around the exhaust. I know this means rich, but I don't know the carb model it has two screws, one is above the bowl and protrudes out in a cylindrical housing the other screw is to the left of the bowl which is nearer the output side of the carb. Any pointers of carb setting info would be a great help. I think you may say take off the carb and clean, but I don't know if I have the skill and don't want to mess it up. Also as mentioned without a model number getting new gaskets isn't easy. thanks

Answer
Hello Steve:

How Goes it. Good I Hope. If this is an Old Tecumseh, then the Model Numbers Should be STAMPED into the Blower Housing above the Spark Plug or Above the Muffler. You May have to Scratch the Paint or Rust to See them. However, I have 2 Tecumseh PDF Site Addys that Might Assist and Interest you. 1- http://www.tecumsehpower.com/CustomerService/BSI.pdf and 2- http://www.cpdonline.com/692509.pdf Here is How the Carburetor Air Mixture Screws are Set. ****The Operational/High RPM Air Mixture Screw is In the Carburetor Bowl in the Center of the Nut or Some Carburetors have Both the Idle Air and Operational Air Adjustment Screws Side by Side on the Top Side of the Carburetor. The Screw Closest to the Engine Is Usually the Operational Air Mixture Screw on this Type Carburetor.The Idle Air Mixture is the One at Top on the Side and Goes Into the Carburetor.   Some have the Idle Air Mixture Screw Going Vertically Into the Top of the Carburetor. The Engine Idle Screw is the One that Moves the Throttle Plate. The Following is the Directions for Setting these Screws. **If the Carburetor was Soaked Overnight in Carburetor Cleaning Solution and the Passages Blown Out with Compressed Air and a New Kit Installed,  then we can Set the Air Mixture Screws. Turn Both Air Mixture Screws In Until Seated. **DO NOT JAM  SNUG ONLY** Turn Both Screws Out 1 1/2 Turns. Set the Throttle to Full Open and Start the Engine. Override the Throttle by Pushing the Governor Arm by Hand and Turn the Operational/High Air Mixture Screw(bottom of the bowl) In Until the Engine is Revving Properly. Now Slow Engine to Idle. Set the Idle Air Mixture Screw so there is No Hesitation when Throttling from Idle to Full Throttle. Now Set the Engine Idle if Required.**** Hope this Helps. I am here if you Require more Assistance. Let me know what Happens, Please. Thanks.


Good Luck

Respectfully

John