Small Engines (Lawn Mowers, etc.): kohler engine performance problems, blown head gasket, simplicity product

Question
I have a 1979 allis chalmers model 712 Shuttle.The problem i am having occurs when the tractor is under load.The engine starts slowing and then begins to knock and rattle then quits.In an attempt to correct this problem i have adjusted the points, rebuilt the carb, changed the plug and coil wire and converted the fuel pump to  an electric.None of these have had any effect on the problem.The engine runs great without being under load.It doesn't use oil or smoke.Any help with this problem would be appreciated.

Answer
Hello Roderick:

Allis Chamblers is a Simplicity Product. What is the Attachment you are Using when this Occurs? If it is the Cutter Deck then: 1- Check the Idler Pulleys and the Bearing and Tube Assemblies for Stiff Bearings. If the Tension Spring on the Deck is Good, the Belts Won't Slip and it will Drag the Engine Down as You Described. Stiff Bearings are Bearings that are in the Process of Locking. It is a Rare Occurrence, but it Does Happen. 2- Check the Compression when Cold. Then Run the Engine at Full Throttle (no Load) for 5 Minutes and Then Check the Compression Hot. BE CAREFUL, PLEASE. HOT ENGINE!! The Compression Should Be No Less than 80psi Cold and 100psi Hot. 3- A Blown Head gasket will Sometimes Effect an Engine This Way also.  You can View a Breakdown of the Mower at this Site Addy, http://www.jackssmallengines.com/ and Scroll Down to the Parts Look Up Button and Click. Then Select On Line Parts Look Up. Then Select the Model Tab. Then Select the OEM (Simplicity). Then Enter the Model Numbers 712S. Then Double Click on your Model in the Results Area. Hope this Helps. I am Here if You Require More Assistance. Let Me Know What Happens, Please. Thanks.

Good Luck

Respectfully  

John