My murray rider 13.5 HP runs good in place but when I get going and it comes under stress (engaging blades, cutting longer grass or going uphill) it begins to stall and die. I've clean the carb and oiled all the linkage.

Hi,
Check for something that would reduce the power like a blown head gasket(or valve adjustment if OHV.)
Peace,
Clarke

Adjust carb to give it more gas