I have a 8 year old lawn boy two cycle mower. I changed the plug and cleaned the air filter at the beginning of the mowing season. It ran fine for the first few cuttings. Then one day I was cruising along and the mower just throttled down, and killed. I tried to restart, and it eventually did, but at full throttle it would run like it was set at low throttle eventually it would rev up to full throttle. I started cutting again, and got half of a row cut and the same thing happened. It is just getting worse and worse now. I can't even use the self propelled now under a cutting load without it throttling down. I have heard the exhaust ports on Two cycles need to be cleaned on occasion. Could this be the problem? Or is it a carb. issue?

i forgot to include my specs with my question. Murray Ultra 15.5 hp Overhead Valve 42" cut Briggs and Stratton Model 28n707 Type 0115 01 code 9602204A When trying to crank the carburator blows oil and gas out of the carb in vast amounts, also fills cylinder with fuel rapidly. Please help.