i have walk behind self propelled model 675 troy bilt 21 in mower i cleaned the carburetor twice the jet does not look like it is removable The mower is probably a couple of years old It runs for a about half hour but sputtering and then it stops running . Do you think i need new carburetor I cleaned the air filter . I have been cutting long grass at a friends house . for a couple of months now . i dont see under the deck any obstructions from long grass build up . Any ideas would be appreciated . it has run good for two years or so up till now. Thanks