Last year the engine in my ~2002 13HP 38" MTD Yard Machine lawn tractor threw a rod among other damage. I eventually bought a used engine of the same family and age, however I've noticed it runs poorly. It's got an exhaust leak as well, as the old muffler rusted out and the new one may not be tight enough. Around the exhaust port it's quite black, leading me to believe the engine could be running rich? Curious for your opinions on what needs to be done.
Like ILENGINE said, make sure the choke is not on while using the engine.
Darn near every rider or anything with a throttle/choke lever I buy, the linkage is alllll messssed up. Choke doesn't work. Over revs to 5,000rpm. Won't start........
Also check the carb float height. If all is solid, put a high altitude main jet in your carb. That will lean it out a touch. Someone might of been in there with drill bits or welding torch tip cleaning rods. That will make it run rich.
Probably need to decarbon the cylinder and clean some cooling fins. Intake port could be clogged up not flowing enough air into the cylinder. I've seen carbon deposits on the back side of the intake valves darn near closing off the port. Installing a new OEM air filter wouldn't hurt.
Spark plug heat range could be incorrect. My latest mower grab had the wrong plug in it. Blade on upside down. Got to love it....
