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Sears uses several manufacturers to build tractors to sears specs. Most are made by Husqvarna and MTD. They recently partnered with Simplicity for the new high end 30 horsepower, I believe the model is CTX9500 and retails for about 6,500.00 dollars. Most rear engine riders are made by Murray or Snapper.
The guy that said "Throw it away" is a moron. I have a Husq 225 blower, 12 years old, gets LOTS of use and still going. This idiot told everybody to buy 4 stroke equipment exclusively. 4 stroke blowers? No such thing. It's all about proper use and servicing. The PB251 starting problem is probably a sheared flywheel key. If it's less than 5 years old, any Echo dealer will fix it free with the possible exception of freight. Any gas engine has to have 3 things to run: Spark (at the right time) compression and fuel/air. Take away any one of those and it won't start. Listen to the experts...ME!