Sure it's not a PB251?Echo lb251 echo fuel premix
How do I check for this?Shorted wire.In any case, the most likely problems are a broken or cracked fuel line or a shorted/grounded kill wire. They're not only the most likely, they're the cheapest to fix!
The simplest thing to do is disconnect the kill wire from the magneto coil. If it starts with the wire disconnected, then the wire is shorted to ground somewhere. It's up to you to figure out where.How do I check for this?Shorted wire.