For the past few months I've been struggling with a very rich running condition on my 2009 John Deere X300R mower. The engine is a Kawasaki FS541V with 500 hours on it. The carburetor is a Walbro model that has only a low speed mixture adjustment. While the engine is running at full throttle it will often shut off suddenly and backfire out the exhaust. When this happens it can be difficult to restart, and when it does a cloud of black smoke blows out of the exhaust with a strong smell of raw fuel. If I remove the air filter I can see a little gas in the intake tube just ahead of the choke. The spark plugs have a sooty, black coating indicating a rich condition. Here are a few of the things I've tried to mitigate this problem with no lasting success:
Checked for sticking choke (it's not)
Replaced air filter & foam pre-cleaner
Cleaned & rebuilt the original carburetor (new needle & float, the seat was non-replaceable)
Replaced the original carburetor with a new carburetor
Checked ignition coil gaps and tested spark
Performed a valve job (resurfaced old valves, cleaned the seats, lapped them in & adjusted per factory service manual).
In spite of all this it refuses to lean-out and run properly. The behavior is very erratic. Sometimes it backfires within 10 minutes of a cold start while other times you can mow for over an hour before it happens. I'm at my wit's end trying to diagnose this, so I am hoping ya'll can give me some ideas on what to check next. Any thoughts?
That description is indicative of a float not sealing properly
Unfortunately looks like no one answered Crow so we don't know if it was a genuine carb or a lucky dip ebay carb .
So pull your carb off tip it upside down and try to blow through the fuel intake .
You should not be able to do it and if you have a small pump or can turn down you compressor that low, it should hold 5 to 7 psi forever
