Lean fuel mixtures have lots of causes including, but not limited to adjustment settings---if your initial settings are as spec. by the mfg., then look elsewhere for the cause---restricted plumbing---air leaks both before & after the carb (fuel supply circuit & mixture supply circuit (intake manifold)--- fuel draw distance, both vertical & horiz. (not so critical with YS) . The YS uses a pressurized fuel system to deliver fuel from the tank to the regulator---the fuel tank is pressurized with engine crankcase pressure through a one way check/valve---be sure your lines are connected correctly & your tank is holding pressure---when you disconnect the check/valve after an engine run, you should hear a distinct pressurized air escape from the tank side---again, hope this helps.......