You don't say what propeller and fuel you're using. If you have a too-large propeller, then that could cause difficulties. If your fuel is old, that could cause difficulties.

We'd use a 10 x 7 or 8 propeller or even an 11 x 7 or 8 prop. Nothing larger because you want the engine to turn up. It should turn over 12,000 RPM when leaned to just rich of peak RPM on the ground. Make sure you have fresh fuel with at least 18% oil content. We'd normally use 10% niro, but 0%-15% is OK.

You may need to check for some kind of leak/obstruction or such in the fuel system. If the engine's tending to die when you try to lean it, then you most certainly have a fuel draw problem. Are you using the stock muffler?

2 tanks of fuel will never "break in" an engine. That may give you enough running time to allow you to fly it, but the engine will "come in" over the next gallon of fuel or so. We usually don't fly an engine until we have 45 min. to an hour of running on the bench, but that's just our way.

You need to go over everything carefully. The O.S. Max .50 SX is one of the easiest engines to set up and tune. We'd not know why your's has the difficulty it does.

One thing to consider: if the engine tends to die when you try to lean it, and it's in the model, you're likely getting vibration-induced fuel foaming. That's when air is agitated into the fuel in the tank while the engine's running. You won't likely see the air in the line to the needle because it's in the form of micro-bubbles. They will congregate at the needle and then break off as visible bubbles. That makes leaning a problem. Many modern models have lightweight airframes that just cannot absorb vibration, and they also have very little room for proper fuel-tank isolation. Ideally, you should have one inch of foam rubber around the fuel tank to keep it from getting influenced by the engine's vibration.