Generally, if your engine is quitting in flight....it's because it's leaning out
and overheating. Lets assume the fuel system is OK. You might have a
slight air leak....the carb must be tight down onto the o-ring or it will
leak air, and give tuning problems.

If you are not using a fuel with plenty of castor, your engine will run hot.
That's just a fact. If your engine is over-propped, it will over heat. If the
engine is mounted upright, it can overheat in a climb from leaning out.
Using the wrong glow plug can have an adverse effect on the engine. If
the engine is cowled, it may not be receiving enough cooling.

I wouldn't take the engine apart unless it has suffered compression loss.

FBD.
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Ditto.

If the engine has good compression, the backplate is sealed, the carb is sealed at its base and there isn't a lot of fuel leaking around the front bearing, tearing it apart is a waste of time and it will void the warranty, if it is still in force.

You mentioned that you had a slit in a line and then said that wasn't the problem. Huh?