Hi!
All planes /engines should be able to dive straight down for one to two seconds without the engine cutting out, If you cannot do that something is wrong!
How is you tank set-up?
Most R/C tanks are set up with two fuel lines, one is going to the carb and the other straight up inside the tank is for pressure /telling when the tank is full. The fuel line going to the carb should be so long that the clunk is set apprx 10mm from the back of the tank.

A better solution is to use the two clunk lines ,called "Uni-Flow" (one going to the carb and the other for pressure) running side by side,the pressure line 10-20mm shorter then the line going to the carb. The third line is going straight up as usual and it's for telling when the tank is full.
Having the pressure below the fuel surface makes a more even pressure distribution for the carb and thus makes the carb setting more even.