Hi!
It sound as if you have a foaming in the tank! But it could be other things to . Hav eyou checked the tank for leaks!

Here is my recommendations and it is for every .46-.50 two-stroke engine on the market.
Use a 11x6 APC prop! Most sport airplanes prefere this size (at sea-level)
Use a OS 8, Enya 3-4 , Rossi 3 or 4 Nova-Rossi 3-4 glowplug,
Use fuel with 5-15% nitro.
Use a 240cc-300cc (8-10oz) fueltank with lots of foampadding around it to hinder vibrations to foam the fuel inside the tank.
The tank should be placed so that the centre is in line with the middle of the carb orrifice when the plane sits on the ground with its stabilizer and wing level with the horizon

If you want the best in fueltanks get a Tettra "bubbleless tank" from Dave Shadel at Performance Specialties in Nevada (he is former worldchamp in F3D pylonracing). This Tettra tank has a silicon bladder inside where the fuel is and pressure from the silencer is comressing this bladder so no air gets in the fuel cell.
This tank could be is glued in the fuselage with silicon glue and it sure will provide better performance than anything you have seen before in.... racing planes that is ....where there is lots of vibration .....But in ordinary sport airplanes an ordinary Du-Bro or Sullivan tank will work just as good

As for throttle drum set up! The idle opening in the carb should always be 1,5-2mm open from fully closed position!
If you use the above set-up then it's just amatter of adjusting the carbs high and low speed needles.....and go flying...remember always to point the nose of the airplane straight up after you have started and adjusted the carb ..slight adjustments are nearly needed before every start, If you want max performance...
Why you held the plane straight up??...it's to check that it can suck fuel without cutting out in the worst possible position (take off position).


Good luck!
Jan K
Sweden