Hi Airacer,
Would believe you do not have much experience with engines?
The problem you mention probably would not exsist if the engine fuel
mixture was adjusted prperly to start with, not knowing otherwise try
this method,
With the engine level and running at full throttle lean the mixture (turn
the needle towards the closed position, usually to the right) 8ntil the
engine wants to stop. Quickly richen the mixture (open the needle valve,
turn to the left) until peak RPM is observed.
With peak assured open the needle more until you note a slight drop in
rpm. That will be the operating mixture.
There are two styles of carburetor adjustments, a 2nd kow speed needle valve or an air bleed adjusting screw. The needle valves would
be in line with each other. The air bleed screw would be to one side
These are low speed, idle, adjustments and such adjusting is done with
them alone, the high speed setting is NOT TOUCHED once adjusted.
Idle speed procedure> close the throttle to the slowest steady rpm.
From that, quickly open the throttle.
Did the engine run rough, burp, want to stop?
If so the mixture is to rich and can be leaned by turning the low speed
needle in (to the right) about 1/2 revelution at a time.
Repeat until the change from low to high is immediate and clean.
If lower idle speed is desired and possible close throttle more and
repeat prodedure.
If with the initial quick throttle opening the engine ran smoothly and
slowly died, quit. Then the mixture is to lean. Repeat the previous
method except turn the low speed needle out (to he left)
If the engine has an air bleed carburetor and the initial low speed try
indicates a rich mixture the air bleed screw is opened, turned to the
left. Follow same procedure as with he needle valve. If mixture is lean
turn he air bleed screw in, to he right.

When proper low speed setting is attained the engine will transverse
from low to high smoothly time after time with no tendicy to stop.
If not increase the low speed rpm very slightly until action is proper.
If when an engine has been set up with this routine and you still see
the problem you describe the trouble is proably in the fuel supply system
not the engine.
Things such as leaky fuel lines, kinked fuel lines and/or fuel tank is to
far from engine.
Also if muffler pressure is used there could be leaks in muffler attachment or itself.
As you probbly have noted the engine comes with operating instructions
but the box can not contain needed experience from useage!
Good luck, have fun!

Hal deBolt