It not that there isn't enough tank pressure, the real problem lies in the POS carburator (the engine itself is just fine). There is not a lot of consistency in the fit between the throttle rotor & the carb body. I believe that the body casting is the chief trouble-maker, as I find it difficult to believe that the machined rotor can have significantly variable dimensions. I have seen these carbs with both tapered & oval rotor-bores in the casting. This results in air-leaks.

The faults result in constantly changing vacuum in the throttle-bore, depending upon throttle position, rotor-bore wear, rotor-spring tension, vibration, air density, airframe attitude & even fuel type (fuels with castor oil seem to work better, particularly if the lube percentage is increased by adding additional castor).

Common symptoms include hard starting, poor idle, changing mixture in flight (usually leaning-out), failure to shut off, flame-outs & significant sensitivity to atmospheric conditions.

There are a couple of fixes --

1) bore the rotor hole true & machine a new throttle rotor (beyond most modellers)

2) swap the carb to another brand -- I have seen good results with a variety of carburetors

3) buy something else.

Not all of these motors suffer from these problems, and a "good" one is a nice motor -- but why bother in the first place?