Cobra

Depending on how old the engine is you may want to inspect/replace the rubber o-rings on the carbs. If these things are leaking air the engine will be a bear to run/tune.

Also, is your helicopter experiencing any extreme vibration? If so that will introduce small air bubbles into the fuel line which will cause the engine to run too lean, and it will once again be a bear to tune. The solution here is to sort out the vibration, or get a header tank, or both.

Is the fuel tank pressurised from the muffler? Otherwise the tank will make a vacuum and starve the engine of fuel.

You shouldn't really have this type of hassle with the setup you have mentioned ie Hawk & OS.