Well mdfogg probably not the sort of idea you like but if it was me I would attach the engine to a fishing line about a foot and a half above the hook and use it as a sinker. Have seen a few of these engines and they have been nothing but trouble. There are plenty of people on the forums who will howl me down but I would go buy an OS or Thunder Tiger equivalent and replace the engine.

The list of things to check, start with all your fuel lines both inside and outside the tank you should not have any bubbles in the fuel line. It is possible that there is a pin hole size leak in the line and you have not found it. Make sure that your tank is secure and not moving about or vibrating and wrap it in foam if necessary to avoid fuel foaming in the tank. Make sure that the tank is sealed properly and no air is finding itsway in and breaking the seal

If non of these work wind the high speed screw out and richen up the fuel mix and see if that helps, if it doesn't find the low speed idle screw and open that up a little (just a bit at a time maybe 1/8th of a turn. If non of these suggestions sort out the problem go back to 1st paragraph and get another motor.