So its really loose and free with spark plug out? And no petrol shot out when you pulled the cord?
If so then you need to be looking at your valve settings (on the engine) maybe even a sticky valve and possibly blocked exhaust? (Long shot)
The problem is the compression in the engine is not being released out... also I believe the gx engines have an automatic decompress or in the engine somewhere, this could be at fault...
In any case I would start with the valve clearances as that's easiest, then I'd take the exhaust of if that doesn't work... After that your looking at taking the valves out and lapping / cleaning them... and that's as far as I'd go personally