Mine SPE 26cc did the same today, the problem is leaking air in the blocked metal tubing on the carburator, that second tubing is used for pump pressure to the caburator directly from the crankcase, in the SPE engine it is not used, because it gets the pump presure from the plate where he carburator is mounted, i just blocke the air leaking there and the engine returnned to perform well, i used a little piece of silicone tubing with the end blocked.
The spe 26cc ia a good engine, but i think it is to weak fot a 120 plane, maybe in a 90 plane it will be good.
hope that helped with yor problem.
Good landings