Mine SPE 26cc had the problem, i discovered it was 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 returned to perform well, i used a little piece of silicone tubing with the end blocked.

Please let me know if the same solves your problem.