You could try flying it without the engine running (Oh no!) directly into the wind or better yet on a calm evening and try a gentle smooth stall on purpose. See if it ALWAYS stalls to the left. If so then the engine did not influence THAT stall. Solve THIS stall to the left problem. Then when you have it stalling straight ahead try testing with the engine running. I would assume if you notice a change in stall behavior when the engine was running it would more likely be influencing the behavior.