Mc culloch MT225 cls difficult to start?

I have a Mc culloch Mt255 cls which will not keep running. It starts easily when fuel is introduced into the cylinder but fades out. Every thing appears to be ok but it will not start until fuel is put in the cylinder.
I have been trying to start it with 25:1 (already mixed) instead of 40:1. Do you think this would lean the mixture out & cause this problem?