I am planning to build a Sig Hog Bipe and am torn between installing a TT 61 2 stroke or a TT 91 4 Srroke . I have a TT61 in a Four * and love it. Starts with one flip by hand and runs great. Have never had a four stroke and other than a good sound and some gas economy, don't see much advantage. Would like to have others thoughts on this plane/ motor combo. Fred123