Please do not take offense, but did you put the blade on upside down? My brother could not fix his BBC Honda of same vintage as yours. Motor seemed to be locked up. He gave me the mower for parts. When I got it I noticed that the blade had been recently sharpened and was on upside down. I removed the blade and the recoil starter rope easily spun the engine. Put the blade on the right way and all was well. I think when the blade was upside down that the clutch stayed engaged. Bro is an Ivy League grad but he shares some of the same dumbassery DNA that I have.