Not sure if my similar experience will help, but you seem to have tried everything else to solve the no-start problem. First, if your engine has a power pack, this is very likely the problem. My experience is with a 1970 Evinrude 85HP—roughly 3 years after I purchased the motor I was having a similar problem with it not starting until waiting about 25-30 minutes—then it would start perfectly as if nothing ever was wrong. After a couple of unsuccesful trips to the dealer to find the cause, I was finally able to duplicate the problem over and over, and actually control the start/no-start condition whenever I wanted to. It turned out that after running the motor wide open for a couple of minutes, then shutting the engine off, it would not start for 30 minutes. If I ran the engine half or 3/4 throttle and repeated the same process of shutting down and re-starting, the engine started fine. It was only after a few minutes of hard, wide-open throttle that the re-start process failed. After explaining this to the dealer (which he did not attempt to duplicate in a tank, thank heaven), I was told that the only other possibility was that a transistor or other electronic component in the power pack was overheating after a hard run and therefore failing to function properly until a cool-down period occurred (25-30 minutes). Since the power pack was sealed, there was no way to troubleshoot individual components, so I decided to take the leap and purchase a new power pack (at a cost of roughly $100 back in 1973 with no returns on electronic parts!!). It was easy to replace, and that solved the problem for years and years to follow.