> 1. Has anyone else had a problem with leaks at the firewall? Did
> replacing the o-rings solve the problem?
That is a common failure mode for leaks. My 89 Mi16 did that and I know of
various other 405's around here with the same problem. On mine, a new
O-ring solved it seemingly the 1st time. But the years went on, I replaced
the manifold a second time. Your pinched 0-ring may have warped the
manifold when it was tightened at the factory.
-Jim