Mike and all others who were kind enough to reply:
I verified there was no induction blockage (just two screws and three worm clamps to removed the induction box.
Removed the mixing elbow and the elbow was cram packed with hard encrustations. It took an hour to dig it all out. There was no visible opening whatsoever. There are also encrustrations and blockage at the exhaust manifold outlet. I am working on those but I am wondering about the shape of the outlet of that box like exhaust manifold. It almost appears to be a sort of venturi shaped outet. Either that or the scale/encrustations/corrosion have mis-shappen it. Can you or anyone comment on the appropriate shape of the outlet of the exhaust maniforld on the Onan.
I havn't finished up re-assembly but I can't immagine how the genset even ran previously. Pictures to follow when I get better band-width.
All the best,
Gary s/v Liahona
PS I think a new thread with a discussion about running the genset under load vs running the genset with no load to allow the generator to cool down prior to shut-down, and preventative maintenance on exhaust elbows might be be in the offing.