350 sterndrive exhaust back pressure problem

[150aintenuff]

working on a bayliner trophy with a 350 (new 300hp) and an omc stringer 800 outdrive (also new from glm) and running into an exhaust back pressure stopping cooling water flow problem on the risers, below a planimg speed the exhaust is backing up the watter passages in the riser and causing the flow of cooling water out of the heat exchanger to stop thus hot running and over heat after 20 or so min at idle speeds..


need to know where to look to elininate the issues OR a quiet way to rerout the exhaust to reduce the exhaust back pressure.. this is NOT REVERSION!!! also this engine has the seirra 18-1997/18-1999 barr center riser exhaust with a custom built 3.5" round 90* down tube to mate to the drive not the stock end riser omc exhaust

other than doing thru transom and capping off the drive to eliminate the problem...... where do i need to look as the through transom is not an option for this person... to loud and not martini cruise compatible...

[150aintenuff]

tracked it down to the drive.. restriction in upper/lower anyone know where to do exhaust reliefs to open up the exhaust path on the old omc

[Darkside's Calling]

My old 800 has a grate installed in the intermediate drive housing that dumps the exhaust at idle. When you raise the drive look at the bottom of the intermediate (the one in the boot, not sure if I'm using the "correct" term). if you need a pic I can get one later today.