Gen IV 7.4L Mercs.

We bought a 1990 350 Sundancer last Feb. I found an invoice for replacing the original engines with Gen IV's. Got no engine manuals so I called Mercury to see if they might have one. The lady I talked to had nothing on Gen IV engines. What have I got? They were advertised as 350HP. Thanks.

mark 4 is the "original" big block chevy (excluding the "W" motors) next were the gen V and then gen VI - the changes were minimal - things like non adjustable valvetrain, revised cooling passages, different front cover an rear main seal - also updated manufacturing process (pm rods, etc) most likely you have original style engines....unsure if you would have forged or cast internals or 4 bolt mains though same on oval vs rectangular port heads...

Copy down the serial numbers and call Merc or do a search on one of the parts sites by SN and it will tell you what you've got. Your boat would have likely come with 7.4 MIE 340hp (never heard of 350hp).

http://www.mercruiserparts.com

1990 should be the last year for the mark iv. The gen v came out in 1991. The easiest way to tell is look at the fuel pump. The gen v eliminated the mechanical fuel pump provision.