1997 7.4 mercruiser mpi blue water inboard

[otherseason]

Hello,
owned this 1997 330 sundancer for 9 yrs. Port engine getting slightly lower oil pressure and compression is down to 120 on most cylnders but 2 middle cylinders are at 80ish. Always maintained, plugs, wires, oils, manifolds, riser, pumps etc.
How can I improve/fix this engine or do I need to look for a remanufactured replacement?

Thanks,

 

[dubs283]

Probably best to look into a local engine rebuilder. Pretty tough to buy a reman 454 these days, they are rare

Curious as to what caused the low compression, with routine care and maintenance 454 ought to last thousands of hours with great performance

 

[alldodge]

Welcome
I would do a rebuild of what you have. Look real close at the motor, it probably is a 7.4 mpi, but might be a 454 Mag mpi. If it's the Mag then it has 385hp with 502 heads

 

[Scott Danforth]

I would pull both motors and take them to a local engine builder. That will be the most cost effective solution

 

[tpenfield]

7.4L (454) Gen 6 rollers . . . nice engines, probably time for some engine work . . . might just be the top end, but maybe do a cylinder leak test on them before pulling the engines.

If they are closed cooling or run in fresh water I'd re-build the existing engines.

 

[Scott06]

Would do a leakdown test. Could just be valve related and a valve job could fix it, but loss of oil pressure if accurate , also indicates bearing wear.

michigan motorz offers remans if you don’t have a local machinist

 

[kenny nunez]

Wherever you go if you decide on a rebuild be very sure that they are familiar with reverse rotation engines. As well as keeping the port and starboard engines‘s parts separate.

 

[alldodge]

Wherever you go if you decide on a rebuild be very sure that they are familiar with reverse rotation engines. As well as keeping the port and starboard engines‘s parts separate.

Kenny, it's a 1997, they should both be LH rotation

 

[kenny nunez]

I guess it depends on the transmission to provide left and right propeller rotation. I cannot imagine both propellers turning the same rotation. Now I am curious. Does Merc have transmissions that provide L&R propeller rotation?

If it was a Volvo then it is easy to have both standard rotation turning L & R propellers.
Educate me.

 

[alldodge]

All Merc twins use opposite rotating props and like VP it's easy to change. Most Merc's used Hurth/Velvet drives and it's simple to change the pump to have it rotate in different directions in FWD

 

[dubs283]

Does Merc have transmissions that provide L&R propeller rotation?

From that vintage mercury used zf hurth straight and v drive transmissions with gear reduction. Pump orientation determines output rotation as 454 from that era was LH rotation only

 

[kenny nunez]

I did not know that Merc was using Hurth transmissions. That definitley keeps it simple and uncomplicated.