View Full Version : mercruiser engines

02-07-2011, 10:04 PM
Has anybody had much to do with Mercruiser 120hp petrol 4 cylinder engines..Any advice or things to look for??

02-07-2011, 10:16 PM
What would you like to know? The 120's are a fairly old engine now. Essentially just a Chev so parts are obtainable although Mercury may have deleted several items.

Last 120 I rebuilt was about 2004/5. We sourced engine internals through the machine shop doing our machine work. could have purchased parts via Mercury but too expensive.


02-07-2011, 10:27 PM
Not much to go too wrong with them really. If it gradually gets hotter and hotter its a typical sign of a semi blocked exhaust elbow.

The tiller arm typically flogs out on older units (connects the steering cable to the gimbal assembly). To check this have someone rock the drive (taking up the slack) and look inside the boat at the tiller arm. If there is a lot of drive movement before any tiller arm movement then the tiller arm (gimbal top pin) is worn. There is a short term fix or a full fix requires disassembly.

If the engine has trouble with the gears, as in, losing forward or losing reverse and adjusting to get one loses the other this is a sign of component wear in several components and in particular the "shift spool" inside the bottom gearbox. Can get a bit costly.

If the engine stalls on gearshift its a common problem with an easy fix. Most people go looking for an ignition based problem but the problem is caused by a tight lower shift cable.

Make sure your oil pressure is good (minimum 20psi with a warm engine at 2000rpm).

That about covers it I think :)

05-07-2011, 12:09 AM
If salt water cooled, there is a tendancy for the head to corrode through and dump seawater into no. 1 cylinder. I had a hard time finding another head, and had to get one out of the U.S.
Postage cost more than the head ! Thankfully I have an outbourd these days and certainly don't miss the the old 120 Mercruiser. I think mine was late 70s so it had definately run it's race.