Opinion on Mercruser 120

dozerII

Admiral
Joined
Oct 25, 2009
Messages
6,527
I am looking at a 77 starcraft 18' SS with a 120 Mercruser on it, It has just had the lower leg replaced with new one, and has been used one week a year since 1977. Are these a good unit, mostly trouble free? I know the Chev 4 cyl is a simple motor and easy to fix.
Comments would be appreciated
Thanks Glen
 

Kainon

Chief Petty Officer
Joined
Sep 13, 2009
Messages
608
Re: Opinion on Mercruser 120

The Mercruiser 120 is a great engine, a slightly smaller version of the 140.

I would recommend a compression check, also a thorough checking of the spark plug color ( old school way of knowing how the engine is running ).

A compression check will tell you the condition of the cylinder head and piston/ring wear.

The Factory Fresh Compression is 140 on each cylinder. The 120/140 engines have been known to have issues with blowing a head gasket ( you'd see the same compression say 80 on 2 adjacent cylinders, typically 3 & 4 ). Also if your engine says DO NOT USE NO-Lead Fuel then you have a cylinder head that doesn't have "Hardened Exhaust Valve Seats" which over time the Exhaust valve will receed into the head and cause low performance. Newer Heads with Hardened seats can be purchased, and hardened seats can be installed in older heads.

I would replaced the points ignition with a Pertronix Ignitor II Kit ( least costly from Carshopinc.com and some on Ebay) the Kellogg Marine Version is from Pertronix and $50+ more !!

I would want to know when the carb was cleaned/rebuilt/ or last replaced and would also want to know the condition of the fuel tank. I would think about draining the fuel tank completely and replacing with fresh 89-92 Oct Fuel
 

Kainon

Chief Petty Officer
Joined
Sep 13, 2009
Messages
608
Re: Opinion on Mercruser 120

Oh, by the way, I did replace the original Camshaft with a Compcams 262H Cam, ran great, had good power, but dont know the power before so couldn't compare, had a great lope to it to.. though the int/exh manifold needed replacing as it was cracked internally and causing a miss at idle. The engine was also bored .030 oversize.
 

fishinpa

Petty Officer 1st Class
Joined
Nov 2, 2009
Messages
221
Re: Opinion on Mercruser 120

I am looking at a 77 starcraft 18' SS with a 120 Mercruser on it, It has just had the lower leg replaced with new one, and has been used one week a year since 1977. Are these a good unit, mostly trouble free? I know the Chev 4 cyl is a simple motor and easy to fix.
Comments would be appreciated
Thanks Glen

the statement and the pic indicate an Outboard ?

we are talking inboard inline 4 cyl ?????


fish
 

dozerII

Admiral
Joined
Oct 25, 2009
Messages
6,527
Re: Opinion on Mercruser 120

Thanks for the replys, Yes it is a 4 cyl. The picture is in my signiture and is the 16 SS I have now. I drove the 875 miles each way and bought the boat. The lower unit is brand new not rebuilt and it purrrrrs like a kitten. There is a whistle coming from the carb at idle I need to look at but I think a carb rebuild will fix it. I took my compression gauge with me but found out I only had metric atapters so could not test the compression. All the spark plugs look the same as far as color kind of a tan to rusty brown. The prop is in real bad shape which I think was on the previous lower unit and is definatly causing speed and power issues. The boat ran well, gets on plane fast and cruises great, loves the rough water. Any thing special to run this on Muffs?

Glen
 
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