mercruiser 120 bent rod zero compression

[captainnate]

Hey everybody, thanks in advance for any advice/ideas. I have a 1972 18' Starcraft Nova I was given from my father-in-law. The boat sat for about 5 years in a pole barn. I have it running, but very rough.

Today I took the valve cover off, and definitely found some clues. I don't know which is the exhaust vs intake, but the same ones where off/loose on number 3 and 4 cylinders. The rod was bent, and the end was smashed a little on 4. The rocker was totally off on number 3, but rod appears straight. Did the valves get rusted in one position, is that why this might have happened? I also noticed some almost foamy looking light colored residue on the inside of the valve cover; does that mean anything?

I did a compression test yesterday. It was my first ever, and I found out I didn't take all the spark plugs out like I was suppose to. Instead I took the plugs out one at a time. I rented the tool from the auto parts store. These are the numbers I got:

#1 - 122
#2 - 126
#3 - 0
#4 - 0

I checked twice on 3 and 4, and then did 1 again to see if I screwed up, but got the same numbers. Also, the 1 and 2 had very black (like coal) spark plugs. 3 and 4 had very wet, but otherwise clean plugs. The engine idles, but very rough at times, and with quite a bit of vibration.

I just had some work done by a mechanic last week. He's an auto mechanic, but does some boats, snowmobiles and such on the side. He set the points, replaced the rotor, distributer cap, spark plug wires, and spark plugs.

Here are some pics, what do you think?

 

[captainnate]

Re: mercruiser 120 bent rod zero compression

another picture

View attachment 116853

 

[Don S]

Re: mercruiser 120 bent rod zero compression

You have some serious problems, might as well pull the head, no matter what the problem is, it has to come off. Then you can see what is wrong.

You also have the wrong spark plugs.

 

[Fordiesel69]

Re: mercruiser 120 bent rod zero compression

First off, see if the valves open & close properly.

Secondly, replace the push rods and ADJUST PROPERLY.

Thirdly, do a leakdown test to see if HG is blown.

If none of the above, replace the engine, they are a dime a donzen.

 

[captainnate]

Re: mercruiser 120 bent rod zero compression

Thanks for the advice, I will definitely use it. About the spark plugs, is this the right one: AC#CR44N? I have to check the serial number to make sure that is the right plug. If these engines are a dime a dozen, is there a good place to get one? I know I could look on ebay and craigslist, just wondering if there is something else.

 

[Don S]

Re: mercruiser 120 bent rod zero compression

Thanks for the advice, I will definitely use it. About the spark plugs, is this the right one: AC#CR44N? I have to check the serial number to make sure that is the right plug. If these engines are a dime a dozen, is there a good place to get one? I know I could look on ebay and craigslist, just wondering if there is something else.

They are not a dime a dozen, and don't listen to someone that has to ask every move they make on their own engine on how to do things.

Next, you need short reach plugs, but at this point, spark plugs are the least of your problem. Pull the head and find what types of problems you have. Putting in new push rods is kind of odd since you still need to pull the head to find out why the compression is zero.

 

[fmalott]

Re: mercruiser 120 bent rod zero compression

take the advise of Don S i'm not as smart as he is i'm pretty sure they are not a dime a dozen and to me just rebuild the engine and use the parts that are still good if you can find parts i'm sure someone knows where to find them . just my 2 cents