120 merc hitting on 2 cyl?

[imastevesmith]

I have a merc 120. It is a 1967 4 cyl. eng. It got hot last fall d/t drain valves being open. Now runs rough at low rpm. Runs better at higher. I have changed plugs, wires, condenser, cap, points. When I pull the front two plug wires there is no change in eng. idle. When I pull the back two wires it does start missing.

 

[fletcher0780]

Re: 120 merc hitting on 2 cyl?

run a compression test, but I'd guess bad head gasket or warped head.

 

[imastevesmith]

Re: 120 merc hitting on 2 cyl?

If I had a bad head gasket would the oil get milky looking like in a car?

 

[Silvertip]

Re: 120 merc hitting on 2 cyl?

Depends on where the gasket leak is. Are you saying the engine oil is milky? If so, yes -- it may be a head gasket. Or a cracked head or block.

 

[imastevesmith]

Re: 120 merc hitting on 2 cyl?

No the oil is fine. After doing the compression test how will i know if it is a head gasket or something else?

 

[Maclin]

Re: 120 merc hitting on 2 cyl?

If it got really hot then the head could have warped and the gasket could not keep sealing due to too much gap.

The compression test will show if cylinders are roughly the same compression or if one or more cylinders are low. If two adjacent cylinders are low and show almost the same numbers then the head gasket could be compromised between those cylinders. The next thing to do (if the compression is low) is to do a leakdown test. That test pressurizes a cylinder thru a fitting screwed into the sparkplug hole and then you listen to the block and the intake manifold and the exhaust manifold to try to determine where the airs is leaking into. The test is performed on each cylinder after the engine is rotated to the point where the cylinder is in the down portion of the power stroke.

If the head gasket has let go then removing the head of course is in order. The head should be checked by a machine shop for a straight dek surface and they can also resurface it and check for cracks as well. Freshening up the valves and seats is probably advisable then as well.

Sometimes overheating can cause valve or valvetrain problems. The leakdown test will help detemine if the intake or exhaust valve or both are no longer sealing for a given cylinder.

 

[imastevesmith]

Re: 120 merc hitting on 2 cyl?

Well I did that compression test this am. 3 of the 4 cyl. have right at 120. 1 has nothing. I'm sorry but I didn't understand that other test. Can we tell if it is a head or just the gasket without pulling it apart? This explains to me why the eng. had no change in idle when I pulled that one but why does the second plug wire when pulled make no change. Remember the other two do make it miss as I would expect.

 

[Don S]

Re: 120 merc hitting on 2 cyl?

If you had a blown head gasket or a cracked or warped head, or block, you would have SOME compression.
You have either a hole in the piston or a bad valve. Either way, the head has to come off.

 

[imastevesmith]

Re: 120 merc hitting on 2 cyl?

Well I pulled the head off. Gasket doesn't look like it is the problem. Took it over to a friend of mine who does mechanic work. He is sure the valve is bent. Guess I take it to a machine shop.

 

[imastevesmith]

Re: 120 merc hitting on 2 cyl?

Got the head back from the machine shop today. They replaced bent valve, planed, cleaned, and pressure checked so we're ready to go back on. I have a paper on the tightening sequence and torque of the head bolts but do I need to put something like Anti-seize on the head bolts? How do I set the rocker arms and what is the gap on them?
The machine shop said this is a GM 2.5 so any specs or step by step info would be greatly appreciated.

 

[Fishermark]

Re: 120 merc hitting on 2 cyl?

If memory serves me correctly those head bolts go into the water jacket, so you will need to put some sealer (not anti-seize) on the threads. Also put some oil just under the head of the bolt to ensure correct torque values. I don't have the torque numbers in front of me right now however.

You really need a manual - someone will probably be along shortly with a link to a manual to help.

 

[imastevesmith]

Re: 120 merc hitting on 2 cyl?

How do I set the rocker arms?

 

[imastevesmith]

Re: 120 merc hitting on 2 cyl?

Ok guys....someone PLEASE help. I really need to find a link, and I've been serf'in the net for hours. I need to know the rocker arm settings for the 2.5L GM motor ( Merc 120) AND any advice on how to set them. I have the torque settings and sequence.

 

[ziggy]

Re: 120 merc hitting on 2 cyl?

ya need sm #1 from this link..... very large file so give it time to load. save to disk..... the sm will have all the info in it ya need.....

http://forums.iboats.com/showthread.php?t=251571

 

[Fishermark]

Re: 120 merc hitting on 2 cyl?

For your rocker arms, basically you tighten the nut down till all the slack is taken out between the arm and the push rod. Then you tighten it an additional 3/4 turn. You need to do this while the lifters are at their lowest - in other words, have the piston at top dead center on the compression stroke. Do this for each cylinder.

It can be difficult to determine exactly when the slack is removed. Go slowly and turn the push rod between your fingers as you do it. You will get a "feel" for it pretty soon.

 

[Fishermark]

Re: 120 merc hitting on 2 cyl?

One more note... hopefully you marked each rocker arm and push rod and lifter. They all need to go back to the same piece. In other words, it's not good to switch them around as they have individual wear patterns.

 

[imastevesmith]

Re: 120 merc hitting on 2 cyl?

Trying to put motor back together and in my gasket kit is 8 little rubber "o-ring's" and 4 little rubber "washers". Could someone please tell me where these go? Never seen any of these when I took it apart so I don't know if they were stuck someplace or if they're for another motor.

 

[imastevesmith]

Re: 120 merc hitting on 2 cyl?

I know I already tried this question but no one replied and I'm hoping to find someone who knows. I really want to put this engine back together and see if it actually works. My head gasket kit came with two differents sets of little tiny rubber washers and rings. Where do I put these?

 

[Fishermark]

Re: 120 merc hitting on 2 cyl?

Sounds like you are describing valve stem seals.

 

[imastevesmith]

Re: 120 merc hitting on 2 cyl?

got head put back on and it sounds like rocker arms are "chattering" quite a bit. Turned over motor till valves on 1 were closed, tightened till lash was gone and went 3/4 of a turn more, then repeated on 3,4, & 2. Sounds good otherwise so how do I adjust to quiet them.