Compression readings after rebuild

[Slacker411]

OK so I have a mercury v1500 v6 outboard. I think the year is 79. Anyhow before I blew cylinder 1 all cylinders compression tested at about 90 psi. I believe the cause of the engine blowing to be the fact that there were no thermostats installed which I wasn't aware of.. And my main card Jets were a smaller size for higher altitude and I live at sea level so this caused the motor to run lean also. So after rebuild cylinders 4 and 6 test at about 110 psi and the others are all at 90. Should I be concerned about this? The boat seems to run well.

 

[Slacker411]

Carb*

 

[Slacker411]

I do not want to tear into this thing again if I don't have too.. The wife will kill me if I keep feeding this thing my money.

 

[Slacker411]

Darn auto correct on my phone.. To*

 

[racerone]

Rebuild means what ?--------Did you install new pistons / rings or used parts ?---------O/S new pistons in bored out cylinders ?----------Did you resurface the warped heads ?

 

[Slacker411]

I had the problem cylinder bored and the other cylinders honed. Cylinder 1 is bored to .30 and the rest are .15. I also had the heads re decked. They were perfectly flat after machining. I replaced a total of 3 pistons including the one that failed obviously but 2 others had some scuffing that I could catch a nail across. New rings on all 6 pistons.

 

[Slacker411]

The pistons are all oversized to the correct bore

 

[Slacker411]

Now that I think of cylinders 4 and 6 were the 2 port pistons that are brand new that have 110 psi..cylinder 1 also has a new piston but had 90psi also. Again all pistons have new rings

 

[Slacker411]

I am just curious if the variation in compression is going to spell trouble down the road.. I know it's probably sage to assume it could.. But the boat seems to run nicely now.

 

[tommarvin]

I think it's 10% or 10 psi all should be within that range. I would use 100% synthetic two stroke oil, and 100% synthetic marine gear lube in LU, and go have some fun. Don't worry about it.

Did you put the sea level carb jets back in, and the correct t-stats ? 50-1 fuel mixture ? If it's oil injected, make sure the filter is clean.

 

[ondarvr]

Differences in compression can cause poor idling and other issues, or no problems at all. Just run it and see what it does, it won't self destruct from the imbalance unless there were real issues with the lower compression cylinders to start with.

 

[Slacker411]

I did get the new Jets and installed them.. Awaiting tstats in the mail

 

[Faztbullet]

Did you mike the cylinders as the Gen 1 blocks wear at a taper, this sound like the bores as past the wear limit (.003)

 

[Slacker411]

The guy who did the machine work used a micrometer. He would have told me if any other cylinders needed to be bored further OS. Although the compression has a relatively low reading for the cylinders at 90..they tested that low even before I had cylinder failure and the boat was pretty damn fast. I have a feeling the cylinder failed due to running lean and hot in the cylinder combined with colder water circulating outside the cylinders in the water jacket due to no thermostats. I am waiting to take it out for test drive under load and to run it for more than 2 minutes once I have the thermostats in which should be tomorrow. I will just have to keep my fingers crossed and not go miles out into the gulf.. This is an old motor and some would have questioned the 700ish I spent on rebuilding as best I could rather than invest in a newer outboard. The plan is to buy a new boat in about a year anyhow when the wife is done with her masters.

 

[Texasmark]

Why don't you seat your rings and then rethink where you are. For one thing the hone marks need to be rubbed off. Besides, the engine is telling you how it feels. If it feels fine, fine something else to worry about. Grin!

 

[Faztbullet]

If he didn't know what wear limit was all he could have checked for was out of round. Since the bored cylinder has higher compression I would suspect it needed more than 1 cylinder bored,, If it was honed with a rigid hone it would have shown imperfections after first few stokes and if it was honed with a ball hone these will follow the cylinder out of round or taper and cylinder will look great and still be bad.Also how much run time does engine have???

 

[Slacker411]

To be honest I am not sure how he honed the cylinders they came back very smooth and shiny. I could not see any obvious honing marks. This guy was referred to me by our largest boat shop in town. They send all their machine work to him and I drove 2 hours to get it to him. I am assuming he knew what he was doing but we all know what happens when u assume. I have had the boat almost 8 years taking it out 1-3 times a week to fish plus some days of heavy use towing skiers etc. It is a 1979 motor so I am sure it has quite a few hours on it. By the way the one cylinder that was re bored is at 90 psi.. The two at about 110 are new pistons but the cylinders didn't need to be re bored. Having not done any machine work myself I am a little confused by the last 2 responses.

 

[Slacker411]

I do know he used a micrometer and he cleaned up my cylinders as best he could without re boring all of them..which was not in the cards financially.

 

[Slacker411]

Also what is meant by re seating the rings? I assume you mean tearing it back apart..hoping to avoid that. I will have to see how it runs on the water first. Like I said I only care if it lasts another year or so.

 

[racerone]

Seating of piston rings is done as the motor runs / wears / breaks in as they say.-----If there are imperfections in the cylinders then piston rings may not touch the cylinder wall at all.-----Just took a 3 cylinder motor apart that had been bored 0.030" O/S.-----Wear marks in the cylinder indicate that rings never touched cylinder walls in some areas.----Not sure how that could happen and block is going away for scrap !