'84 Merc. 150 - Low Compression - Now What?

[MAtkins]

I've got a Mercury Black Max 150 XR2 that runs.
I had trouble getting it up to speed on the water but once I did it ran over 50 mph.
It ran that fast just last week.
I started it again today with little difficulty.

It doesn't like to idle and has a lot of trouble getting up to plane so I asked & researched here.
When I try to kick it into gear it'd rather stall. I have to baby it to get it to take off.
The resounding response was - engine trouble - need to check compression.

I did and got the bad news I expected.
Here are the compression ratings I got for each cylinder.
Looking from the prop:
TOP
105 - 80
89 - 97
105 - 90

So, no zero compression cylinders but it looks like 4 of 6 cylinders aren't right.
What would your first guess as to my problem be?
I'm thinking - most likely worn rings?

Here are the plugs:
They're NKG BU8H


I don't want to turn this into any more of an 'over-haul' than I have to.
Budget / Money is a major factor.
I've got about $1K into it now and I figure it's worth $2K to $4K if it's running right.
If I can say 'rebuilt' it should be worth closer to 4K.

I hate to trash the motor because of something like worn rings.
So, what should I do now?

 

[Iceman66]

Re: '84 Merc. 150 - Low Compression - Now What?

First thing to do is taking heads off and inspect cyl. walls, in best case honing and new rings will get your engine fine again.
Even 105 psi is low, mine Blackmax has appr. 125 on all cyl.

 

[JB]

Re: '84 Merc. 150 - Low Compression - Now What?

The first thing to do is a thorough decarbon treatment followed by a new compression test. I would not turn a screw or nut until I had verified that there is really a compression problem, not just a bit of carbon. Your max span is only 16psi. That is awfully close to spec.

Don't get paranoid about psi. Gauges, conditions and technique vary greatly. The important value is the span among cylinders.

A tummyache does not justify surgery. It is probably going to be just fine after a decarb treatment.

 

[MAtkins]

Re: '84 Merc. 150 - Low Compression - Now What?

OK, thanks. What is a de-carb treatment? How do I learn how to do it.
I've got a Seloc manual (as mentioned) but I don't see anything about that in it.

 

[JB]

Re: '84 Merc. 150 - Low Compression - Now What?

Here are some good stuff, MA. http://forums.iboats.com/engine-frequently-asked-questions-faq/decarbing-faq-158076.html There are also good instructions on the containers of products. Seafoam is a very popular product, but there are others on the shelf at your marine supplies store.

 

[MAtkins]

Re: '84 Merc. 150 - Low Compression - Now What?

OK, thanks JB: I've done some research here and other places.
I think I've got this:

1. Mix (premix gas/oil +/- 2/3 gallon) + 16oz. Seafoam
2. Run motor gas line from the gas can with the above mix in it:
run 15 minutes - wait one hour - repeat until gas can is JUST empty.
Don't run the mixed gas all the way out of the carbs.
That is, don't run the motor until it runs out of gas.
We want the mixture to sit in the carbs, lines, etc. over night.
3. Trim the motor up to a horizontal position (or as close as you can).
4. Take the plugs out and pour Seafoam directly into the cylinders through the spark plug holes.
5. Manually turn the motor to work the fluid into the cylinders and under the rings (careful not to start it).
6. Leave sit over night or longer (one testimony was over a week and it worked out great for him).
7. Reconnect the gas line to the boat's gas tank.
7. Take the boat out and run it but not at WOT. Keep running at a leisure pace until the smoke clears.
8. Run it flat out at WOT for a bit to blow any remaining carbon out of the motor.

Apparently, I should expect it to (and hope it does) smoke a whole lot as the carbon is expelled from the motor.
Should this work or do you see any problems with this procedure?

 

[JB]

Re: '84 Merc. 150 - Low Compression - Now What?

Sounds like a winner to me, MA.

 

[Chris1956]

Re: '84 Merc. 150 - Low Compression - Now What?

Do the Seafoam treatment and just run the bloody thing. It runs 50MPH, just what would you expect from a 29 year old motor.

 

[Iceman66]

Re: '84 Merc. 150 - Low Compression - Now What?

Any luck with the seafoam treatment?

 

[MAtkins]

Re: '84 Merc. 150 - Low Compression - Now What?

I've done it once so far and no, the compression on the 2 best cylinders actually went down.
CharlieB, in another thread on this suggested a carb clean and then do it again.

So, I've cleaned the carbs and got them back on the boat.
Next I'll do the seafoam treatment again.

I did it just like I described above. I was talking to a friend who's had several boats and he suggested pretty near exactly the same thing, without my prompting or influencing my question. So, I've got 2 strong votes on that procedure now.

I'll try it again, hopefully this weekend and see what happens.
I'll keep ya posted.

 

[mrklean]

Re: '84 Merc. 150 - Low Compression - Now What?

I've done it once so far and no, the compression on the 2 best cylinders actually went down.
CharlieB, in another thread on this suggested a carb clean and then do it again.

So, I've cleaned the carbs and got them back on the boat.
Next I'll do the seafoam treatment again.

I did it just like I described above. I was talking to a friend who's had several boats and he suggested pretty near exactly the same thing, without my prompting or influencing my question. So, I've got 2 strong votes on that procedure now.

I'll try it again, hopefully this weekend and see what happens.
I'll keep ya posted.

Have you tried testing the compression with wet rings? In auto engines that means dripping a few drops in the cylinder to get oil on the rings that will make a better seal and reveal a higher compression number, noting that the rings are actually junk.

I've heard on 2 cycle you can use fogging oil in the same manor. I would try this and see if the numbers change much. If they do then I'd start point a bit more to rings.

Also for the carbon removal treatment. Get the motor running for a while, operating temp, then take the can of sea foam and pour it into a spray bottle (windex bottle or something, it won't hurt the seals in the bottle like gas will so you can even store it in there) and spray the sea foam into the intake directly while the engine is running. Pump a lot of sea foam into the carbs and intake while its running. It's going to bog down or even get close to dying. Try to keep it running until you pump about half the bottle in. Then for the last blast, try to kill the motor by choking it out on sea foam. The soaking of sea foam will soak the carbonand it'll break it down. Then wait ten minutes or more. (I wouldn't let it sit more than a half hour to an hour) And run it varying wide open throttle (i.e. lots of accerlation to full open). Repeat process till the smoke lessens. (According to sea foam the liquid burns pretty much smokeless and the smoke you see is contaminants burning up)