Compression numbers

[TOY BOAT]

I did a search but am not really seeing what I am looking for. So I am sorry if this has been answered many times.
I have a 85 75 hp merc. I want to know in general what is good compression, marginal compression, and poor compression?
What are the real numbers from good to bad?
Mine is a 4 cyl, and it read:
Reading from top down.
1. 120 lbs
2. 120 lbs
3. 115 lbs
4. 105 lbs

Thank you

 

[Maxz695]

Re: Compression numbers

there should be no more than 15 Lbs veriance between cylinders 120 is good 105 is fair to poor but still within the range. Try using some sea foam (Can get it at autozone ) in the gas mixture to decorbonize and it may be a big help in feeing up the rings

 

[JB]

Re: Compression numbers

There are simply too many variables to specify psi, TOY BOAT. Any change in the gauge, temperature, battery or testing technique can change the psi reading.

If, however, all of those things remain the same for all cylinders tested they will all test within 10-15% of each other. That is what we look for. If any cylinder is weak or damaged it will test outside that 10-15% range. Do not get hung up looking for psi numbers. Look for the relationship between cylinders

Your #4 cylinder is barely within the 15% number. I like 10%, so I would declare #4 as needing action. I do not think the problem is serious; most likely carbon causing slight ring sticking. That should clear up with a good decarbonizing treatment.

Though Seafoam is very popular (it must work) there are other products for the same problem. Used according to directions they all work.

 

[TOY BOAT]

Re: Compression numbers

Well
Thanks for the response guys. You answered my question.
I did however do each check with the other three plugs in, and would take out the next one and move it to the last checked cylinder. I did look at the plugs prior, and they all looked the same, and ok. All four checks were done with a two second turn of the key.

 

[Capt Ken]

Re: Compression numbers

Well the first problem was taking the readings with any spark plugs installed. Remove all plugs and check again. Your numbers will improve greatly.

 

[Maxz695]

Re: Compression numbers

Again I would be slightly concerned but it will take some running time for the decarbonizer to do it,s job. It doesn,t sound to me you are the kind of guy that up and downs the throttle constantly jerking the motor out of wack in fact just the opposite. I,m fairly confident you consistanly use a well ablance 50 to 1 ratio as you seem to care deeply for your engine just by inquiring about the issue. No matter how well you treat it it will in time build up carbon. Using the additive to clean the carbon greatly reduces as well as eliminates cabon build up believe it or not. You should use it at least every three tank fulls after clearing it as a future preventitive measure. Miricals won,t come overnight but they hopefully will come if this is done. Wish you the best on your fishing trips and voyages.

 

[JB]

Re: Compression numbers

All plugs should be out and the engine should be turned over at least 5 times or until the reading stops increasing.

See my comment about technique and check "compression testing" in the engine FAQs.

 

[TOY BOAT]

Re: Compression numbers

Well the first problem was taking the readings with any spark plugs installed. Remove all plugs and check again. Your numbers will improve greatly.

Are you saying that with the plugs out of the other three cylinders I am not checking, I will get a higher number on the compression check?
I picked up some seafoam and stabilizer. I plan to run the motor today on muffs to get the chemicals into the carb and beyond. Then while running, I will disconnect the fuel line, and let it run it self dry. I still have 5 gallons in the six gallon tank, so I plan on running it a couple hours.
The plan is to sell the boat since I don't have a vehicle to pull it anymore. The tow rating on my car is 1000lbs, and I think with everything, I am looking at about 1300 lbs. Probably not bad to pull on the road, but I think would be difficult to pull out of the water. So I want to know what is what when I introduce it to a possible buyer. I do not want either one of us to find out there are issues. If there are issues, I want to be up front. If not, I want to be able to say there is no issues.

 

[jbjennings]

Re: Compression numbers

I'm not so optimistic about decarbonizing. I would say your engine has a problem. The #4 cylinder/piston is very likely bad---just my opinion. I personally wouldn't describe it as a motor with no issues.

 

[emckelvy]

Re: Compression numbers

Might have a problem with the crankshaft lower oil seals. This'll usually show up as a problem in the #4 cylinder, due to water ingestion.

But you really can't get an accurate compression check, as mentioned, in only 2 cranks of the key. You should be cranking it over until the needle on your compression gage stops moving.

For most accurate check, the throttle butterflys need to be open fully, you can't compress air if there isn't enough of it.

Simple way to do this, is to have the remote control on full throttle. Make sure the ign is OFF. All (4) spark plugs should be removed.

You'll need to supply +12V to the yellow wire connected to the starter solenoid. An automotive remote starter switch works great for this.

Just watch out for the prop, 'cause she's gonna turn!

When you're doing compression checks, be sure to have your flushing muffs attached and a small supply of water, just enough to lube the impeller. It's not a good idea to turn 'em over dry, even if the motor's not gonna run.

HTH & let us know what you find.........ed

 

[TOY BOAT]

Re: Compression numbers

I am going to rent the compression tool again, but for now I wanted to try some seafoam, and get some stabilizer in it. I am also fighting a carb issue, and a poorly adjusted throttle cable. So this video is it running with the carbs exposed so I could prime them. It will not initially start without doing this right now. Once warmed up, it starts part of the time on it's own. Once running, it will run as long as I keep the high idle about midways. I wanted to get the seafoam in the carb, and hopefully clean out some deposits as suggested above.

Here is a video of it running. It had been running for about an hour when I shot the video.
[URL=http://s66.photobucket.com/albums/h251/IMPALADAKID/?action=view&current=stuffandboatrun024.mp4] [/URL]

 

[TOY BOAT]

Re: Compression numbers

I did not want to leave this thread unattended. I got some sea foam, and ran the motor a couple times at an hour a shot, letting the engine sit for days between start ups finally I tried the compression test again and got much better numbers with the plugs out. Across the board the numbers were at or near 130. I sold the boat yesterday, and it was a big help in knowing what I was selling either way.

Thanks again for the help everyone