Compression test results??

S

South of Heaven

New member
Guys,
I finally got the boat on the water last week after ALL the spring commissioning and new engine bits were installed (2 new batteries, thermostat, external and internal anodes, internal and external fuel filters, new water pump kit and probably more things that I'm forgetting).

Anyway, we did a compression test BEFORE I took it on the water and essentially the motor had only been run for like 10 or 15 minutes in the past 6 months or so. I was actually there when the mechanic did the test. He took out the spark plugs and put in the gauge. Then he had me put the gear in Forward and give it full throttle (with the motor off, obviously). Then he disconnected a wire on the right side of the engine and touched something else and the motor turned over a few times. He then repeated the process with the wire for the other 3 cylinders.

The readings which he got were:

1st cyl---225
2nd cyl--225
3rd cyl--225
4th cyl--275

WHY ARE THEY SO HIGH?? Most outboards are usually around 150-175psi, right? Is this a Yamaha F80 thing?

He told me the 4th cylinder was definitely "off" a bit but still "okay" and just within the spec. He advised me to run a LARGE amount of ethanol blocker and a SeaFoam or comparable product for the next few times I use the boat.

He also suggested that the compression should be tested AFTER you run the boat on the water for a few hours. We had tested it essentially "cold" and the numbers may have been better if we checked it after running for a good period of time. Another thing he told me was that we could do a "leak down test". But all in all he gave my motor a satisfactory rating and said that we could work out the "bugs" as the season goes along and I use it more. He said it's hard to tune a motor when you're not on the water and using it and getting/giving feedback to the person working on it. That makes sense and looking at the current Boston weather I won't be on the water for a few weeks. Frustrating...

SO, after I got the compression test I immediately launched the boat. Solo launching and recovery were a breeze! I was happy about that anyways. Lol. My car and trailer performed perfectly. I never stepped in water once. Even the rear wheels of my car were like 2 or 3 feet away from the water.

I started the boat and ran it at the dock for about 10 minutes or so. It was pumping water okay..not the best telltale pressure in neutral but that stream increased once I was underway (just like my old boat).

It sounded great in neutral. Once I put it in reverse and then powered away around 1000 RPM it also sounded great. After I left the no wake zone then I started playing around. I went through all the different RPM ranges and was trying to figure out the trim. I have a Permatrim on the outboard, I don't have trim tabs. She ran great! I was blasting around for about 2 hours or so. I think I did about 8 miles in total. I found a good cruising clip was around 20 knots or so and I think 4300-4400 RPM? Maybe less. I can't remember.

Now the "bad" part, although it's not that bad I hope. Between like 2000-2500 RPM or so the engine kinda sounds like it struggling, like it's not getting fuel or something else. Although it never stalled. I could run it at those RPM's for as long as I wanted to but it didn't sound 100%. But below that it's fine. I was running at between 1000-1500 RPM for a long time in the no wake zone and she was great. The same thing when I went above 2500. She was awesome! I also ran a lot around 3500 that day. That was a good, slower cruise speed too.

I just have to figure out that issue now. The motor is an 2003 Yamaha, 4 stroke, carb'd motor. Both of the fuel tanks were drained last month and fresh fuel was filled up with ethanol block and Yamaha RingFree additive.

Now I just need some Mother Nature to cooperate so I can start using the boat regularly.
 
Compression tests are never about compression being too high. It can be about being too low. However it is about making sure that all cylinders are operating the same relative to each other. By the same, I mean withing 15% or so of each other. One cylinder being higher by even more is not normally a thing to worry about. I think your engine is doing fine in the compression department.
 
potter water":2889ja1s said:
Compression tests are never about compression being too high. It can be about being too low. However it is about making sure that all cylinders are operating the same relative to each other. By the same, I mean withing 15% or so of each other. One cylinder being higher by even more is not normally a thing to worry about. I think your engine is doing fine in the compression department.
Click to expand...

Okay, well that's a good thing. Thanks. The mechanic told me not to worry either. Now I'm more worried about the way the motor runs at 2000 RPM's. I can't wait to run more cleaner and ethanol block through her....
 
I'm not very experienced with outboards/mechanics, so take this with that in mind. But anyway, when I bought my C-Dory with Yamaha F80 I knew it had been sitting for some time. The carbs had been "cleaned" (looking at the receipt, it was via just putting in some cleaner type stuff, hence the quotes). It started up instantly and ran like a top, except....

At about 1500 RPM (might have been slightly higher) there was a hesitation/miss. My guess was that the carbs needed more than just an easy cleaning. IIRC, around that RPM when a secondary carb circuit (probably wrong term) kicks in and you would then expect a problem if there was a carb issue. I bought the boat with a price negotiation for engine work, then had a Yamaha mechanic do the works. The works included completely rebuilding the four carbs. I had lots of other maintenance done too (wanted a baseline), so I can't be SURE it was the carbs, but that would be my guess. It has run beautifully ever since then. When I put the boat away for more than a couple of weeks, I not only run it out of fuel, but I also drain the bowls of the four carbs (that doesn't happen just by running it dry). Takes five minutes or so, which is a small percentage of "putting the boat away."

I forget if you have had the carbs rebuilt - if so then I'm probably off base.
 
During the last years of Yamaha carburetors they really tried to make them lean...to use less fuel and produce less emissions.... to an extreme.

I have twin Yamaha 40 motors...and have always had problems with the carburetors... they would foul up anytime the boat would sit for more than a few months... and so badly that I ended up having to rebuild them three times before I got mad and decided to fix them or break them.....I even looked into replacing them until I realized they have no resale value...

What I did was some research and rejetted them....a bunch... and amazingly it did not affect my fuel mileage but I am sure they will not now get the three star emission rating that they once had.... they were starved for fuel...badly...Now they perform well and seem much happier...

Possibly you can check into changing your mid-range jets and go larger...you will be amazed at what the results will be...sure worked on mine...

Joel
SEA3PO
 
Sunbeam":3ejwhvo5 said:
I'm not very experienced with outboards/mechanics, so take this with that in mind. But anyway, when I bought my C-Dory with Yamaha F80 I knew it had been sitting for some time. The carbs had been "cleaned" (looking at the receipt, it was via just putting in some cleaner type stuff, hence the quotes). It started up instantly and ran like a top, except....

At about 1500 RPM (might have been slightly higher) there was a hesitation/miss. My guess was that the carbs needed more than just an easy cleaning. IIRC, around that RPM when a secondary carb circuit (probably wrong term) kicks in and you would then expect a problem if there was a carb issue. I bought the boat with a price negotiation for engine work, then had a Yamaha mechanic do the works. The works included completely rebuilding the four carbs. I had lots of other maintenance done too (wanted a baseline), so I can't be SURE it was the carbs, but that would be my guess. It has run beautifully ever since then. When I put the boat away for more than a couple of weeks, I not only run it out of fuel, but I also drain the bowls of the four carbs (that doesn't happen just by running it dry). Takes five minutes or so, which is a small percentage of "putting the boat away."

I forget if you have had the carbs rebuilt - if so then I'm probably off base.
Click to expand...

No, I haven't had any carb work yet. I hadn't even run the motor for more than a few minutes until I launched it last week.

I did see the 4 screws for the carb bowls. I may start draining them as well if the boat is gonna sit for a while. It looked like the very bottom screw was kinda a pain to access. What do you use a small handled screwdriver at an angle or do you have to remove the piece in front?
 
SEA3PO":cbv6agm3 said:
During the last years of Yamaha carburetors they really tried to make them lean...to use less fuel and produce less emissions.... to an extreme.

I have twin Yamaha 40 motors...and have always had problems with the carburetors... they would foul up anytime the boat would sit for more than a few months... and so badly that I ended up having to rebuild them three times before I got mad and decided to fix them or break them.....I even looked into replacing them until I realized they have no resale value...

What I did was some research and rejetted them....a bunch... and amazingly it did not affect my fuel mileage but I am sure they will not now get the three star emission rating that they once had.... they were starved for fuel...badly...Now they perform well and seem much happier...

Possibly you can check into changing your mid-range jets and go larger...you will be amazed at what the results will be...sure worked on mine...

Joel
SEA3PO
Click to expand...

Interesting..
 
I had an '02 Yamaha 40 on a RIB. Never had any problem with it. When I put it up for the winter, I just ran the motor without the fuel line attached until it quit.

During the summer it would sit for about a month at a time. I didn't do anything to "store" it.

My '07 Honda BF90D had issues in the mid rpm range. At low speeds and above 4000 rpm it ran fine. In the middle it would constantly surge a few hundred rpm. Turned out to be a bad ECU.
 
I just have to figure out that issue now. The motor is an 2003 Yamaha, 4 stroke, carb'd motor. Both of the fuel tanks were drained last month and fresh fuel was filled up with ethanol block and Yamaha RingFree additive.

Now I just need some Mother Nature to cooperate so I can start using the boat regularly.[/quote]
The readings which he got were:
1st cyl---225
2nd cyl--225
3rd cyl--225
4th cyl--275
WHY ARE THEY SO HIGH?? Most outboards are usually around 150-175psi, right? Is this a Yamaha F80 thing?

Re: South of Heaven's F80 issues quoted above:
Before I retired I worked as a Pow. Eng in a large thermal gen. plant and we had 100s of test gauges for various pressures ranges. They were all periodically tested for accuracy--and some were way off scale. Drop a gauge and it was not used again until it was retested.

S-O-H --These numbers are not believable-to me- and #4 cylinder would have to have a compression ratio of about 23-1 -!! only some diesels reach that ratio. I think its 175psi not 275 psi on #4 cyl too.
Lets start with the 2003 Yamaha compression ratio which is, as I remember, a 9.6-1 ratio which should relate to 150-175 psi when tested. My BF 50 Honda on the other hand has 12.6 -1 ratio and factory cyl. pressure 199-225 PSI. So no way would I believe that mechanic's test results. Get some other shop to test your engine or do it yourself is my advice.
 
BlueBack: I couldn't believe the numbers were so high either. If I didn't see him do it in front of me then I definitely wouldn't have believed him. Lol.
I think my next step is to coordinate with another local shop and test it shortly AFTER I've used the motor for a few hours. Maybe the cold reading didn't help?
 
I expect the values will be decrease after the boat is used for hours then warmed up before compression is checked.\

High compression can be due to carbon build up
 
I Think you better have another mechanic try it again with a different
compression gauge (someone that knows what their doing) because these readings are all too high. I think his compression gauge is defective also
when the test is being done hold the throttle wide open so the engine gets as much air as possible. Your cruising speed vs rpms might be effected
by how much gear in the boat, or if it isn't trimmed out correctly plus if your at 20 knots that is 22.5 mph which might be near/close to the top speed
as you are running an 80 hp engine which is 20 hp under the max for
a c-dory 19. Well anyway change gauges and mechanics first and try
it again.
 
Don't worry about the compression and get your carbs synced . If the mechanic doesn't know what sync and link is , go somewhere else.
The use a vacuum gauge and adjust the carbs to work properly in unison.
Marc
 
Wefings":18vobbzp said:
Don't worry about the compression and get your carbs synced . If the mechanic doesn't know what sync and link is , go somewhere else.
The use a vacuum gauge and adjust the carbs to work properly in unison.
Marc
Click to expand...

Will do, thanks. I'll be dreaming of carbs for the next couple of weeks now. And I'm not talking about bread and pasta! :)
 
Ya know the carbs does not have Anything to do with the compression period. First try another compression gauge and check the results.
If the boat is running OK then drive it around for awhile and see if it
does not get any better. You said the boat/motor has been sitting for a while
maybe the gas has gone kind of bad , check/change your fuel filter
drain your carburetor bowls maybe they have water or dirt in them
are you running a water separator filter ? You should be. If the carbs
have never been messed with then they should be fine . The Dealer would
love for you to bring in your engine and have you drop a thousand
synchronizing those carbs. But solve one problem before you jump to another and you will save a bundle.
 
I'm not saying your engine has this problem, but a lot of times carbon build-up can give really high compression results. When this is the case the engine will often run on when shut off, pings, and generally runs like crap due to pre-ignition. Due to strange numbers, I would run the compression test again with a different gauge - something is goofy.
 
SEA3PO: Just curious, when you had trouble with your Yamaha 40 carbs after storing, were you draining the bowls or just running it dry (which leaves a bit of fuel in the bowls).

South of Heaven: Yes, the bottom screw is a bit less convenient to get to than the top three. I don't remove anything else. I can't remember what I use now, but it was just some small tool that I could fit in there. Nothing specially bought or fabricated. After running the engine out of fuel you basically only need a bit of paper towel to catch the fuel - or at least it's not a large amount on mine.

I didn't have a compression test on my engine (it only had 50 hours and so I focused on getting the maintenance base-lined and the carbs just so). But.... I vaguely remember that one was "supposed" to do a leak-down test if one wanted to know the compression. I think (?).
 
Fish Commander":1gce87ex said:
Ya know the carbs does not have Anything to do with the compression period. First try another compression gauge and check the results.
If the boat is running OK then drive it around for awhile and see if it
does not get any better. You said the boat/motor has been sitting for a while
maybe the gas has gone kind of bad , check/change your fuel filter
drain your carburetor bowls maybe they have water or dirt in them
are you running a water separator filter ? You should be. If the carbs
have never been messed with then they should be fine . The Dealer would
love for you to bring in your engine and have you drop a thousand
synchronizing those carbs. But solve one problem before you jump to another and you will save a bundle.
Click to expand...

Yeah, for sure. I'm not in PANIC mode yet. Yes, both of my fuel tanks were drained and filled with fresh STABILIZED fuel. I do have a fuel/water separator and I changed it a few weeks ago.
 
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