low compression on all 3 cylinders in 84 yamaha 40 hp

[woopigsooie24]

I've been dealing with overheat problems on my 84 yamaha 40 hp for a while now. I replaced the water pump and thermostat and after taking it out yesterday the problem seemed better but was still there. Also, ever since I got the boat (2 years ago) the motor would start missing/cutting out at about 2/3 throttle. Yesterday when we were out we tried giving it a little bit of choke when it started missing and that made it stop but I couldn't really rev it any higher. I'm now thinking it's been running lean and that's what's causing it to overheat.

I did a compression test (cold) and got about 60-70 psi on all three cylinders. I poured almost a cap full of oil into each cylinder then got 90, 100, 110 psi from top to bottom cylinder. I'm guessing this means either the rings are shot or the cylinder is scored or both. If the cause of the miss at high revs is from running lean then I'm a bit worried about the cylinder as the PO told me that it had done that for two years and he couldn't figure out what it was.

BTW, when I replaced the thermostat the other day it was the first time those bolts had ever been removed. All the powerhead bolts still have the factory paint on them.

 

[Toddboat]

Re: low compression on all 3 cylinders in 84 yamaha 40 hp

First, to do a compression test properly, the motor must be heated up first. Considering you got reasonably high numbers (for a 40 horse) while cold, you probably don't have a problem with compression. I only have 110-115 on my 50hp Yamaha and it's from 2003. I don't think the numbers have to be much higher than 100psi for the motor to run strong. Run the test again with the motor heated up and see what you get then. Of more concern is that the difference between 2 of the cylinders is more than 10%. This may be attributed to the way you're doing the test. If you're only turning the key once, you may not get the full reading. Heat the motor up, remove all of the spark plugs. Zero the compression tester and put it in one of the cylinders and turn the key a few times. After turning the key about 3 times, whatever reading is on the tester is not likely to be able to be pumped up any higher with more turns of the key. Do this same thing for the other cylinders.
If there is still a considerable difference between two of the cylinders, you can remove the exhaust cover on the side of the block and see into the cylinders. Look at the condition of the cylinder walls and piston rings. I don't think you'll even have to do this, but you may want to do it anyway because it segways into your other issue... the water line.
Before discussing that, you should know that many people swear by decarbonizing their motor every season (every 50-60 hours in fact). carbon buildup in the motor can contribute to a loss of compression. You can do this by running Seafoam through the motor for 10-15 minutes. There are some good posts on this forum about how to use Seafoam, and also videos on youtube of people doing it. It's an inexpensive products that people swear works wonders to decarbonize the inner workings of the motor. It may also clean out your carbs, although there's no substitute for disassembling the carbs and cleaning and soaking them manually.
Back to your overheating issue... the water line and/or cooling jackets may be caked up with salt and other debris. Changing the impeller is only part of the cooling system. You have to scrape the gunk out of the water jackets and clean or replace the water tube.
Overheating can also be a symptom of low oil. Make sure your oil injection system is working. If you don't have oil injection, then you should be running the motor on premix. Check the manual for the recommended amount. It's usually 50 parts fuel to 1 part TCW-3 oil.
As for the motor cutting out, this is far more common at lower RPM's because carbs have a low speed circuit that is easily clogged. At 2/3rds throttle, the butterfly plates in the carbs should already be open enough that you're running off of the high speed jet in the carbs, not off of the low speed jet. However, as it still sounds like a fuel starvation issue, you should clean the carbs and also make sure all of the butterfly plates open and close in sync. Try priming the bulb when the motor starts cutting out. If it helps, it could be a fuel pump issue. Priming the bulb makes you become your own fuel pump, so it is a good way to diagnose fuel pump issues. If the fuel pump is the issue, you can either replace the membrane/s in it, or better yet, just replace the whole thing.
While cleaning the carbs, replace any iffy looking gaskets. A torn gasket between the carbs and the intake manifold can let in excess air and cause a lean condition.

 

[woopigsooie24]

Re: low compression on all 3 cylinders in 84 yamaha 40 hp

Toddboat, thank you for such a lengthy and informative post. I've heard of Seafoam before but never thought to use it and had no idea it could increase compression. I will do a compression test with the engine warm this weekend (pre and post Seafoam) and report back with the results. Here were my results with the first test. I used a tester I had and got 75, 70, and 80 psi on a cold engine. Thinking I might have a faulty tester I borrowed one from autozone and got 65-70 psi on all three cylinders. Then dumped the oil in and got 90 100 110. With my tester the results were about 10 psi under that.

The motor is oil injected but I don't know how to test whether it's working. I see that many people bypass it so I'm assuming they can stop working?

We've tried squeezing the primer bulb when underway and it's almost firm but not completely. It doesn't seem to make a difference. Although I just noticed a couple days ago that squezing it hard will make fuel come out the top of the fuel filter housing so I need to get a new gasket for that.

 

[Toddboat]

Re: low compression on all 3 cylinders in 84 yamaha 40 hp

Seafoam can increase or decrease compression, but it's good to decarbonize the motor either way. Carbon is the enemy of 2 strokes. The carbon can actually coat the walls of the cylinders, closing the tiny gap between the rings and the cylinder walls. When the carbon burns off, the compression can drop a bit because the seal is not as tight. However, in the case of a motor with less than perfect compression, it can actually increase it. This is because the carbon can also get behind the rings and make them stick to the piston instead of having them move around freely. Yahama has a product called ringfree which is good for the prevention of carbon buildup. Not a bad idea for maintenance.
Do the compression test with the motor warm, both with and without adding oil in the cylinders before using Seafoam. I think the numbers will improve to the point that you will find that compression loss is not your issue.
People do bypass the oil injection system, but in general it is a bad idea. At least for my motor, a 2003 50hp, Yamaha recommends starting it on a 50:1 premix until all of the air gaps have left the oil lines. Check your oil lines for air gaps.
If any exist, you might want to run on premix until they go away. If they go away, you know the oil injection system is working.
If priming the bulb did nothing to improve performance, then your fuel pump is probably not the culprit.
I think it's in the carbs.

 

[woopigsooie24]

Re: low compression on all 3 cylinders in 84 yamaha 40 hp

Here's my results after doing a test with the motor warm:
t m b
before oil -- 80 80 89
after oil -- 110 105 100
after 15 min of seafoam -- 82 82 91

notice the bottom was the strongest initially but saw the least increase from the oil. It seems like it must be the rings, but also seems strange that all cylinders would be so close to each other.

Btw the motor ran fine for over 15 minutes without getting much more than warm. When it first started the stream was strong, then it got weak to the point it was almost just going straight down but still not very hot, then for the rest (95%) was about a medium stream and felt cooler than bath water. It seems to do this cycle everytime I run it on the hose.

 

[woopigsooie24]

Re: low compression on all 3 cylinders in 84 yamaha 40 hp

I hope to pull the head sometime this week when I get the chance. What should I look for in the pistons to see if there's an issue with the rings? Also I've read about people taking the head off to clean the water passageways to fix overheat issues but those seem to all be from salt use. Mine's never seen saltwater, but is that something I should do while I have the head off?

 

[Toddboat]

Re: low compression on all 3 cylinders in 84 yamaha 40 hp

When you remove the exhaust cover, look at the cylinder walls for damage (scoring), and look at the piston rings to see that they move around freely and aren't stuck to the pistons. I don't think you'll see any problem in there.
Once the cover is off, you'll see the water passages. If you've only run in fresh water, odds are they're very clean, but check for any blockages anyway. You'll be able to trace the path water takes from the LU to the pee hole.
You're removing the exhaust cover located on the side of the motor, not the head. You'll be looking at the sides of the pistons, not viewing them from head on. I say this because you said you are going to remove the head. Just making sure you know the distinction.

 

[woopigsooie24]

Re: low compression on all 3 cylinders in 84 yamaha 40 hp

Thanks again Toddboat for your help. I removed the exhaust cover and believe I found the culprit for the overheat issue. I should have known this is what I'd find judging by how the impeller looked when I replaced it. The rings seemed to move freely but it was hard for me to tell. I'm going to pull the head tomorrow to get a better look at the cylinders.

 

[Toddboat]

Re: low compression on all 3 cylinders in 84 yamaha 40 hp

Wow! That was all in the water jackets? No wonder water wasn't flowing through there! Now that you have it open, you can get the salt out of there, too. Too see the rings more clearly, you can turn the flywheel by hand to move the pistons.
It's advisable to disable the ignition system before doing this as a precaution, although I never do that myself.