Morkelkhan
11.06.2021 18:04:48
Compression testers can be extremely affordable. We talked a little earlier about getting a compression tester at any auto parts store for sometimes under $50. [links]
See full version: Everything You Need To Know About Outboard Compression Tests
Compression testers can be extremely affordable. We talked a little earlier about getting a compression tester at any auto parts store for sometimes under $50. [links]
What the compression test tells us, is what kind of shape the mechanics of the engine are in. Whether or not the engine is creating the proper amount of compression to run properly.
Sometimes this is a simple fix and other times it is catastrophic. So if you do see low compression results, try to warm the engine up if it will run and check again. [links]
Someone might say, the engine won’t run, it was running fine. Forgetting to tell you that they were backing down on a sailfish and the engine was partially submerged underwater! more
You will need a basic set of sockets and a wrench to remove any kind of plastics or covers that protect the ignition coils. You will need your socket sets to remove the coils and expose the spark plugs.
You might be wondering this question, as I’ve had to answer it many times over the years to newer mechanics and boaters. Which is a great thing to me, because it shows a person’s willingness to learn! more
Understanding this chart:
Example: Grn = Green Wht/Grn = White/Green Stripe Pk = Pink
"Wire to wire" means take measurement between those colors of wires on the component
Light green columns are reference readings for measured ohms of the specified component
Light yellow columns are reference readings for measured DVA output of specified component
While it's possible a component can be within specs and still be faulty, any component found not to be within specs should be replaced. [links]
[links]
2. Extract all the spark plugs from the motor and keep them in the order of which they were removed. Examine the plugs, inspect the plugs for inconsistencies in color and for sign of damage and corrosion near the tip. here
Secondary Compression Test here
1. Start by installing the adapter plates over the exhaust ports and the intake ports to provide a complete seal of the powerhead. You must make sure that you do not cover the water jacket holes.
CAUTION: Extracting all the spark plugs, then cranking over the engine may or will lead to an explosive situation if oil and raw fuel should spray out of the spark plug holes. here
9. Obtain a can of fogging oil and spray the perimeter of the cylinder walls and pistons. After the oil has been applied, retest that cylinder.
From my research, cylinder compression values that people say are “acceptable” for low horsepower outboards seem to vary. They not only vary from the reporting individual but from manufacturer to manufacturer. For instance, a fully functional Mercury 25 Hp cylinder compression value may be significantly different than that of a Johnson 25 Hp. Its also rather hard to find where a manufacturer will say the compression range should be from this psi to that psi. What I have found unanimously is that the relative compression values between the cylinders should be within 10%. This source says modern two-stroke outboards should be in the 90 to 105 psi range. However if you watch the end my video below you will see that my 1979 Evinrude 6 Hp cranks easily cold ( 2 pulls) with 60 psi compression This source says the older two stroke outboards below 25 Hp should be between 75 and 85 psi. This source seems to confirm with my understanding that 60 psi in older outboards can sometimes be OK. Bottom line, if I was buying an older two-cycle outboard I would probably want to see compression values above 75 psi ESPECIALLY if I could not see it run.
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Its hard to find good data supporting what an appropriate measurement of cylinder compression should be in older outboard boat motors. I have owned several Johnson / Evinrude outboard motors, a couple of Mercury’s, and at the time of this post one Yamaha four stroke. I have started this post as a repository of my findings regarding cylinder compression and hope to expand it over the years as my habit for tinkering with outboards continues. more
Four cycle outboard motors produce much higher compression than two cycles. However, four cycle engine cylinder compression should still be within 10% cylinder-to-cylinder.
Older two cycle outboards, especially low horsepower, can run fine around 60 psi however above 90 psi is optimal. Four stroke/cycle engines produce much higher compression. The main thing to check is the difference between cylinder compression. Cylinder-to-cylinder compression readings that differ more than 10% reveal trouble.