Is that really the case though? Results vary depending on whether the outboard is a Four-Stroke or a Two-Stroke. Here is everything that you need to know about outboard compression tests.Outboard boat engine - How to do a full Compression Test - FAST \u0026 EASY!
How to do them and what the results mean for you! A compression test for an outboard is a simple test that can be performed on any combustion engine.
It is the very basic test that can provide the tester with the viability of the engine itself. It will prove whether the engine has a good powerhead or not. As in any combustion engine, the power of the engine is made when a piston moves upward in a cylinder. Compressing a fuel and air mixture to a certain point, and then setting it on fire. When the mixture explodes inside of the cylinder, it will push the piston back down.
Driving the pistons of other cylinders up and then doing this in a specific sequence to keep all of the pistons going up and down and creating the power of the engine. What the power is used for depends on what the engine is placed in. In a car, it will turn the transmission and then, in turn, spin the wheels of the car around. For us in the boating world. It will turn the gears in the lower unit, which transforms this motion to the propeller.
That drives the boat forward. 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. Depending on what brand of engine and what kind of engine you are trying to test. Will determine what tools you will need. 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. This is where you will need a couple of specific tools. There are really only two things you will need once you get to this point. You will need a compression tester, and a way to turn the engine over. We have a couple of compression testers that you can get from Amazon on our resources page here.
Or you can always go down to your local auto store, napa, advanced auto, AutoZone, or whatever is in your area. Most compression testers come with several hoses with a variation of fittings on the ends of them that will fit the most common spark plug threads on almost all manufacturers. As long as it fits your engine, you are good to go! Bringing us to the next tool that you will need.
Depending on the brand and manufacturer of the engine, it will determine what you can do at this point. If you are by yourself, you are going to want a remote key switch or a starter button.You want to get a stable reading of the max pressure so you need more than just the one compression stroke. Pressure can be lost through bad valves, bad gaskets or bad piston rings.
So after the first set of tests, if your readings seem low, you squirt a very small amount of oil into the cylinder and retest. If you have one cylinder with a reading much different from all the others or very low readings in general you likely have a problem.
Remove a sparkplug and insert a compression tester. While in neutral crank the engine several times. The compression will build to to lbs. Lower than that will indicate some form of loss.
If your compression leaks down you can add a small amount of oil to the cylinder and retry. If the compression goes up then you may have bad rings. If it is unchanged you have a bad Valve. On some bikes and outboards the cdi box will be damaged if it is pulsed whilst no plug is attached.
So simple remove the power wire from the coil before cranking the engine. It will have no effect on the compression test. If you are to test dry then follow the other procedures. If you are to do a test wet then spray a little lube or oil into the cylinder under test and then apply the tester and crank the engine for a few cycles. That way you are keeping at least one variable constant. You should do the test both dry and wet and possibly do it twice to confirm your results.
All cylinders should show similar readings and any that are considerable lower than the others should be investigated. You typically remove a spark plug and attach a compression tester to the hole. You then turn the engine and take note of the point of highest compression. You repeat this for each cylinder and compare the end results against the specifications provided by the manufacturer.
Firestorm is correct about the open throttle. The other thing is to have the engine warmed up first. Pistons expand when hot and it's common to have readings higher on a hot engine. Since engines are hot when running, that's how you want to test the engine.A compression check can tell a lot about the health of your outboard engine. Here's why and how it's done.
There are four distinct phases in a four-stroke cycle: induction, compression, power, and exhaust. An outboard gas engine needs air, fuel, spark, compression, and unobstructed exhaust — in the right amounts and at the right time. If the piston and rings can't compress the air-fuel mixture in the cylinder so the spark can ignite it, the engine won't run well, if at all. In addition, if the rings aren't sealing properly against the cylinder walls, they're not doing another job of transferring the heat from the pistons to the cylinder walls.
Everything You Need To Know About Outboard Compression Tests
Without that transfer, the piston would grow in size from the heat and "stick" inside the cylinder, causing damage. A compression check can indicate that the piston rings are working properly and that the cylinders are in good condition.
The test measures how much pressure is built up by the motion of the piston inside the cylinder, given in pounds per square inch PSI. Even if you're not a trained technician, you might be mechanically inclined enough to do this diagnostic test. And even if you'd rather not attempt this yourself, understanding how the test works will let you evaluate what a mechanic tells you. There are other diagnostic tests, such as a leak-down test, in which you measure the rate of air pressure leaking from a cylinder while the piston is in top-dead-center position.
This also can help you determine whether an expensive teardown is needed. In principle, the compression test is simple, but the many different types of outboards can add numerous important complexities. The following steps are only general guidance.
Your engine may require different steps depending on factors such as whether it's a two- or four-stroke, has fuel injection, has computers onboard, is hand cranked or has a starter motor, and how its ignition can be disabled. These variations and other issues mentioned below will bear on your ability to do this yourself.
Pros tend to use good compression gauges from high-end companies like Snap-On and Mac. But you can also buy a reasonably good compression gauge at almost any local auto-parts store.
It should work fine for occasional use. Get the right adapters to fit the spark plug holes in your engine.
There are advocates for checking compression on a cold engine, and those that advise doing it only on a warm engine.Its hard to find good data supporting what an appropriate measurement of cylinder compression should be in older outboard boat motors.
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. As an Amazon Associate and member of the eBay Partner Network I earn commissions from qualifying purchases using the links below at no additional cost to you. 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. This source says modern two-stroke outboards should be in the 90 to psi range. However if you watch the end my video below you will see that my 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. Four cycle outboard motors produce much higher compression than two cycles. Older two cycle outboards, especially low horsepower, can run fine around 60 psi however above 90 psi is optimal. The main thing to check is the difference between cylinder compression. Two-Cycle Outboards. Four-Cycle Outboards. Tags: 2 stroke4 strokeantiquecompressioncylinderevinrudefour strokejohnsonmotoroldoutboardrepairtwo stroke.
Those 3 things, in order of importance, are compression, spark, and fuel. After those 3, most everything else comes down to the lower unit. Compression is created within the cylinder when the piston moves forward and back. During the stroke, the piston pushes air toward the cylinder head, which creates pressure. This is measured in psi, or pounds per square inch, and we refer to it as the compression reading.
On a 4 stroke engine, most mechanics and dealers will perform a leak down test, to see how much pressure escapes over a set period of time. On a 2 stroke engine, a compression check is sufficient. Yes - If you have a set of battery cables and a battery, you can ground the negative lead to the block and use the positive red lead to jump directly to the starter post.
This will jump the starter and spin the flywheel. This question is almost too involved to answer in simple form. Johnson and Evinrude made a Looper, or 90 degree engine starting in that would generally read psi on the cylinders, which was fine.
Mercury inline 3 and 4 cylinder engines run around when broken in. The older Mercury engines would usually be between and Pull start engines will run anywhere from Like I said, this is a very involved question. Most mechanics say yes. Good judgment should be applied here. Skip to main content. Steps to performing a compression check: Obtain a compression gauge. As earlier stated, you can purchase a compression gauge at most any auto parts store for about 30 dollars, give or take.
Remove all spark plugs. Ensure that your engine is in neutral. Ensure that your battery is fully charged. If your battery is weak, your starter turns over more slowly, which will cause a low reading. Screw the end of the compression gauge into one cylinder. Go to the key and turn the engine over for 3 to 5 seconds. What matters is that you turn it over for a consistent time on each of the cylinders. Repeat steps 5 and 6 on each of the other cylinders. Removing the head will answer that question pretty quickly.
Worn rings: This will usually cause low compression across the board. Rings will wear after enough hours.Login or Sign Up. Compression Test Posts Latest Activity. Page of 2. Filtered by:. Previous 1 2 template Next. Compression TestPM. In a different thread townsend had mentioned a compression test, which had me thinking. If you were to google "how to perform compression test", for every link that provides instruction there are just as many conflicting procedures. Some say ground the plug cap, some don't.
Some say warm engine first, some don't. Some say have neutral throttle lever engaged during test, others don't. It's really amazing how one differs from another. Searching this forums threads, i found a bunch advising to do test but none describing step by step. Perhaps one of you old crusty pros can give us amateurs the end all be all method of performing comp test. And for the sake of you having to type less, let's assume the lanyard is pulled so engine will not start.
Tags: None. Here's some info that I use. I do not ground the plug wires as the ignition is disabled by the pulled safety lanyard. I'm sure you'll get a lot of other recommendations by other members.
Outboard Cylinder Compression
Joined Sep 1, Messages 20, Re: Suzuki 4 Stroke Compression Test compression test on that engine is a teat on a boar hog. Joined Jun 3, Messages Re: Suzuki 4 Stroke Compression Test see why I say a compression test on small 4 strokes is a teat on a boar hog?
I found a Suzuki mechanic yesterday, who showed me the shop manual. Warm up engine. Remove throttle linkage and place the throttle body leaver in full open position. Specks: psi to psi with a 14 psi delta maximum between all cylinders.
Re: Suzuki 4 Stroke Compression Test dunno I dont run compression tests on 4 stroke outboards. I only use leak down tests as the accuracy and speed is so much greater. Joined Feb 2, Messages 1, Re: Suzuki 4 Stroke Compression Test We winterize a lot of four strokes and we always do a compression check. Joined Sep 7, Messages 7, Re: Suzuki 4 Stroke Compression Test But like Rod said, if it is easier and faster to do the leak down test and gives more info as to what any problem could be, intake valves, exhaust valves, rings.
Why would you waist time on a compression test unless you do not have the equipment, especially if you see a problem and then have to do the leak down test anyway. Re: Suzuki 4 Stroke Compression Test i need to post a pic of the dry stack area and of maybe 4 strokes we deal with. Re: Suzuki 4 Stroke Compression Test I have the forklift driver move the hull on a rack near the shop.
Re: Suzuki 4 Stroke Compression Test I do not need to practice diagnosing an 4 strk engine with a leak down tester. Re: Suzuki 4 Stroke Compression Test no its not crap. I have options, option 1 takes two people one cranking one in the back moving the compression tester, option two unplug the ign harness and use my boat in a box and monkey around with it rissking possible connector pin damage.
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