Compression Test - Why is this necessary?

[kbranden]

Just curious, lots of 'how to's', but not alot of 'why's'. One of the steps in the sticky within 'Compression by JB' says this..

4. Rig a jumper to engage starter solenoid without turning ignition on. Pull the emergency kill switch on rope start engines.
5. Screw guage into #1.
6. Turn engine over until reading on guage stops increasing or 5 times, whichever is more.


Why do you do number 4?

And 6, does that mean turn engine over via the starter?

Thanks.

 

[jay_merrill]

Re: Compression Test - Why is this necessary?

You do #4 so the engine doesn't start. You can accomplish the same thing by pulling all of the spark plug wires and making sure that they are far enough away from the plugs to prevent arcing to them



???

 

[BigB9000]

Re: Compression Test - Why is this necessary?

Compression gives you a good idea of the inner working of the engine.
Lets say you have a 2 cylinder engine, one cylinder is putting out 100 psi and the other is 20 psi.

You know there is a problem internally.

Lets say you have both cylinders at 100 psi.
Its probably fine, and now you can start dumping money into it to get it running.

Another test would be a leak down. This pumps a cylinder full of air, and see what leaks out.

 

[5150abf]

Re: Compression Test - Why is this necessary?

If you hit the starter without firing the ignition you don't have to do anything with the spark plugs, been told it is bad to not ground the plugs when they are out of the engine.

Getting acomprssion reading is kind of like taking your temperature, it gives you a general idea about the health of the engine, I check mine once of twice a year.

 

[lindy46]

Re: Compression Test - Why is this necessary?

Or you can remove the plugs, leave the boots on and ground the body of the plugs on the engine block. Then fire away without causing any damage. Just make sure there is no gas in the cylinders, as those firing plugs will ignite it with explosive results.

 

[1946Zephyr]

Re: Compression Test - Why is this necessary?

I used my lawn mower as a flame thower once. The cylinder filled up with gas, so I pulled the spark plug out and yanked on the rope. Well, the spark plug lead arced out on the cylinder head, therefore making a nice flame shoot out the spark plug hole for about 10 feet. Once I put the fire out, I cracked a cold beer and started laughing my *** off.

 

[F_R]

Re: Compression Test - Why is this necessary?

Long story made short: JB is telling you to disable the ignition system to prevent sparking. Failure to heed can result in several things, including ignition system damage and fire---as already pointed out.

If you want to do it the easiest way, simply ground the plug wires to the engine block. If you work on very many motors, it would pay you to make up some grounding wires with alligator clips on both ends. Stick one end in the boot and ground the other end. Any wire will do, doesn't have to be spark plug wire.

 

[kbranden]

Re: Compression Test - Why is this necessary?

Long story made short: JB is telling you to disable the ignition system to prevent sparking. Failure to heed can result in several things, including ignition system damage and fire---as already pointed out.

If you want to do it the easiest way, simply ground the plug wires to the engine block. If you work on very many motors, it would pay you to make up some grounding wires with alligator clips on both ends. Stick one end in the boot and ground the other end. Any wire will do, doesn't have to be spark plug wire.

Got ya, thanks to all. It's nice to know why things are done along with what needs to be done. For those that are familiar with working on motors these things are probably logical, for those that aren't (such as me), these details are important.

Thanks Again.

 

[Craig-]

Re: Compression Test - Why is this necessary?

You can yank the kill lanyard to kill ignition and crank with key. Easy way to go about it is to run on muffs, yank lanyard to kill motor, this way you know kill switch worked. Leave lanyard out then install gauge and crank with key. Best to do one cylinder at a time, replacing plug as you go. Anti seize grease on plug and gauge threads. Most gauges have a release button to hold the needle at highest reading, be sure to release in between cylinders. A more accurate diagnosis can be made by watching gauge rise to see how fast pressure builds. Cranking with a remote switch at motor makes this a one person job, unless you have really long arms or a close by key-switch.

 

[F_R]

Re: Compression Test - Why is this necessary?

Did somebody say he even has a lanyard???

 

[Craig-]

Re: Compression Test - Why is this necessary?

Did somebody say he even has a lanyard???

Obviously it won't work if you don't have one, but works fine if you do.
The question wasn't about a specific motor or year.

 

[Tim Frank]

Re: Compression Test - Why is this necessary?

Getting acomprssion reading is kind of like taking your temperature, it gives you a general idea about the health of the engine, I check mine once of twice a year.

Your compression...or your temperature?....

 

[ground_pounder]

Re: Compression Test - Why is this necessary?

Where does leak down compression come into play on a 2 cycle, for instance, turning over by hand you don't really get a reading or feel the edge of the compression within the stroke, (unlike a 4 stroke where a whole stroke is dedicated to compression and pressure relative to valve timing and the amount of cc's compressed, or like kick starting a bike or pulling a lawnmower, generally you can bump up against it) but the motor turning over reads healthy compression numbers, how does this affect performance then? Turbulance in air flow? Idle? Granted things can affect ring seal and compression on a motor otherwise, like engine temp, and how much oil is around the rings. With compression being one of the basic necessities for a motor to run, what about having compression at more than starter rpms, but not really at slowly turning the crank or a leaky cylinder, which is nil in rpms compared to idle. Not looking to highjack, just elaborate on what you all are already discussing.

 

[Overkill82k5]

Re: Compression Test - Why is this necessary?

If you have a lot of blowby (slow building compression, bad leakdown, etc.) your motor will be down on power. The pressure that is supposed to push the piston down and turn the crankshaft, resulting in motion, escapes around the piston and is basically wasted. Also, lower RPM operation could be compromised if the blowby is bad enough, resulting in crappy or erratic idle but ok running at higher RPM's.

 

[kenmyfam]

Re: Compression Test - Why is this necessary?

The air and fuel mixture fed into each cylinder is ignited by a spark from the spark plug. The compression is vital for giving a good explosion when the spark ignites it. In a nutshell if the compression is very low it will ignite with a little "Poof" but if the compression is high it will ignite with a "Boom". Moving on from that the timing of when the spark ignites the fuel/ air mixture is also critical but that is another chapter.

 

[ElwoodsDad]

Re: Compression Test - Why is this necessary?

I hear all of the conversation and none seem to hit my problem. I have a 'Evinrude 120hp. My compression is 132, 115, 114 and 115. I was told that the unbalance will eventually cause problems. I dont run the engine hard 3/4 throttle max. Can someone explain the problems cause by unbalance psi? thanks!

 

[kenmyfam]

Re: Compression Test - Why is this necessary?

I hear all of the conversation and none seem to hit my problem. I have a 'Evinrude 120hp. My compression is 132, 115, 114 and 115. I was told that the unbalance will eventually cause problems. I dont run the engine hard 3/4 throttle max. Can someone explain the problems cause by unbalance psi? thanks!

Partly for the reasons stated in the post above yours.
Best to start a new thread of your own on your particular issues.
Welcome to iboats.

 

[ElwoodsDad]

Re: Compression Test - Why is this necessary?

Will do! thanks

 

[Craig-]

Re: Compression Test - Why is this necessary?

Where does leak down compression come into play on a 2 cycle, for instance, turning over by hand you don't really get a reading or feel the edge of the compression within the stroke, (unlike a 4 stroke where a whole stroke is dedicated to compression and pressure relative to valve timing and the amount of cc's compressed, or like kick starting a bike or pulling a lawnmower, generally you can bump up against it) but the motor turning over reads healthy compression numbers, how does this affect performance then? Turbulance in air flow? Idle? Granted things can affect ring seal and compression on a motor otherwise, like engine temp, and how much oil is around the rings. With compression being one of the basic necessities for a motor to run, what about having compression at more than starter rpms, but not really at slowly turning the crank or a leaky cylinder, which is nil in rpms compared to idle. Not looking to highjack, just elaborate on what you all are already discussing.

2 strokes have an power efficiency advantage over 4 strokes because 4 strokes waste a stroke pushing exhaust out the valves at the top of the cylinder. 2 strokes put every stroke to work making power. The intake charge enters the crankcase when negative pressure is created by the upstroke, providing lubrication before transferring to the cylinder through ports after reeds(intake valves to the crankcase) close. 2 strokes also have a supercharging effect designed into the intake charge. Some of the intake charge actually escapes through the exhaust port during port crossover. This escaped charge is slammed back into the cylinder by an exhaust pressure wave after the transfer ports close, creating the supercharge effect by filling the combustion chamber with more fuel/air charge that can be achieved with atmospheric pressure alone.

Not sure if that answers your question, but that's basically how a standard 2 cycle motor works. There are other 2 cycle motors that have an oil filled crankcase, cylinder intake valves and exhaust ports. There is also a diesel fuel 2 stroke version of this that detonates with compression, a super efficient design.

 

[ground_pounder]

Re: Compression Test - Why is this necessary?

That gives me a much better idea of how that works, and about the reeds and the supercharger effect like you said. That makes more sense to me why compression is so little til you start cranking, and the reeds keeping it all from spitting back out the carburator. If you had a bad reed for some reason, it would be spitting as much fuel out the carb as it sucks in. I imagine compression would be lower typically if leakdown and blow by was taking place, and with the cylinders standing alone on different crank timing, why the health and compression numbers of the cylinders have to be close for balance and power curve