Compression test How-To's?

[DavesInvader]

Hi folks,
Any tips on exactly how to do a compression test on a '58 Lark? I checked the FAQ and couldn't find it. So do you just take a plug out, put in the tester and give it a few cranks? With or without fuel hooked up? Do you need to use the electric starter to give it a good go or is a few pulls on the starter cord enough? Sorry if this is elementary. Thanks!
Dave

 

[jonesg]

Re: Compression test How-To's?

remove all plugs ( if theres more than 1), don't connect gas can, but don't crank it for 30 minutes either, just get the readings.

you'll probably get higher readings using the elec start if the battery is good.
a 2-3 second crank is usually enough, you'll see the guage stop increasing and thats it.

watch a video
http://www.youtube.com/watch?v=SgavzDPDAy0

 

[MikDee]

Re: Compression test How-To's?

Just a couple of things to add here, open your throttle all the way, and ground out your spark plugs (or remove the lanyard) before doing the test.

 

[JB]

Re: Compression test How-To's?

Okay. Here is how to do it:

1. Fully charge battery.
2. Start engine and warm her up.
3. Remove all spark plugs and disconnect fuel line.
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. Open throttle all the way. Turn engine over until reading on guage stops increasing or 5 times, whichever is more.
7. Repeat #5 & #6 for all cylinders.

All psi readings should be within a 10-15% span.

* Don't get psi hysteria. Remember this is a 2 stroke engine and you are an amatuer at compression testing.
* Rope start engines will usually give lower psi readings than your know-it-all neighbor claims are required. Same for big JohnnyRude V6s

 

[mostlysnow]

Re: Compression test How-To's?

Just a couple of things to add here, open your throttle all the way, and ground out your spark plugs (or remove the lanyard) before doing the test.

I am reading this because I am trying to learn how to compression test my 72 Evinrude 25 horse....
what do you do in a case where the engine wont turn over while in gear, there is a plastic thingy just under flywheel that keeps wheel from turning while in gear and the only way I can get my throttle full is by putting in gear? Any helpful hints?

 

[MikDee]

Re: Compression test How-To's?

I am reading this because I am trying to learn how to compression test my 72 Evinrude 25 horse....
what do you do in a case where the engine wont turn over while in gear, there is a plastic thingy just under flywheel that keeps wheel from turning while in gear and the only way I can get my throttle full is by putting in gear? Any helpful hints?

Apparently there are some people here that don't feel it matters to open the throttle all the way, so then just keep it in neutral, and set the throttle to the fast idle position. The compression reading should be accurate enough.

 

[crxess]

Re: Compression test How-To's?

Opening throttle is simply to allow free air flow through intake.
Anything 1/4 or beyond should do fine.

Remember you are not turning anywhere near WOT RPM's

 

[tashasdaddy]

Re: Compression test How-To's?

i never open the throttle for the test, it will be accurate enough to know if you have problems.

 

[aclmvh]

Re: Compression test How-To's?

I would think the wide open throttle is to pump more oil mix into the cylinders to protect them as they grind during the test....and to provide more of a wet compression test. I split the difference and just give it a little high idle throttle when compression testing.

mvh
Jax/Fl

 

[krogie]

Re: Compression test How-To's?

if you feel that the carb should be opened up fully you could do this manually while at the motor and jump the solenoid to get the engine to turn while inturn read the gauge. but i allways do it with the carb closed , like said before the engine is not turning at wot.

 

[alangf]

Re: Compression test How-To's?

Hi folks,
Any tips on exactly how to do a compression test on a '58 Lark? I checked the FAQ and couldn't find it. So do you just take a plug out, put in the tester and give it a few cranks? With or without fuel hooked up? Do you need to use the electric starter to give it a good go or is a few pulls on the starter cord enough? Sorry if this is elementary. Thanks!
Dave

There is a lot of variety in the technique people use to do compression tests. The main thing is to do exactly the same thing for each of the cylinders. This is important because the most meaningful information you are looking for is the variation between the cylinders and not so much the number itself.

 

[Pain'n'da'boat]

Re: Compression test How-To's?

Should you run muffs or in can to have water on the impeller while testing for compression??? I was told to NEVER crank without somekind of water going through the engine, is this true for compression testing too?

 

[crxess]

Re: Compression test How-To's?

Should you run muffs or in can to have water on the impeller while testing for compression??? I was told to NEVER crank without somekind of water going through the engine, is this true for compression testing too?

Never RUN without water. No damage will be done turning the motor over for a compression test. The motor is not spinning fast enough to generate damaging heat.

 

[Vic.S]

Re: Compression test How-To's?

There are those who even say dont crank the engine dry.
BUT if you have warmed the engine up as JB says you will have been running in water or on muffs so there will be enough water around still to lubricate the pump impeller during a compression test but you can easily leave it in the water or on the muffs while doing the test.

 

[tashasdaddy]

Re: Compression test How-To's?

read this: http://forums.iboats.com/showthread.php?t=324830

 

[Phoenixfirewalker]

Re: Compression test How-To's?

where do I get a compression testing thingy?
what does it look like?
how much does it cost?
what do you eat it with?

thanks in advance

 

[Pain'n'da'boat]

Re: Compression test How-To's?

where do I get a compression testing thingy?
what does it look like?
how much does it cost?
what do you eat it with?

thanks in advance

You should be able to pick one up at any automotive chain...I got mine from AutoZone...I use it on car engines but it worked just fine on the boat motor (had 116-117 psi across all cylinders).
It looks like a pressure guage connected to some air line that has a screw adapter that screws into your spark plug hole. (make sure it is short enough so the top of the cylinder can't hit it-mine has 2 different adapters)
Ohh and I eat buffalo hot wings dipped in ranch with mine chased with an ice cold Shiner-

Thanks for the replies, I love iBoats!

 

[hoosier]

Re: Compression test How-To's?

Why is it nessesary to remove all plugs instead of one at a time? Not trying to second guess anyone Im just trying to learn.

 

[Home Cookin']

Re: Compression test How-To's?

cost about $25

 

[MikDee]

Re: Compression test How-To's?

Why is it nessesary to remove all plugs instead of one at a time? Not trying to second guess anyone Im just trying to learn.

This way your engine is freewheeling, not laboring, it gives the best reading, and is easier on the battery, & starter.