How do you check compression?

[Algielong32]

I have a 1984 Johnson 140hp engine and I want to check the compression on all 4 cylinders but do not know how. Is it something easy? Was told that one of my cylinders had 135psi on #1, 125psi on #2, 40psi on #3 & 135psi on #4. The boat runs great. Plugs look okay after heavy use. What are the signs of low compression?

 

[JB]

Re: How do you check compression?

Howdy, Algie.

1. Get a compression tester. Best is the type that screws into a spark plug hole. It is best to warm the engine up before the following, but a cold test is usually reliable.

2. Remove all 4 spark plugs. Screw the tester into top left plug hole.

3. Prepare a jumper to jump from the hot lug on the solenoid to the small lug. DO NOT turn on the ignition. Set the throttle at wide open.

4. Jump the solenoid. The starter will turn. Let it turn at least 5 revs. Record the reading on the guage.

5. Repeat #4 for each cylinder.

All cylinders should read over 100psi and be within a range of 15%.

A very low reading on one cylinder is usually damage. Low reading on 2 or more cylinders is often carbon sticking the rings.

Good luck.

 

[BoatBuoy]

Re: How do you check compression?

In addition, use a fully charged battery. A battery that is partially discharged will turn the engine slower, resulting in lower compression readings. Although, the relative difference is more important between cylinders, you really want good readings.

 

[JUSTINTIME]

Re: How do you check compression?

also make sure u open the carbs are in WOT (wide open throttle) u will get false readings
u want to be within 10%, bu really 5% is less is perfect, 6-10% is OK anything over 10% is check out things like head gasket,rings, pistons etc

if u have a emergency stop switch (ESS aka Kill switch) u can pull that and use your key switch to turn over the motor

using the jumper is not the best thing to do, sparks and gas are not a good combo

the best is go buy a remote starter

invest in a good gauge!

taking plugs out are good, no wear on the starter, but bad u got fuel and oil flying out

leaving plugs in, more wear on the starter, no oil/gas flying out, and u have normal operation conditions with them in

when i test i leave them in

does your motor use them with them in or out8)

 

[i386]

Re: How do you check compression?

Make sure your tester doesn't screw farther into the hole than your spark plugs do. They come with a couple different thread lengths. :$

 

[imported_MattG]

Re: How do you check compression?

I tested mine that im restoring. She hasnt ran since I dont know when. She reads about 70psi +/- 3 in every one. Is this too low?

Thanks Matt

 

[JUSTINTIME]

Re: How do you check compression?

I tested mine that im restoring. She hasnt ran since I dont know when. She reads about 70psi +/- 3 in every one. Is this too low?

Thanks Matt

what year and hp

 

[rickdb1boat]

Re: How do you check compression?

Run some Seafoam through her. And you might want to try a diiferent gauge. BTW, Richland Chambers Lake is nice. Have family that live in Corsicana and I keep my little runabout down there...

 

[imported_MattG]

Re: How do you check compression?

Thanks! Its really nice when the lake is full. Right now we are 11 feet below pool. Everyboat house is out of the water. That doesnt stop me though. lol I have my ways!

Whoops sorry about that its a 1963 Evinrude 75hp Speedifour.

I plan on running some seafoam once I get her fired up. I just rebuild the carbs, changed the impeller, New Points, New Oil. etc etc.

I know this is off topic put what is the correct Oil mixture ratio I cant find it in my manual.


Thanks Matt