Compression test

[smokin_dodge]

so i did a compression test on a 1979 johnson 140 im working on. i was reading around earlier and found that all cylinders should be over 100 psi and no more than 10% variance. well today here are the numbers i got.

in cylinder order from 1 to 4
1. 70psi
2. 80psi
3. 75psi
4. 90-92psi

to get these numbers i took all the spark plugs out and placed the tester in each sparkplug hole from 1 to 4 and turned the motor over 5 times.

since my numbers are so low, what does this mean for the motor. i was already expecting a semi-rebuild, replacing a few parts, rebuilding the carb, etc. but with low compression should i just junk it or will be along the lines of a full rebuild?

 

[tx1961whaler]

Re: Compression test

Does the motor run?
If it runs, do a decarb on it and run the compression test again.
Are you sure the compression gauge is good? Those numbers seem darn low for a engine that actually runs...

 

[smokin_dodge]

Re: Compression test

it wasnt running, it turns, but we havnt actually gotten the motor to fire yet. brand new compression gauge, spend about $35 on it at the local autoparts store

 

[james williams]

Re: Compression test

Quote--`turned the motor over 5 times.``
Did you turn it by hand? If you did,I think you`re supposed to turn it with the starter.

 

[smokin_dodge]

Re: Compression test

turned the motor over with the starter hooked up a new battery to the motor, it turns very slow though, maybe turns a complete360 twice a second, maybe a little slower.

 

[james williams]

Re: Compression test

120 rpm.Pretty slow.Could be leaking down,like worn or stuck rings.

 

[1946Zephyr]

Re: Compression test

Yes. Open the throttle wide open and crank it with the starter about 5 times. I would cycle some fresh fuel mix through the engine with the plugs all removed. If it has sat for any length of time, the rings aren't working the way they should be and you'll get false readings. Ground the ignition system and crank it over with the plugs out. You'll see a mist of fuel mix blowing out the holes. Smaller engines, I lay them on their backs and dump a small amount of gas into the carb and crank it, till the gas goes all the way through. About this time, the rings should be lubed good and ready for a compression test.
Once you get a compression test done, then I would try to get it to fire. See how it runs and go from there.

 

[HybridMX6]

Re: Compression test

On a new battery that seems awefully slow. Also, is there a fuel line hooked up to it? If not, at least spray some oil in the cylinders before turning it over, no lubrication is going to make it worse.
The decarb is a good suggestion, but you will need to get it running first to do it. Different gauges show different #'s. I've always heard as long as they are within range of each other then it should be ok, as long as we're not talking like 10-15psi (total comp. each cylinder, that would obviously show a major problem). I am rebuilding a 3cyl. that was 85, 115, 115, and it still ran, although pretty badly. Carbs were gunked up, so it wouldn't idle at all, but if I held it open it would run, just didn't like it.

 

[smokin_dodge]

Re: Compression test

Ground the ignition system and crank it over with the plugs out.

where should i ground the wires to, should i just attach them to a head bolt?

 

[james williams]

Re: Compression test

I would say yes.

 

[1946Zephyr]

Re: Compression test

agreed. I never did it, because I've worked on enough of these things and never had a problem. but I would always suggest it to others. I usually twist the boots away, so they can't arc out and start nothin'. I love flames shooting out of a spark plug hole.

One time, I had a lawn mower that flooded out and filled the cylinder with gas. This baby was so full, I could not pull the rope. So I removed the spark plug and pulled the rope as hard as I could. The coolest flame thrower I ever had my hands on. Half of my lawn was singed, so I didn't have to mow it. I got the fire out and went on my way to start it up. My neighbors laughed when I told them the story.

 

[emdsapmgr]

Re: Compression test

In stock form that original 1979 140 engine should have had well over 130 lbs compression on each hole. Do the recommended decarb which was mentioned and then recheck to see what you get.

 

[F_R]

Re: Compression test

No matter what, if it cranks that slowly with all the plugs out it will never start until you find and fix the cause of the slow cranking. It needs to crank at least 250-300 RPM (plugs in) in order for the ignition system to work.

Slow cranking and low compression may be related. Rust or binding within the powerhead. That is the main reason you are doing the compression check before spending time and money on a dead horse.

 

[tashasdaddy]

Re: Compression test

90% of starter problems are bad connections. remove and clean both ends of the battery cables, so that they are shiney, also the cable from the solenoid to starter. check for nicks in the cables. and make sure the connectors are on the wire good. the cable tend to corrode from the inside out, if nicked, corroded wires, and connections, heat up and cause resistance to the follow of electricity, thus the starter doesn't get enough. you can also take jumper cable pos battery post to large post on starter. with a good connection, the engine should spin. then if the starter is good clean everything and retest. then trouble shoot solenoid. starters can be rebuit at a starter/alternator shop, much cheaper than a new one.

also have the battery load tested at the auto parts store, free, i've had new batteries go bad

 

[smokin_dodge]

Re: Compression test

ok, so i just got a hold of the owner of the boat. he has given me a little past history on the motor.

5 years ago, motor was completely rebuilt, ground up.

has no more than 200 hours on rebuild. and the last time it was run, he turned off the key and put the boat in storage.

so the owner thinks that if i pull the motor and put some tcw-3 in the cylinders that might free up the pistons.

any advice on pulling the motor? i have an overhead 3ton lift and a 2ton movable engine lift.

 

[Dhadley]

Re: Compression test

Why would you "pull the motor" to just put oil in the cylinders?

Start at the begining. Oil the cylinders, roll it over by hand to get the oil on the cylinder walls and find out why the starter is so slow. Could be a bad battery, bad / loose / coroded connection or a bad starter. Once you get it spinning, check compression again. 5 revolutions probably isn't enough, especially at that low rpm.

 

[crb478]

Re: Compression test

If it is turning that slow with a new battery I would check my cables and connections first. If the cables are corrioded inside it will never spin fast enough even with a new battery. With the plugs out it should really spin. Try using jusmper cables directly to the starter. It is fine to soak the rings, but I doubt that you compression is actually low, you just have not gotten an accurate reading yet.

 

[smokin_dodge]

Re: Compression test

Status Update:

This evening i went and got a quart of TCW-3 and a small plastic syringe. i put about 4tsb of oil in each cylinder and turned the motor by hand with spark plugs out.

hooked up the battery and turned the motor. about 10 revolutions it started spinning faster and faster, so i put the compression tester on it. over 110psi in each cylinder, with one near 120.


so now my question is should i continue to let the motor set and put more a little more oil in the motor, or should i go ahead and try and start it.

also when should i seafoam the engine, before or after i get the motor running?

 

[Dhadley]

Re: Compression test

Ok, compression good. Next clean the carbs (soak & new kits) and change the water pump kit (not just the impeller). Then start it, not before.

 

[Dhadley]

Re: Compression test

And change the gear lube while you're working on the lower unit (pump).