Lowest compression you’d buy

[jitterbug127]

I bought a motor this week for $50. I didn’t do a compression test like I normally would because I didn’t have my tester. Got it home, had a bunch of acorns/fur under the hood. I even found acorns and fur in the lower unit. Motor turned over and shifted freely. I did a compression test and I was getting 60-70 in both cylinders. My spark tester broke, so I was unable to test for spark. I hooked the electric up to my battery and didn’t have any luck getting it started. I sold the motor due to the low compression/lack of electrical with the starter. Motor was in rough shape to say the least. I sanded the points, put on new spark plugs and changed the impeller. I never actually tired to start the motor. I was wondering what was the lowest compression motor someone should buy? For reference motor was 1972 25 hp electric. Carburetor was also seized up even dumping some 3:1 on it. I sold motor for $150 and thought someone else could probably use it for parts. I unfortunately was unable to have success with this motor.

 

[flyingscott]

Well the buyer got a good deal. The electric start bracket was worth more than he paid for it. Those motors on the healthy side should be about 140 psi.

 

[jitterbug127]

Well the buyer got a good deal. The electric start bracket was worth more than he paid for it. Those motors on the healthy side should be about 140 psi.

I’m glad buyer got a good deal then. I had little to no use for motor, so I was happy to pass it along for cheap. I’d have liked to got it running, but it wasn’t in the cards.

How low if compression is with running? Like at what point does the power head need rebuilt?

 

[racerone]

Depends on how you want it to run !-----Had a 30 Johnson here a few years ago.------Showed owner the scored piston.-----Did not want it fixed.-----Had no idea how serious it was.-----Says his motor runs great.-----Most folks think that if a motor " can run " there cannot be anything wrong with it !!

 

[jitterbug127]

Depends on how you want it to run !-----Had a 30 Johnson here a few years ago.------Showed owner the scored piston.-----Did not want it fixed.-----Had no idea how serious it was.-----Says his motor runs great.-----Most folks think that if a motor " can run " there cannot be anything wrong with it !!

That’s basically exactly what I assumed. This week is I begin the process of trying to put a different motor back together.

 

[909]

I grabbed a parts motor with 80 PSI , and got it to run @ 130 PSI

 

[jitterbug127]

I grabbed a parts motor with 80 PSI , and got it to run @ 130 PSI

I talked to the guy that bought it. He said started right up 2nd pull and he’s gonna sell it

I was reading yesterday and looks like I made a big mistake selling it. I didn’t realize compression can’t effectively be tested until motor is running. I’ve always run a compression test before I’ve bought them or before I ever stated a motor, but this isn’t the most accurate test.

Does anyone have a check list of things to do before selling a motor? I really feel like I messed up on this one, and would like to try to keep learning.

I know check compression, check 5/16” gap for spark (I need to get a new tester), make sure new plugs, new water pump, change gear lube, test lower unit seals for leaks, I bought a timing light and I’m going to get one of those heat laser things. I also sand and set the points.

 

[ShoestringMariner]

I grabbed a parts motor with 80 PSI , and got it to run @ 130 PSI

Did a decarb bring it back up, or did it need a re-ring etc?

 

[jitterbug127]

I talked to the guy that bought it. He said started right up 2nd pull and he’s gonna sell it

I was reading yesterday and looks like I made a big mistake selling it. I didn’t realize compression can’t effectively be tested until motor is running. I’ve always run a compression test before I’ve bought them or before I ever stated a motor, but this isn’t the most accurate test.

Does anyone have a check list of things to do before selling a motor? I really feel like I messed up on this one, and would like to try to keep learning.

I know check compression, check 5/16” gap for spark (I need to get a new tester), make sure new plugs, new water pump, change gear lube, test lower unit seals for leaks, I bought a timing light and I’m going to get one of those heat laser things. I also sand and set the points.

I meant to say starting not selling.

 

[RMClark]

The one thing I learned from my first adventure with compression testers is that cheap compression testers may or may not give you an accurate compression reading.

I had an old Johnson 30HP that came with an even older Lund I bought. I could get it to start but it wouldn't run well or very long. As part of the things I did to try and get that motor running right, I headed down to the local Harbor Freight and bought one of their compression testers. IIRC, I paid about $25 for it.

Took the new tester home, hooked it up, and pulled on that rope to check the front cylinder. 70 PSI was the reading. Hmmmm...too low I thought. Checked the second cylinder. 70 PSI on the dot. Hmmmm. Compression in this engine is too low, I thought. But it would occasionally start. I checked compression several times. Both cylinders were always consistent right at 70 PSI.

I eventually sold that motor and replaced it; a number of factors led to that action.

When I sold the motor, the guy who came to look at it had me attempt to start it on the stand. It started on second pull, revved a couple of times and then soon died. His first comment was, "OK, so I don't need to test compression." He gave me the asking price, loaded the motor and left.

I took two things from this event. First, my cheap HF compression tester is not accurate. At all. Second, the compression tester did give me a clue that if it was consistently measuring by cylinders at the same exact reading, and I could start the motor, then I probably didn't have a low compression problem.

 

[jitterbug127]

The one thing I learned from my first adventure with compression testers is that cheap compression testers may or may not give you an accurate compression reading.

I had an old Johnson 30HP that came with an even older Lund I bought. I could get it to start but it wouldn't run well or very long. As part of the things I did to try and get that motor running right, I headed down to the local Harbor Freight and bought one of their compression testers. IIRC, I paid about $25 for it.

Took the new tester home, hooked it up, and pulled on that rope to check the front cylinder. 70 PSI was the reading. Hmmmm...too low I thought. Checked the second cylinder. 70 PSI on the dot. Hmmmm. Compression in this engine is too low, I thought. But it would occasionally start. I checked compression several times. Both cylinders were always consistent right at 70 PSI.

I eventually sold that motor and replaced it; a number of factors led to that action.

When I sold the motor, the guy who came to look at it had me attempt to start it on the stand. It started on second pull, revved a couple of times and then soon died. His first comment was, "OK, so I don't need to test compression." He gave me the asking price, loaded the motor and left.

I took two things from this event. First, my cheap HF compression tester is not accurate. At all. Second, the compression tester did give me a clue that if it was consistently measuring by cylinders at the same exact reading, and I could start the motor, then I probably didn't have a low compression problem.

I appreciate your insight! Well thought out response. I’ve got a good compression tester because it’s read well on ny other motors. I had similar reading on both cylinders, so I probably should’ve assumed compression was ok: I’ve read I want compression 100/100 or better l, and this 25 hp was 60/60 at worst 70/70 at best.

Is compression in both cylinders being similar more important than the actual rating of the compression?

 

[ShoestringMariner]

I have a 70hp that I thought had a worn out powerhead with low compression. My mechanic said it was full of carbon because the previous owner removed the thermostat and it could not get up to temp. He did a decarb and got the compression up to more acceptable numbers. Still lower than I like but it should run ok

 

[racerone]

The 2 stroke outboard engine is unique.----It operates / boat floats in a supply of cooling water..-----Unlike an automotive engine that carries a sealed water supply INSIDE the block. ----Folks do not maintain / replace the impeller on a regular basis !-----Results in lack of cooling.-----If water supply to an outboard block is interrupted at full throttle damage occurs in less than 30 seconds.----The carburetors are also the oil pump on most older 2 strokes.------If a carburetor is plugged on a multi carburetor engine you end up scoring a cylinder due to lack of oil.-----Most small motors do not shut down / warn the operator of an overheat condition.-----It is easy to buy an outboard motor.----But it seems to take time and an empty wallet for folks to learn how to operate and maintain them !!-----Folks loan them out to friends who have no concept of 2 strokes.-----The comment --" I didn't know you had to mix oil in the gas " has been heard more than once.---Or --" did not know that you had to maintain the oil injection system " has been heard more than once.---Motors with " expensive to repair " issues are put on the market all the time.------People come up with new ways to destroy these fine motors all the time!

 

[jitterbug127]

The 2 stroke outboard engine is unique.----It operates / boat floats in a supply of cooling water..-----Unlike an automotive engine that carries a sealed water supply INSIDE the block. ----Folks do not maintain / replace the impeller on a regular basis !-----Results in lack of cooling.-----If water supply to an outboard block is interrupted at full throttle damage occurs in less than 30 seconds.----The carburetors are also the oil pump on most older 2 strokes.------If a carburetor is plugged on a multi carburetor engine you end up scoring a cylinder due to lack of oil.-----Most small motors do not shut down / warn the operator of an overheat condition.-----It is easy to buy an outboard motor.----But it seems to take time and an empty wallet for folks to learn how to operate and maintain them !!-----Folks loan them out to friends who have no concept of 2 strokes.-----The comment --" I didn't know you had to mix oil in the gas " has been heard more than once.---Or --" did not know that you had to maintain the oil injection system " has been heard more than once.---Motors with " expensive to repair " issues are put on the market all the time.------People come up with new ways to destroy these fine motors all the time!

Racer your responses and knowledge are unparalleled on the boards in my opinion. If you had someone just starting out today knowing literally nothing about outboard motors and they wanted to learn what would you recommend? (My guess AOMCI + iboats+ YouTube + reading service manuals) I would like to find a mentor in my area to help learn a few techniques from I think the AOMCI may be helpful for that.