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Low compression on a 1974 85 hp evinrude

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Derrickw87

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I have a 1974 85 hp Evinrude and all 4 cylinders have 82 psi compression. I am wondering if it could just be bad head gaskets or do I need to do a ring job also? Thanks.
 
Derrickw87 said:
I have a 1974 85 hp Evinrude and all 4 cylinders have 82 psi compression. I am wondering if it could just be bad head gaskets or do I need to do a ring job also? Thanks.
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Also it runs good on the water 28 to 30 mph but at low speeds it does run a little rough but at full power runs great
 
Only kidding. These are great V4's. Is the paint discolored anywhere around the heads? This should have minimum of 110 psi with all plugs out, open throttle, and nice fast cranking. There are some that tell me the throttle can be fully closed on a compression test, but let me know if you gain any by having the butterflies tied or wedged open, I am still researching this idea. Thanks.
 
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Thanks timguy I will try that in a few weeks I'm out of state for work right now should be back in 2 weeks or so ....and let u know the compression results
 
Excellent information, however, I am so sorry about your motor. Maybe you had better pull the heads and have a closer look. At this compression you will have about 1/3rd less horsepower. It may likely only get worse.
 
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timguy said:
Excellent information, however, I am so sorry about your motor. Maybe you had better pull the heads and have a closer look. At this compression you will have about 1/3rd less horsepower. It may likely only get worse.
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I had just replaced head gaskets today and got same results.....am I in need of piston rings?
 
Derrickw87 said:
I had just replaced head gaskets today and got same results.....am I in need of piston rings?
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The first thing to do is to get another gauge somehow, somewhere and compare..... it may simply be your gauge that is failing.

Also... keep in mind that the psi reading will vary with the speed that the engine is cranking over... think about that! If cranking the engine over with a rope, pulling it slowly thru... you might have 10 psi if you're lucky. Cranking speed of roughly 300 rpm is desired.

The fact that you have the identical reading on all four cylinders indicates (my opinion) a cranking speed problem. Normally, with the engine cranking at a normal rpm (all spark plugs removed)... if there's a compression problem, it's going to pertain to one or two cylinders... IT IS NOT going to show up as identical low psi readings on all cylinders.

Now.... Pertaining to compression. Throttle butterflies open or shut, leaf/reed plates broken, welded shut, whatever... it doesn't matter. You could remove the entire intake manifold assembly, everything... it wouldn't matter. There's plenty of air for compression available via the exhaust ports... and compression does not begin until the exhaust ports are closed.
 
joereeves said:
The first thing to do is to get another gauge somehow, somewhere and compare..... it may simply be your gauge that is failing.

Also... keep in mind that the psi reading will vary with the speed that the engine is cranking over... think about that! If cranking the engine over with a rope, pulling it slowly thru... you might have 10 psi if you're lucky. Cranking speed of roughly 300 rpm is desired.

The fact that you have the identical reading on all four cylinders indicates (my opinion) a cranking speed problem. Normally, with the engine cranking at a normal rpm (all spark plugs removed)... if there's a compression problem, it's going to pertain to one or two cylinders... IT IS NOT going to show up as identical low psi readings on all cylinders.

Now.... Pertaining to compression. Throttle butterflies open or shut, leaf/reed plates broken, welded shut, whatever... it doesn't matter. You could remove the entire intake manifold assembly, everything... it wouldn't matter. There's plenty of air for compression available via the exhaust ports... and compression does not begin until the exhaust ports are closed.
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Thanks for the answer so low cranking speed problem could that just mean my starter is getting weak? And the psi readings are with another guage
 
Low cranking speed can be contributed to a good but discharged battery, a good fully charged battery with either loose connections...OR... tight but dirty connections, or yes, a failing starter.

The place to start (all spark plugs removed), although you're absolutely sure that the connections are okay... is to remove all cables from the battery, clean those cable terminal ends and the battery posts so that they look better than new... then do the same with all the cables/wires at the engine such as the cables connected to the starter solenoid, the electric starter, and also (Important) the black battery cable where it grounds out to the powerhead.

When you have accomplished the above... and the battery is known to be good and fully charged... see if there is an improvement in the cranking speed of the engine. And if not...........

The second task (if it were me) would be.... with a good known fully charged battery and known good jumper cables... to run those jumper cables directly to the electric starter... and if the cranking speed is still slow, dismantle that starter motor for inspection.

The absolute clean and tight cable routine is however a must!

If any of the heavy duty cables related to the electric starter system becomes very hot (battery, solenoid,starter, ground) that normally indicates that either the cable has a bad connection at the shortest distance from that hot spot... OR... the internal wires of the cable at that hot spot has become corroded and not capable of carrying the current, in which case, replace it.
 
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joereeves said:
Low cranking speed can be contributed to a good but discharged battery, a good fully charged battery with either loose connections...OR... tight but dirty connections, or yes, a failing starter.

The place to start (all spark plugs removed), although you're absolutely sure that the connections are okay... is to remove all cables from the battery, clean those cable terminal ends and the battery posts so that they look better than new... then do the same with all the cables/wires at the engine such as the cables connected to the starter solenoid, the electric starter, and also (Important) the black battery cable where it grounds out to the powerhead.

When you have accomplished the above... and the battery is known to be good and fully charged... see if there is an improvement in the cranking speed of the engine. And if not...........

The second task (if it were me) would be.... with a good known fully charged battery and known good jumper cables... to run those jumper cables directly to the electric starter... and if the cranking speed is still slow, dismantle that starter motor for inspection.

The absolute clean and tight cable routine is however a must!

If any of the heavy duty cables related to the electric starter system becomes very hot (battery, solenoid,starter, ground) that normally indicates that either the cable has a bad connection at the shortest distance from that hot spot... OR... the internal wires of the cable at that hot spot has become corroded and not capable of carrying the current, in which case, replace it.
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Very good information I'll try connections this weekend thanks!
 
For some reason, I have no idea why..... I've always been lousy for remembering just exactly what gauge this or that wire might be. I'll leave that up to another member with a better memory.

When shopping for wire, I always take a sample of it with me.
 
Copper clad is aluminum which is only coated. It has less conductivity than pure copper and usually less flexible. For instance you can achieve almost equal conduction with #4 copper clad vs. #6 pure copper. So don't be fooled.
 
Ok so I'm thinking my piston rings will need to he replaced. Got new cables and the 1 and 3 cylinders are 92 psi and the 2 and 4 are 88. Cylinders 1 and 3 went up 4 psi
 
When you had the heads off, how did the cylinders look? Maybe you can get by with a hone and std pistons.
 
Cylinders look good would need a hone for sure when I replaced the exaust gaskets I could see small pits on the side of number 3 piston.....so you think just a hone pistons and rings?
 
Alright and I know the job is pretty easy but other them headgaskets is there any other gaskets I would need or just the pistons and rings?
 
The powerhead needs to come off.----A gasket set is in order.--Crankshaft seal are needed.------O-rings as well.-----It appears they have 3 standard pistons on this site at $50 each.----Rings are a bit pricey too.----Take it apart see what you need.
 
Derrickw87 said:
Alright and I know the job is pretty easy but other them headgaskets is there any other gaskets I would need or just the pistons and rings?
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... no offense intended but that question leads me to wonder if you really know what's coming your way with a rebuild.
A "ring job" as you put it, is going as deep as it gets in the motor, as racer said, the power head has to be completely torn apart, and you don't just need one or two gaskets here but the whole power head gasket kit.

Maybe I misinterpreted your question, and in any event, best of luck with this project!
 
No offense taken just trying to see what I'm getting into.....n will prolly do the rebuild next fall or year after bc or still runs alright just knew it had low compressuon
 
I really appreciate everyone's help with this subject. Has been a great help. One more question a little off subject but the boat this motor is on never had a tach. I have a tach guage and wondering what wire on the motor do I wire into to get the rpms?
 
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