40% Compression, is it worth the overhaul?

[birks]

1973 9.5hp Johnson outboard.<br />Model #: 9R-73B, Serial #: J3749033.<br /><br />The motor runs great but last time I took it in for a professional checkup, they said the compression was only 40-50%. I'm told that the cylinder walls and/or piston rings are most likely bad. What would I be getting myself into if I re-honed the cylinder walls and installed new piston rings. Do they even sell these parts anymore? What kind of costs am I looking at? The motor always starts and runs fine but really lacks hp. If I take on this work do I have to rebuild anything else? Would this affect other parts of the motor?

 

[cvx35]

Re: 40% Compression, is it worth the overhaul?

384073 is the part number for the .030 oversized piston rings. I don't have that part number in my price book. I would guess that they're not available. Also, each piston has 3 rings. I would try to find a later model engine. Good luck

 

[WillyBWright]

Re: 40% Compression, is it worth the overhaul?

Did they tell you what the compression readings were? Those are low compression motors. I wouldn't worry if they were 70 to 80psi, in fact that's what I'd expect. Below 60 and it needs an overhaul and it's not worth that.

 

[birks]

Re: 40% Compression, is it worth the overhaul?

If I remember correctly one of the pistons was at mid-50's% compressiion while the other was at high-40's% compression. Is there anything I can do to try and increase the compression? The motor runs so good I just hate to get rid of it.

 

[alcan]

Re: 40% Compression, is it worth the overhaul?

Hi Birks<br />It has been my expirence that when a motor looses compression is doesn't run well, if at all. I would probably go head and run this motor until it quit. You might try a decarb treatment and see if this brings up compression a bit. To overhaul or not depends. If you can do all or most of the labor yourself, you may come out almost even with market value. I'm sure you could beat brand new price. This sized motors are always in demand, I feel that they tend to hold their own, value wise, most of the time.

 

[Chinewalker]

Re: 40% Compression, is it worth the overhaul?

Hi Birks,<br /> Do you me 40 PSI and 50 PSI, or do you mean 40-percent of recommended and 50-percent of recommended? If it's at 40 or 50 psi, then yes that's low. If it's at a percentage, then what is the mech using as a baseline for 100-percent?<br /> I would give it a decarboning and see if that loosens things up a bit. Might simply have a stuck ring or two and that would help it out. Might be able to squeeze a few more seasons out of it if you tear it down, hone the bores and install new top rings. It isn't necessary to replace all the rings and that will likely add to wear in the bores. Replacing the top ring helps compression without contributing to excessive wear in the alread well-used bores...<br />- Scott

 

[birks]

Re: 40% Compression, is it worth the overhaul?

Now I'm not sure if the serviceman said PSI or % of compression. This weekend I'll try and establish exactly where the PSI reading is at. This Decarbonization sounds interesting what does it consist of. Can I do it myself? Monday, hopefully I have a better handle on the pressure each cylinder is creating. Thanks everone.

 

[birks]

Re: 40% Compression, is it worth the overhaul?

After talking to a friend does anyone know what material the cylinder is made of? Is it a aluminum block with a steel sleeve? or is it a aluminum and silica block. He said if it's option #2 you cannot hone the cylinder walls. Is this true? 30 years ago which way was Johnson making it's outboards?

 

[Chinewalker]

Re: 40% Compression, is it worth the overhaul?

Hi Birks,<br /> Your motor is an aluminum block with steel sleeves. The sleeves are plenty thick allowing oversize boring (at least back 30 years ago when oversize pistons and rings were readily available at a dealership).<br /> Decarboning is as simple as picking up some DeepKreep, Sea Foam or OMC Engine Tuner from your local boat shop. All do pretty much the same thing - loosen up and remove carbon from the piston rings, cylinder head, exhaust ports, etc. It's a relatively cheap and easy tune-up in a can, so to speak. It can't fix everything, but it can help some things...<br />- Scott