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questions about Merc 70 pistons/rings.. worth fixing?

B

bulletbob

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1987 2 stroke,,, Has about 120 PSI on cyl 1 and 3 and about 90 on #2.. At some point #2 carb must have been lean.. I have cleaned all carbs, and tried getting some comp back with various ""treatments"" to no avail..Motor starts fine , actually runs pretty well, and can probably be run as is for years... I Know these motors don't have a cyl head per se, and that you need special tools to install the rings.. i was thinking about tearing the engine down, honing #2, new rings, new piston on #2 if needed,, and having a good marine tech with the proper gear just install the pistons for me for whatever they consider a fair price, and then I would re assemble the powerhead.. I can't even get $300 as it sits even though its still running pretty well, as NO one wants a motor with uneven compression at any price.. i thought just the good lower unit and power tilt would be worth $300 but no bites... Is it worth trying to save this motor?/. Run it as is?.. Take whatever I can get as is and run?... I don't need this much power in a 900 pound boat, and would really rather have a 40 HP, but hate to junk whats still a decent running 70.. Not sure if fixing it and then selling it would be worth it.. Its probably still only a $700-800 motor locally even if rebuilt.. most guys want nothing to do with older 2 strokes of this size from what I can see.. 20-30 HP 2 strokes get sold fast for good money in my area, but the larger and older the motor, the tougher the sell.. A clean 2 stroke 25 HP remote.electric start goes for more money than a clean 100 of the same year for some reason... Just want some on ideas on what to do with a motor thats not an easy sell, running right, and even tougher when it has issues.....
 
So are you saying that the piston/rod and ring assemblies will slip up into the bore due to a taper in the design??... Not sure I follow.. I know the pistons go "up" into the sleeve, and not downward as they would in an engine with a conventional cylinder head design.. Hoping for a bit more clarification... bob
 
Pictures of your motor.----Are you sure it is an 87 model ?----Serial # of your motor.-----Mechanic in a can does not work.----Clearly he is saying that piston can be put into the bores without special tools !------Do not overthink this simple repair on this motor.
 
1987 was first year for "small bore" 70/80/90 engines. If it is a looper the sleeves are tapered and you will not need ring compressors. Post a picture of motor if possible.
 
Some guys and gals love (and understand) the older 2-strokes. Not too much tougher than rebuilding your old 2-stroke snow-sled or dirt-bike. Hobo-frieght has a cheesy and cheap ring expander and compressor tools. Both can be purchased for under $30 for both.assuming you don't have piston/ bore damage or excessive wear, rings & gaskets won't cost you the farm either. You won't know for sure until you get in there and inspect it.. In top-nick or needing some help, it's a vintage 70 2-stroke and it has value to someone who needs it and can appreciate it. The value it holds for you would be fully realized when you break out your hand-tools and the ole FSM and fix it yourself, the parts are easy to find and affordable, paying someone to do the work for you is where it gets costly. Just my opinion but a 70 2-stroke on the right boat can be a blast! Sounds like you have a cool project on your hands, have fun with it!
 
actually, i wanted to go smaller and lighter anyway, as I feel this motor is a bit too heavy on the transom.. Boat is less than 900 pounds.. I was never that much of a Merc guy, I prefer OMC and understand them a lot better, having owned many of them over the years, and only a few mercs.. I am planning on finding a 2 cyl omc 2 stroke, maybe a 40 or 50 tops as thats all this aluminum boat needs.. Shortly after this post, i got a buyer for the merc at $350 which is fair considering the #2 cyl is down on comp.. Still runs pretty good, but no way i would try and sell it as a problem free motor.. thanks to all for the good advice... bob
 
Started and ran this motor today.. Started right up, no issues ran pretty well actually.. This thing always sounds like a fast street car with a hot cam.. Kind of a "lopey: sound at idle, but cleans up quick with some throttle... So ran it till warm, took a compression test and got 1-105
2-90
3- 105
certainly not great, but based on todays startup and run after sitting 10 months or so,, not sure if this is a "parts motor" just yet.. Based on the comp pressures, its obvious there is some wear in the motor, but I am not much of a Merc guy at all, so don't know what a healthy one should have for compression readings on a warm engine... there is a 15 psi difference, but I dunno, It seems it would be tough to replace it, for the $350 I was asking for it... I am not seeing anything that runs decent in a 50 or so HP for less than about $1200-$1500, and most of those are 30-40 years old just like this Merc... Parts any problem for a merc this old?.. Might reconsider bailing on it....bob
 
I know there are no specs for these engines concerning compression, but are they typically say 120 or above for a good one?. its seems ro me that 105 in each "good cyl" is still on the low end, but I'm not sure what compression readings these mid size Mercs usually show when in prime condition.. It is tired I suppose, but it still runs pretty well, even at low speed.. Not sure a rebuild is the way I want to go yet, as long as its starting and running ok which it is at this point... I want to go down to a 40 HP anyway eventually, .. I had this sold as a "parts motor", but I dunno, not sure if its that far gone just yet.. I'll know more when I get it out on the water soon... bob
 
How confident are you that your compression gauge is accurate?
If you have a history with the gauge and know it to be reasonably accurate, those readings are what I would consider to be on the low side.
Even so, if it powers up for you on the water, run it the way it is.
 
bulletbob said:
1987 2 stroke,,, Has about 120 PSI on cyl 1 and 3 and about 90 on #2.. At some point #2 carb must have been lean.. I have cleaned all carbs, and tried getting some comp back with various ""treatments"" to no avail..Motor starts fine , actually runs pretty well, and can probably be run as is for years... I Know these motors don't have a cyl head per se, and that you need special tools to install the rings.. i was thinking about tearing the engine down, honing #2, new rings, new piston on #2 if needed,, and having a good marine tech with the proper gear just install the pistons for me for whatever they consider a fair price, and then I would re assemble the powerhead.. I can't even get $300 as it sits even though its still running pretty well, as NO one wants a motor with uneven compression at any price.. i thought just the good lower unit and power tilt would be worth $300 but no bites... Is it worth trying to save this motor?/. Run it as is?.. Take whatever I can get as is and run?... I don't need this much power in a 900 pound boat, and would really rather have a 40 HP, but hate to junk whats still a decent running 70.. Not sure if fixing it and then selling it would be worth it.. Its probably still only a $700-800 motor locally even if rebuilt.. most guys want nothing to do with older 2 strokes of this size from what I can see.. 20-30 HP 2 strokes get sold fast for good money in my area, but the larger and older the motor, the tougher the sell.. A clean 2 stroke 25 HP remote.electric start goes for more money than a clean 100 of the same year for some reason... Just want some on ideas on what to do with a motor thats not an easy sell, running right, and even tougher when it has issues.....
Click to expand...
I would cut your losses and trade it in on a new 40HP. I went down the road of trying to fix a 40 HP 1982 loads of 5 or 6 mm bolts. I think I had seven busted bolts just trying to get the inspection plate off. I struggled with that engine for 3 years never did run really good finally traded it in on a new 40 HP.
 
steve_t said:
I would cut your losses and trade it in on a new 40HP. I went down the road of trying to fix a 40 HP 1982 loads of 5 or 6 mm bolts. I think I had seven busted bolts just trying to get the inspection plate off. I struggled with that engine for 3 years never did run really good finally traded it in on a new 40 HP.
Click to expand...
yeah, no trade in value on a 36 year old motor without good compression readings.. Right at this time, I am just going to run it as is. it starts fine, actually runs pretty well despite that middle cyl being somewhat lower in compression, it DOES use a lot of gas. I mean way out of line.. I have had OMC 115 OB's that used half the gas this merc 70 does on boats that weighed 500 pounds more than the aluminum Grumman 17 the merc is mounted on.. this boat only gets used a few times a year in fresh water, and I have a little 4 HP kicker on it as well, so I will leave it for now, and see what happens this season... thanks ... bob
 
If it is using that much fuel there is something wrong.----Not related the low compression.----Should be an easy fix for that.
 
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