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How can I prevent this - Force 70 Low Compression

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  • How can I prevent this - Force 70 Low Compression

    Hello. I picked up 2 parts motors, 1995 70 hp forces over the weekend. One of the motors was purchased new by seller, who admits he used quicksilver oil, mixed correctly for its life. Motor did well for years, but eventually lost power. I pulled head just to peek at it, and top cylinder seems to have a damaged top ring land (actually looks like ring broke piston in a small area. What causes this? The second motor I believe also suffered a similar fate. It seems that Ive seen a fair amount of this on forum pictures.


  • #2

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    • #3
      Actually second motor has detonated piston at cylinder 2.

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      • #4
        Originally posted by atengnr View Post
        Hello. I picked up 2 parts motors, 1995 70 hp forces over the weekend. One of the motors was purchased new by seller, who admits he used quicksilver oil, mixed correctly for its life. Motor did well for years, but eventually lost power. I pulled head just to peek at it, and top cylinder seems to have a damaged top ring land (actually looks like ring broke piston in a small area. What causes this? The second motor I believe also suffered a similar fate. It seems that Ive seen a fair amount of this on forum pictures.
        I think JerryJerry has explained this in your other post (Force 70hp general question)
        "Success is walking from failure to failure with no loss of enthusiasm." - Winston Churchill

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        • #5
          I believe the rings had an update due to failures (cracked & got caught on the cylinder-ports)
          "Success is walking from failure to failure with no loss of enthusiasm." - Winston Churchill

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          • #6
            Running the wrong prop for the boat / load.-----Running without a tachometer.------Running wide open throttle at the wrong RPM.-------Running wide open throttle before motor is warm.----Etc, ETC.------Folks want to have fun and do not know the fine details of operating a boat motor.

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            • #7
              When did design change occur to correct this ring and port issue?

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              • #8
                I believe on the '95 units Mercury has done the update. regardless of the update done or not; I think racerone has pretty much sum it up in post #6, he really knows his stuff, so you can take his word for it. (lord knows I do)
                "Success is walking from failure to failure with no loss of enthusiasm." - Winston Churchill

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                • #9
                  Originally posted by racerone View Post
                  Running the wrong prop for the boat / load.-----Running without a tachometer.------Running wide open throttle at the wrong RPM.-------Running wide open throttle before motor is warm.----Etc, ETC.------Folks want to have fun and do not know the fine details of operating a boat motor.
                  Would you be willing to elaborate a little because I think I may be running under what is suggested on my 85hp 3 cyl. I have a tach that is accurate and i have been getting 4.2k @ wot. Using a 17 pitch prop. So looking up the specs i believe i should run around 4.5k-5.5k range. If I am just under the range i assume im doing damage?

                  I dont want to hijack this thread to help with my issues Im just curious if you could elaborate a little to help better understand this issue. Not everyone is an expert with these engines.

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                  • #10
                    You should be running over 5000 RP.-------If not you are " lugging " the motor !----You have the wrong prop.----Or there is something wrong with your motor.---Or something wrong with your boat.-----The decals say 85 HP but a dynamometer or test prop will tell you what it is putting out today !

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                    • #11
                      The 70 has one carb.
                      The top cyl. isn't getting the lubrication the other cyl. are.

                      The 85 is actually a 90hp. they rated it at the prop.
                      WOT should be 53-5500
                      What's the speed?

                      The boat and load and if the motors operated right can affect the rpms.
                      You using the trim?
                      Get a tiny tach make sure the one you have is reading right.

                      Load, 2-3 chunky people, dog, cooler full of beer and the regular equipment.
                      Might be too much?
                      Size boat?
                      Water logged foam? (Cut up a 21 and the foam had about 700# of water soaked in the foam.
                      Turns out the manufacturer made this boat without a way for the water to drain.? )


                      Buy American made Cars and Trucks!!!Towing insurance for a boat,a GREAT thing.
                      ​Tools to make my life easier: Battery tester, die grinder, temp.gun, compression tester, DVA meter.

                      In Scenic , Beautiful San Sebastian by the sea Florida

                      88 Bayliner 21CC 2001 Mercury 225 HP EFI

                      PLEASE,NO PERSONAL MESSAGES IF THE QUESTION CAN BE ASKED ON THE OPEN FORUM I WON'T ANSWER AS MY SYSTEM CAN'T REPLY TO YOUR EMAILS

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                      • #12
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                        Originally posted by jerryjerry05 View Post

                        The 85 is actually a 90hp. they rated it at the prop.
                        WOT should be 53-5500
                        I've been wondering about that as I own multiple 85 & 90 and all carburetors have the same fuel jets.

                        thanks Jerry for clearing that up
                        "Success is walking from failure to failure with no loss of enthusiasm." - Winston Churchill

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