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  • Low compression

    I just bought a complete top end for a 98 Yamaha XL 1200 from SBT. And after 5 hours of running it under 30 mph I checked the compression and it was 70- 75 -80 psi on the three cylinders. What could have happened? My oil mixture was 4.oz to 1 gallon. This was tested on a new compression gauge. Never rev it more than 6000 rpm, I also let it cool down after every 1/2 an hour to break in. I am an ase master automotive mechanic. I really baby the break in procedure, I know that it takes 8 gallons to break it in, I've only used 7 gallons and my compression is low.

    Could I be using to much oil? And is that why the rings may not be sealing right?
    Last edited by ase mechanic; 06-10-2008, 10:53 PM.

  • #2
    Re: Low compression

    Did the compression test read correctly when first installed, and got progresively worse? What was the cause of the original failure? What do your spark plugs look like? Did the old pistons seize? Did you clean/rebuld the carburetors? If not they are probably partially clogged and caused the engine to run lean, resulting in your low compression readings. Should be about 2.5 ounces per gallon, but it would not stop the rings from seating.
    John Kubiak
    Powersports Technical Training Professional
    Las Vegas, Nevada
    Sea-Doo Tech 13736
    PWC Tech since 1988 (22 years)
    PowerSports since 1976 (34 years)
    NEVER BUY TIRES AT A "BLOW-OUT" SALE
    Please do not use Private Messaging, use the forums.

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    • #3
      Re: Low compression

      I did not do a compression test before I started the engine.the original failure was normal wear and tear. low compression on all 3 cylinders 30-30-40 pistons are still good only needs rings I am saving the pistons for another time. carbs are running good,spark plug are dark probably because of rich oil mix and gasoline mix. I also run 93 oct gas just to play safe for the detonation.and yes I used two compression gauges for the test.

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      • #4
        Re: Low compression

        What was your piston to cylinder clearance on the original parts? Did you use our rings on another brand piston? You only re-ring if the piston to cyl wall clearance are within spec, and you only use same brand to same brand parts. Our rings are made to fit the grooves in our pistons, Wisco is the same as is O.E. their rings fit only their pistons. Tom SBT tech.
        SBT Tech Support is here to help with your problems.
        We try to answer each question quickly and accurately.
        Please do not use Private Messaging for Tech Support, use the forums.

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        • #5
          Re: Low compression

          I bought the complete kit FROM SBT ( cylinders, pistons and gasket set) cylinder exchange.I did measured 1 ring into the cylinder and it looked like a good tight fit .may be 004 clearance.

          Edit: I could not resist my self and took the cylinder head off and cylinders to see what was going on with the low compression. only to find minor vertical scratches on the cylinder wall and some shiny spots on the wall of the cylinder,witch makes me think the cylinders are out of round ,I also saw that the crom part of the piston ring was fading unevenly throughout the circumference. Over all I still think the low compression could be some wear else. What is the c.c.of the dome? on a 98 YAMAHA xl 1200 npv probably some one has changed the dome size in a machine shop making it bigger thus lowering the compression?
          I am a master mechanic I rebuild diesel engines, gasoline engines, 4 and 2 stroke. with 30 years of experience and this low compression problem with my ski really doesn't let me sleep. Only 5 hours on it.
          Last edited by ase mechanic; 06-12-2008, 10:56 PM.

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          • #6
            Re: Low compression

            How does it run, otherwise?

            I know you are an ASE mech.. But something sounds wrong with your gauge.. having ALL three cyls go bad is a VERY rare occurance on Yamaha's.. and to have it happen twice?
            Dan Solie -
            04 RXP (the "Instigator")
            94 xp (freestyle mods)
            95 xp (faster than the 94)
            JS550 (the toothpick & the loaner)

            For FREE Seadoo Manuals - Click HERE!!!!

            http://www.tripleateam.com/water/v/U...ery/Dan-Solie/

            **** Please do not PM me. I don't do PM's.... Please post your question on the correct forum so that all can see and learn. Thanks! ****

            Comment


            • #7
              Re: Low compression

              Originally posted by ase mechanic View Post
              ....the original failure was normal wear and tear. .... carbs are running good.
              Listen, I don't want to crush your stones here, but things that look like normal wear and tear on a heavy duty diesel (2 or 4 stroke) or most auto engines are usually catastrophic failures in a little itty-bitty two-stroke engine...

              The major cause of second and subsequent failures on two strokes are usually caused by a fuel system issue...The crux of the matter is that the system may in fact be a little lean right now, and you may have "masked" the problem by using premium fuel to compensate for any anticipated detonation.

              As a professional in this field, and a former trade school instructor, I assure that there have been many "good running" carburetors out there that have destroyed their share of engines!

              Be safe, take the extra time and the spend the extra money and make sure beyond any doubt that there is clean fresh fuel being delivered through clean fresh fuel lines to clean fresh clean (i said it twice) carburetors.
              Last edited by John Kubiak; 06-13-2008, 03:12 PM.
              John Kubiak
              Powersports Technical Training Professional
              Las Vegas, Nevada
              Sea-Doo Tech 13736
              PWC Tech since 1988 (22 years)
              PowerSports since 1976 (34 years)
              NEVER BUY TIRES AT A "BLOW-OUT" SALE
              Please do not use Private Messaging, use the forums.

              Comment


              • #8
                Re: Low compression

                Originally posted by ase mechanic View Post

                Edit: I could not resist my self and took the cylinder head off ....only to find minor vertical scratches on the cylinder wall and some shiny spots on the wall of the cylinder,witch makes me think the cylinders are out of round, I also saw that the crom part of the piston ring was fading unevenly throughout the circumference.
                Again, I don't want you to think I am trying to crush your stones here.

                I have personally been to SBT's facility many, many times, and I can assure you that the CNC boring and honing equipment that they use does not produce out of round cylinders. The equipment monitors the size of the bore from start to finish and top to bottom during both processes, and automatically compensates for any problems. Plus the quality assurance checks would not pass any cylinders that were not perfect. In the next step, they measure the cylinders and verify the matching pistons and rings.
                John Kubiak
                Powersports Technical Training Professional
                Las Vegas, Nevada
                Sea-Doo Tech 13736
                PWC Tech since 1988 (22 years)
                PowerSports since 1976 (34 years)
                NEVER BUY TIRES AT A "BLOW-OUT" SALE
                Please do not use Private Messaging, use the forums.

                Comment


                • #9
                  Re: Low compression

                  I thank everybody for your suggestions, I am open to any ideas, In the mechanic world you can never know it all, when you think you know it all is when something like this happens.I mesured, the dome and each one is 50 c.c. is that to big? what is the c.c. on a standard dome head? I .think the problem is there, on the domes.If i have average psi of 75,what dome size should I have to obtain 150 or 145, dont want any more psi, not looking detonation, for speed I ride my other ski rxt-sc. 67 mph.
                  Last edited by ase mechanic; 06-13-2008, 06:05 PM.

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                  • #10
                    Re: Low compression

                    If your current head is getting you 75psi, then your dome needs to be 1/2 the size to get 150psi.. I don't think you can machine out a dome that big (75psi) while keeping the squish band and using a stock head..
                    Dan Solie -
                    04 RXP (the "Instigator")
                    94 xp (freestyle mods)
                    95 xp (faster than the 94)
                    JS550 (the toothpick & the loaner)

                    For FREE Seadoo Manuals - Click HERE!!!!

                    http://www.tripleateam.com/water/v/U...ery/Dan-Solie/

                    **** Please do not PM me. I don't do PM's.... Please post your question on the correct forum so that all can see and learn. Thanks! ****

                    Comment


                    • #11
                      Re: Low compression

                      from what I under stand is that if I have 75 psi with 50cc domes, I need to install smaller domes 1/2 size witch would be 25 cc in order to obtain 150 psi is that wright? can some one tell me what is the dome size on a standard head for 98 yamaha xl 1200 so I can find out what is going on with my compression. please!!!!!!! since my engine is apart there is no way to check my compression gauge with it. I got a 3erd gauge from my neighbor that reads exactly as my second gauge,I tried it on my other ski. there is no doubt on the gauge readings.
                      Last edited by ase mechanic; 06-13-2008, 10:59 PM.

                      Comment


                      • #12
                        Re: Low compression

                        What's the other ski read as far as compression then?

                        To measure the exact cc's of the domes, fill up the dome with water using a kids medicine dispenser.. they have CC marks on them. However much water fills the dome to the top, that's your cc's.

                        But, as an example.. Let's say the stock dome is 50cc's.. That's SUPPOSED to get you in the 125-130 range... That would mean you would have 80cc domes now to get the 75psi.. Your head can't go that big..
                        Dan Solie -
                        04 RXP (the "Instigator")
                        94 xp (freestyle mods)
                        95 xp (faster than the 94)
                        JS550 (the toothpick & the loaner)

                        For FREE Seadoo Manuals - Click HERE!!!!

                        http://www.tripleateam.com/water/v/U...ery/Dan-Solie/

                        **** Please do not PM me. I don't do PM's.... Please post your question on the correct forum so that all can see and learn. Thanks! ****

                        Comment


                        • #13
                          Re: Low compression

                          Thank you for your help, I honed the cylinders and cleaned the carburetors and put them back together, my compression is 147-147-150. Just don't know how those scratches got there. Now I can go ride on it and have a good night sleep. Some times you over look the small things and always look for major problems.

                          Comment


                          • #14
                            Re: Low compression

                            did you line up the openings of the rings with the notch on the piston when you put them together the first time?
                            1997 Speedster (twin 720s)
                            London, Ontario, Canada

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                            • #15
                              Re: Low compression

                              yes, I did line up the piston notches to the openings on the rings, what got me worried was the small scratches, so small that I could run my finger nail and it would not stomp. I have bin rebuilding all kind of engines for 30 years.
                              I got the problem fixed by honing the cylinders. That must have taken about 30 hours of life from the engine.

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