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  • compression testing new top end?

    ive whacked a new top end (rebored barrels, pistons and rings) into my rz250r

    out of curiosity after the build before the first ride, i warmed it up and did a compression test and got 115psi with a 1.2mm squish gap- have done 60km on it now, and tested it again.. no change still 115psi.

    i have previously tested this bike before i munched the old top end with ~40,000km on it, at 100psi

    also tested ashley's rz250r which was in original condition and had about 25,000km on it, at 130psi

    so i guess i am wondering why does my fresh top end and tight squish have less compression than a motor with 25,000km on it? of course ashley was not the first owner of that bike, its possible the motor had been frigged with although it had the look of a motor that had never been in bits.

    will the compression come good after some more running in?

    can worn reed blocks contribute to poor compression?

    i didnt bother switching oil for the run in, am still on full synth.
    Originally posted by Bendito
    If we get to a stop and we are missing a dozen bikes and you are last, it was your fault. Don't be that guy. No one likes that guy.

  • #2
    can reed blocks contribute to poor compression?
    A; only if they the flappy thingys are broken off!

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    • #3
      well they arent quite that bad, i did notice maybe 2 out of 8 of the fins were not quite sitting as perfectly flat as the others.
      Originally posted by Bendito
      If we get to a stop and we are missing a dozen bikes and you are last, it was your fault. Don't be that guy. No one likes that guy.

      Comment


      • #4
        Compression tests on 2 strokes are vaguely indicative of jack shit in many ways. Angst over numbers can drive you nuts mate.... are both sides the same, give or take? A 25k motor with a coked up piston likewise will falsely elevate the test. If it's running well it will be all good. The new pistons may have a different crown profile/displacement to the oldies, coupled with a minor change in swept volume there is no datum unless you had taken measurements beforehand..... At 1.2mm it's still on the "safe" side for a street motor but this isn't the only parameter change that effects a compression gauge reading from a 2 stroke. Then there's the gauge and quality therof. At 60km, for all intents and purposes, the top end is run in regarding piston ring normalisation..... Did you measure and note/adjust the end gaps before assembly? You may get a mickey more comp in the the short term but I don't think 115psi is bad for a stocker.... Is there a figure quoted in the manwell for the motor? If it's really bugging you, pull the pipes off and have a looksee for peace of mind. Did the other motor you checked have an identical unswept volume to yours. Careful with this comparison, it's a long bow to draw. Feel free to call me an idiot, it's been one of those weeks!

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        • #5
          yeppers both sides blow exactly the same

          for datum and guage quality- its a super cheap guage.. but all measurements taken with the strength of my kick and same guage on hot motors + WOT

          i didnt measure the end gaps on the rings apart, i did note they were keyed for the locating pin and guess i hoped that meant they didnt need trimming.

          the motor is pulling well i think, compared to others ive ridden.. even with powervalves wired open it pulls away rather than lugging at low rpm and comes on pipe with a crisp ripping tone and feel

          i havent been able to find a reference value in the manual- but it was worth pulling it out.. it has this to say on poor compression 'Low crank case compression caused by worn main bearings and seals''

          i wonder if my crank seal is not as good as it should be.. have not noticed any evidence of leaking f/a mixture though
          Originally posted by Bendito
          If we get to a stop and we are missing a dozen bikes and you are last, it was your fault. Don't be that guy. No one likes that guy.

          Comment


          • #6
            Wait until your rings bed in properly mate.
            You'll still be getting a heap of blow-by.

            Comment


            • #7
              Originally posted by GD Engineering (Gavin) View Post
              At 60km, for all intents and purposes, the top end is run in regarding piston ring normalisation
              Originally posted by Desmogod View Post
              Wait until your rings bed in properly mate.
              You'll still be getting a heap of blow-by.
              i will keep testing after each tank and post the results.. am going to visit nath this w/e and test his freshly oversized 350 top end with proX pistons as well for comparison
              Originally posted by Bendito
              If we get to a stop and we are missing a dozen bikes and you are last, it was your fault. Don't be that guy. No one likes that guy.

              Comment


              • #8
                Sorry, didn't read Gav's post before I posted mine.
                Ignore me, he knows.

                Comment


                • #9
                  im sitting here reading what the haynes manual has to say on the subject

                  blah blah

                  highly advisable to treat the engine gently blah blah

                  ... even greater care is necessary if the engine has been rebored... the engine will need to be run in again as if the machine were new. This means greater use of the gearbox and a restraining hand on the throttle for at least 500miles.

                  ...a rebored 2T engine will require more careful running in over a long period than its 4T counterpart, there is far greater risk of seizure if the engine is wokred hard.

                  do not add extra oil to the fuel, it causes lean condition
                  Originally posted by Bendito
                  If we get to a stop and we are missing a dozen bikes and you are last, it was your fault. Don't be that guy. No one likes that guy.

                  Comment


                  • #10
                    Okeedokee, both sides the same means celubriously great or identically screwed, I'll lean toward the former..... sounds good to me Dave but time will tell. There is a fair bit of conjecture about powervalves and their useful(less)ness, even less,understanding of the importance of the the synergistic action of secondary and tertiary acoustics with the PV........ in contrast to the widely accepted exhaust port timing rationale.Crankcase comp leakage will show up as a slightly lower VE value but only at higher engine RPM (unless the seal has fallen out completely........ not likely) and only on one side.So, the bike pulls well with the valve wired open at lower RPM, definately food for thought on the powervalve menu....... but then, a lot of two stokes will. A great mod mate, certain engine tunes will lend well to a PV too. As for 500mi run in.......... you make up your mind on that one. It won't hurt if done right but totally unnecessary, if it wants to pull up the scale let it, just don't force it if it feels tight. Has Nath got a running bike? What's the punchline?

                    Comment


                    • #11
                      Originally posted by GD Engineering (Gavin) View Post
                      Has Nath got a running bike? What's the punchline?
                      Well.. it ran before took it apart.. I guess it will run again once I put it back together

                      Comment


                      • #12
                        Originally posted by GD Engineering (Gavin) View Post
                        So, the bike pulls well with the valve wired open at lower RPM, definately food for thought on the powervalve menu....... but then, a lot of two stokes will. A great mod mate, certain engine tunes will lend well to a PV too.

                        Has Nath got a running bike? What's the punchline?
                        imho the bike runs perfectly fine with pv's wide open.. maybe a little more urrg urrr uhhh blarrr at low rpm and low throttle openings but as soon as you apply load to the motor it stops complaining

                        i dont think they are worth the maintenance required to keep that collar thingy lining them up perfectly, would much rather tune it with a file to match both sides up with the ports then set and forget it

                        yeah nath has had his 350cc motor running for a while now although i am not sure he has put more than a tank or two through it.
                        Originally posted by Bendito
                        If we get to a stop and we are missing a dozen bikes and you are last, it was your fault. Don't be that guy. No one likes that guy.

                        Comment


                        • #13
                          Fuck.. I hate it when they "urrg urrr uhhh blarrr"

                          This is the noise I imagine you make when constructing poll's.

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                          • #14
                            have you ridden the 350 with greg's pipes yet?
                            Originally posted by Bendito
                            If we get to a stop and we are missing a dozen bikes and you are last, it was your fault. Don't be that guy. No one likes that guy.

                            Comment


                            • #15
                              Originally posted by g0zer View Post
                              maybe a little more urrg urrr uhhh blarrr at low rpm and low throttle openings
                              http://www.perthstreetbikes.com/foru...-noises-34488/

                              2006 Yamaha R1SP;
                              In an interview with the New Scientist magazine marking his birthday, Stephen Hawking was asked what he thought about most during the day, and replied: "Women. They are a complete mystery."
                              Amen, brother. Amen.
                              My nemesis; Barfridge

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