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2 St - Primary Compression Ratio?

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  • Dan McManus
    Would anyone have a guess on what the primary compression ratio would be on a modern 2 stroke trials engine? I m trying to estimate what it would take to
    Message 1 of 24 , Jun 24, 2013
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      Would anyone have a guess on what the primary compression ratio would be on a modern 2 stroke trials engine?  I'm trying to estimate what it would take to achieve the OEM primary compression ratio after increasing size from 200 to 250cc.  Using guesses of 1.5 and 1.2, I calculated the crankcase volume would have to increase 100 to 250 cc.  Does this seem reasonable?  See the spread sheet.:
       
       
      I'm still fighting popping in the exhaust and a dealer suggested that this may be the issue.
       
      Thanks
    • Ken Augustine
      Dan McM,     Do you   NEED    power, torque or practice?     Who says the compression ratios need adjustment from where they are now, why is that and
      Message 2 of 24 , Jun 24, 2013
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        Dan McM,

            Do you   NEED    power, torque or practice?

            Who says the compression ratios need adjustment from where they are now, why is that and in which direction should changes be made?   I'd leave the crankcase compression ratio alone and likely the cylinder compression ratio too.   Try it, note the response then add head gaskets to bring it down a taste and retest.

            For deceleration popping in the exhaust, I'd check for exhaust leaks then change the idle mixture and retest.

        Ken A

        Subject: 2 St - Primary Compression Ratio?

         
        Would anyone have a guess on what the primary compression ratio would be on a modern 2 stroke trials engine?  I'm trying to estimate what it would take to achieve the OEM primary compression ratio after increasing size from 200 to 250cc.  Using guesses of 1.5 and 1.2, I calculated the crankcase volume would have to increase 100 to 250 cc.  Does this seem reasonable?  See the spread sheet.:
         
         
        I'm still fighting popping in the exhaust and a dealer suggested that this may be the issue.
         
        Thanks
        _

      • Ian
        ... There was a time when tuners chased high PCR s, but I recall that it was found that too high a PCR becomes detrimental. ( Obviously too low is a problem
        Message 3 of 24 , Jun 24, 2013
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          Who says the compression ratios need adjustment from where they are now, why is that and in which direction should changes be made? 


          There was a time when tuners chased high PCR's, but I recall
          that it was found that too high a PCR becomes detrimental.
          ( Obviously too low is a problem too )


          Cheers               IAN
        • johnayleng@...
          There was a time when tuners chased high PCR s, but I recall that it was found that too high a PCR becomes detrimental. ( Obviously too low is a problem too
          Message 4 of 24 , Jun 24, 2013
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            There was a time when tuners chased high PCR's, but I recall
            that it was found that too high a PCR becomes detrimental.
            ( Obviously too low is a problem too )


            Cheers               IAN
             
             HIgh CC compression is detrimental to a good expansion chamber. 1.5 is on the high side.
            JA

          • Ken Augustine
            The primary compression ratio choice depends entirely upon in what range the motor is designed to operate.   Higher crank case compression ratios are better
            Message 5 of 24 , Jun 24, 2013
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              The primary compression ratio choice depends entirely upon in what range the motor is designed to operate.   Higher crank case compression ratios are better for low rpm and lower crank case compression ratios are better for high rpm.   The explanations have to do with less cushioning and higher static pressures at low rpm with high compression ratios and less resistance plus higher capacitance at high rpm with low crankcase compression ratios.  

              Ken A


              There was a time when tuners chased high PCR's, but I recall
              that it was found that too high a PCR becomes detrimental.
              ( Obviously too low is a problem too )


              Cheers               IAN
               
               HIgh CC compression is detrimental to a good expansion chamber. 1.5 is on the high side.
              JA
            • Ian
              ... Interesting Ken, I ve never heard it put that way. Still, long time since I ve played with 2 bangers. Cheers IAN
              Message 6 of 24 , Jun 24, 2013
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                The primary compression ratio choice depends entirely upon in what range the motor is designed to operate.   Higher crank case compression ratios are better for low rpm and lower crank case compression ratios are better for high rpm.  


                Interesting Ken, I've never heard it put that way.

                Still, long time since I've played with 2 bangers.


                Cheers                 IAN

              • Brad Heiter
                Years back, about 2000, I asked Kevin Cameron what the best PCR was and he said that modern 2 strokes with modern large volume expansion chambers don t push
                Message 7 of 24 , Jun 24, 2013
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                  Years back, about 2000, I asked Kevin Cameron what the best PCR was and he said that modern 2 strokes with modern large volume expansion chambers don't push the charge up from the CC they pull it up with the exhaust pipe and they do it more effectively with more CC volume.  My question was specific to roadrace engines but it 'probably' applies to most two-strokes.
                  Brad H 


                  On Mon, Jun 24, 2013 at 2:16 PM, Dan McManus <dan.mcmanus5@...> wrote:
                   

                  Would anyone have a guess on what the primary compression ratio would be on a modern 2 stroke trials engine?  I'm trying to estimate what it would take to achieve the OEM primary compression ratio after increasing size from 200 to 250cc.  Using guesses of 1.5 and 1.2, I calculated the crankcase volume would have to increase 100 to 250 cc.  Does this seem reasonable?  See the spread sheet.:
                   
                   
                  I'm still fighting popping in the exhaust and a dealer suggested that this may be the issue.
                   
                  Thanks


                • Ken Augustine
                  Ian,     The key to all this is to realize the crossover from one pressure and resistance source set to another as the crankshaft speed changes.      
                  Message 8 of 24 , Jun 24, 2013
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                    Ian,

                        The key to all this is to realize the crossover from one pressure and resistance source set to another as the crankshaft speed changes.  

                        At extremely low rpm, the compression ratio should be infinite from zero crankcase volume at BDC and thus, all that is drawn in, is compressed which raises the pressure in the crankcase which would ultimately hydraulically lock except the transfer ports open before the piston reaches BDC, thus relieving the lock.   However, as the piston continues down, with zero volume at BDC,   ALL   of the inlet charge must be transferred to the cylinder.   This is how hydraulic pumps, grease guns or bicycle tire pumps function and why they don't work when they are air bound by a bubble of air or extra compressible volume if BDC is not reached and most of what they then do is to slightly compress that bit of pneumatic cushion which reduces the delivery pressure.   All this is understood at extremely low rpm as would be expected in the trial;s engine about which this conjecture thread started.  

                        However, at high rpm, resonances from the expansion chamber completely overcome any primary crankcase pumping pulses and the extra capacitance in larger crankcase volumes plus what has to be considered reduced flow resistance in the larger passages under the piston and along side the crank cheeks simply gives a larger volume of charge on which the pipe can draw with correspondingly less pressure reduction against which the pipe pulses must work.    If you add to that, the now increased volume between the currently ubiquitous inlet reeds with direct passages from the reeds straight to the transfer ports, the further increased volume simply produces less flow resistance and yet more volume from which the pipe pulses can draw.  

                        In high rpm 2 stroke racing engines, all of the steel, aluminum, fiber and rubber pieces are just pneumatic and mechanical linkages from the pipe to the ground.   There are even self regulating resonance tuned stationary 2S pump engines which have no throttle or governor and the resonance alone limits both the lower and upper rpm as the pump load varies with the water height.   They aren't common in motorcycling but that was the market from where the Konig evolved.  

                    Ken A  



                    From: Ian <iwd@...>
                    To: mc-engine@yahoogroups.com
                    Sent: Monday, June 24, 2013 5:17 PM
                    Subject: Re: 2 St - Primary Compression Ratio?

                     
                     
                    The primary compression ratio choice depends entirely upon in what range the motor is designed to operate.   Higher crank case compression ratios are better for low rpm and lower crank case compression ratios are better for high rpm.  


                    Interesting Ken, I've never heard it put that way.

                    Still, long time since I've played with 2 bangers.


                    Cheers                 IAN



                  • hans_o_man
                    Hi there, I made following experience: I increased compression ratio on my 86 TY250-Yamaha(trials) by welding and reshaping the combustion chamber to a more
                    Message 9 of 24 , Jun 24, 2013
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                      Hi there,
                      I made following experience: I increased compression ratio on my '86 TY250-Yamaha(trials) by welding and reshaping the combustion chamber to a more "modern" dome-shape. Before cr was around 11:1, now it's about 13:1. That's what modern competition trialsbikes have (so far I could find out..) Now it's popping badly into the exhaust, what makes slow riding with a bit more than idle-gas problematic . I'm still fiddling around with the carb adjustment, but it's difficult to find jets for the 26mm teikei. Possibly I have to change the slide-cut, but for easier bike-handling simplest would be to remount the old low compression head..
                      So my 5¢: dont't think about primary compression. I belive you'll find the answer in the secondary ...

                      Cheers,
                      Hans
                    • marshengnz
                      One of my racing user forums was postulating about having the crankcase open to the carb all the time. The transfer from crank to cylinder would be done but
                      Message 10 of 24 , Jun 25, 2013
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                        One of my racing user forums was postulating about having the crankcase open to the carb all the time. The transfer from crank to cylinder would be done but the expansion pipe alone. Basically running like a pulse jet but using a piston to trap the energy.

                        The reed valves would only be used for starting, once up to rpm the reed valves could be removed. !!!!

                        I go with adding head or base gaskets and see what happens.

                        If you want to make more power, there are many packages out there to model the motor. We have 2 chaps her in NZ that are very good at modeling motors and I could put you in touch with them.

                        Otherwise here is a good forum
                        http://www.kiwibiker.co.nz/forums/showthread.php/86554-ESE-s-works-engine-tuner/page800?highlight=ese

                        Just short of 1 million views and 12k posts. Mostly about 2 strokes.

                        Cheers Wallace
                      • Swiss
                        Wallace, Tried to register on the Forum but it asks Kiwi questions that I don t know the answer to? Things like City with a soft drink named after it and city
                        Message 11 of 24 , Jun 25, 2013
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                          Wallace,

                           

                          Tried to register on the Forum but it asks Kiwi questions that I don’t know the answer to?  Things like City with a soft drink named after it and city that smells of sulfur? 

                           

                          Swiss

                           

                          From: mc-engine@yahoogroups.com [mailto:mc-engine@yahoogroups.com] On Behalf Of marshengnz
                          Sent: Tuesday, June 25, 2013 2:57 PM
                          To: mc-engine@yahoogroups.com
                          Subject: Re: 2 St - Primary Compression Ratio?

                           

                           

                          One of my racing user forums was postulating about having the crankcase open to the carb all the time. The transfer from crank to cylinder would be done but the expansion pipe alone. Basically running like a pulse jet but using a piston to trap the energy.

                          The reed valves would only be used for starting, once up to rpm the reed valves could be removed. !!!!

                          I go with adding head or base gaskets and see what happens.

                          If you want to make more power, there are many packages out there to model the motor. We have 2 chaps her in NZ that are very good at modeling motors and I could put you in touch with them.

                          Otherwise here is a good forum
                          http://www.kiwibiker.co.nz/forums/showthread.php/86554-ESE-s-works-engine-tuner/page800?highlight=ese

                          Just short of 1 million views and 12k posts. Mostly about 2 strokes.

                          Cheers Wallace

                        • Ian
                          ... Coke and Helios.......... ( You might have to have Wallace on-line whilst you register ) Cheers IAN
                          Message 12 of 24 , Jun 25, 2013
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                              Tried to register on the Forum but it asks Kiwi questions that I don’t know the answer to?  Things like City with a soft drink named after it and city that smells of sulfur? 


                            Coke and Helios.......... 

                            ( You might have to have Wallace on-line whilst you register )



                            Cheers            IAN

                          • Ken Augustine
                            I have heard of something similar.   BTW, if you put a complete carb and reed inlet system from a late model 2/S engine on a flow bench then draw with it
                            Message 13 of 24 , Jun 25, 2013
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                              I have heard of something similar.  

                              BTW, if you put a complete carb and reed inlet system from a late model 2/S engine on a flow bench then draw with it hard enough to produce the flow that 4/S coefficients dictate to make the power that th test configure is confirmed to make, the reeds hardly open.   Even if you draw on it harder to make say 50% more flow than 4/S coefficients state which might compensate for flow lost out the exhaust port and thus the relatively fuel mileage that 2/S engines yield, the reeds still barely open.   However reeds and cages  always show witness and wear marks which clearly show that the reeds have been contacting their stops at pulsed deflections many times greater than are seen on the steady state flow bench even at flow rates double of what 4/S coefficients indicate.  

                              Ken A



                              From: marshengnz <marshland@...>
                              To: mc-engine@yahoogroups.com
                              Sent: Tuesday, June 25, 2013 1:56 PM
                              Subject: Re: 2 St - Primary Compression Ratio?

                               
                              One of my racing user forums was postulating about having the crankcase open to the carb all the time. The transfer from crank to cylinder would be done but the expansion pipe alone. Basically running like a pulse jet but using a piston to trap the energy.

                              The reed valves would only be used for starting, once up to rpm the reed valves could be removed. !!!!

                              I go with adding head or base gaskets and see what happens.

                              If you want to make more power, there are many packages out there to model the motor. We have 2 chaps her in NZ that are very good at modeling motors and I could put you in touch with them.

                              Otherwise here is a good forum
                              http://www.kiwibiker.co.nz/forums/showthread.php/86554-ESE-s-works-engine-tuner/page800?highlight=ese

                              Just short of 1 million views and 12k posts. Mostly about 2 strokes.

                              Cheers Wallace



                            • Swiss
                              Yes, I tried both Coke and Helios for answers (they rotate the question) and it wouldn t accept either. It won t let you view the saved jpg. Images etc.
                              Message 14 of 24 , Jun 25, 2013
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                                Yes, I tried both Coke and Helios for answers (they rotate the question) and it wouldn’t accept either.  It won’t let you view the saved jpg. Images etc. unless you are registered.

                                 

                                Swiss

                                 

                                From: mc-engine@yahoogroups.com [mailto:mc-engine@yahoogroups.com] On Behalf Of Ian
                                Sent: Tuesday, June 25, 2013 4:38 PM
                                To: mc-engine@yahoogroups.com
                                Subject: Re: 2 St - Primary Compression Ratio?

                                 

                                 

                                 Tried to register on the Forum but it asks Kiwi questions that I don’t know the answer to?  Things like City with a soft drink named after it and city that smells of sulfur? 


                                Coke and Helios.......... 

                                ( You might have to have Wallace on-line whilst you register )



                                Cheers            IAN

                              • Dan McManus
                                Interesting. Thanks for this. Published compression for this engine was 11.4 but it would knock until I lowered to about 10.5. I m down to about 9 now and
                                Message 15 of 24 , Jun 25, 2013
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                                  Interesting.  Thanks for this.  Published compression for this engine was 11.4 but it would knock until I lowered to about 10.5.  I'm down to about 9 now and the power feels soft but it still pops.  Interestingly, the new generation engine is published at around 9 also.  Although..... that knock happened in the same exact type of conditions as this pop.  (The pop was also there when it knocked.)

                                  On Mon, Jun 24, 2013 at 11:47 PM, hans_o_man <der_hiha@...> wrote:
                                   

                                  Hi there,
                                  I made following experience: I increased compression ratio on my '86 TY250-Yamaha(trials) by welding and reshaping the combustion chamber to a more "modern" dome-shape. Before cr was around 11:1, now it's about 13:1. That's what modern competition trialsbikes have (so far I could find out..) Now it's popping badly into the exhaust, what makes slow riding with a bit more than idle-gas problematic . I'm still fiddling around with the carb adjustment, but it's difficult to find jets for the 26mm teikei. Possibly I have to change the slide-cut, but for easier bike-handling simplest would be to remount the old low compression head..
                                  So my 5¢: dont't think about primary compression. I belive you'll find the answer in the secondary ...

                                  Cheers,
                                  Hans


                                • Ken Augustine
                                  Have you tried using decent gasoline?   Even using 100 low lead avgas can make the difference you need.   Ken A Interesting.  Thanks for this.  Published
                                  Message 16 of 24 , Jun 25, 2013
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                                    Have you tried using decent gasoline?   Even using 100 low lead avgas can make the difference you need.   Ken A


                                    Interesting.  Thanks for this.  Published compression for this engine was 11.4 but it would knock until I lowered to about 10.5.  I'm down to about 9 now and the power feels soft but it still pops.  Interestingly, the new generation engine is published at around 9 also.  Although..... that knock happened in the same exact type of conditions as this pop.  (The pop was also there when it knocked.)

                                    On Mon, Jun 24, 2013 at 11:47 PM, hans_o_man <der_hiha@...> wrote:
                                     
                                    Hi there,
                                    I made following experience: I increased compression ratio on my '86 TY250-Yamaha(trials) by welding and reshaping the combustion chamber to a more "modern" dome-shape. Before cr was around 11:1, now it's about 13:1. That's what modern competition trialsbikes have (so far I could find out..) Now it's popping badly into the exhaust, what makes slow riding with a bit more than idle-gas problematic . I'm still fiddling around with the carb adjustment, but it's difficult to find jets for the 26mm teikei. Possibly I have to change the slide-cut, but for easier bike-handling simplest would be to remount the old low compression head..
                                    So my 5¢: dont't think about primary compression. I belive you'll find the answer in the secondary ...

                                    Cheers,
                                    Hans




                                  • Caparo
                                    ... Swiss, soft drink is paeroa according to Google :-} and yes it is correct. -- TTFN Caparo.
                                    Message 17 of 24 , Jun 26, 2013
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                                      On Tuesday 25 June 2013 22:57:52 Swiss wrote:
                                      > Wallace,
                                      >
                                      > Tried to register on the Forum but it asks Kiwi questions that I don't know
                                      > the answer to? Things like City with a soft drink named after it and city
                                      > that smells of sulfur?
                                      >
                                      > Swiss
                                      >
                                      >
                                      Swiss,
                                      soft drink is paeroa according to Google :-} and yes it is correct.

                                      --
                                      TTFN
                                      Caparo.
                                    • Swiss
                                      Thanks, got in with the Paeroa answer. From: mc-engine@yahoogroups.com [mailto:mc-engine@yahoogroups.com] On Behalf Of Caparo Sent: Wednesday, June 26, 2013
                                      Message 18 of 24 , Jun 26, 2013
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                                        Thanks, got in with the Paeroa answer.

                                         

                                        From: mc-engine@yahoogroups.com [mailto:mc-engine@yahoogroups.com] On Behalf Of Caparo
                                        Sent: Wednesday, June 26, 2013 4:03 AM
                                        To: mc-engine@yahoogroups.com
                                        Subject: Re: 2 St - Primary Compression Ratio?

                                         

                                         

                                        On Tuesday 25 June 2013 22:57:52 Swiss wrote:

                                        > Wallace,
                                        >
                                        > Tried to register on the Forum but it asks Kiwi questions that I don't know
                                        > the answer to? Things like City with a soft drink named after it and city
                                        > that smells of sulfur?
                                        >
                                        > Swiss
                                        >
                                        >
                                        Swiss,
                                        soft drink is paeroa according to Google :-} and yes it is correct.

                                        --
                                        TTFN
                                        Caparo.

                                      • marshengnz
                                        Things like City with a soft drink named L&P - Lemon and Paeroa ... Rotorua I even had to do a web search to get the spelling right. We have Jan Theil on the
                                        Message 19 of 24 , Jun 26, 2013
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                                          Things like City with a soft drink named

                                          L&P - Lemon and Paeroa
                                          > that smells of sulfur?
                                          Rotorua

                                          I even had to do a web search to get the spelling right.

                                          We have Jan Theil on the forum from from Denmark or the Netherlands.

                                          Cheers Wallace
                                        • marshengnz
                                          ... Just been talking about this recently. The flow bench is just a simulation. Think on this. 1 m3 of air weighs 1.2 kg. Now take your intake charge with fuel
                                          Message 20 of 24 , Jun 26, 2013
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                                            --- In mc-engine@yahoogroups.com, Ken Augustine <kineticanalysis@...> wrote:
                                            >
                                            > I have heard of something similar.  
                                            >
                                            > BTW, if you put a complete carb and reed inlet system from a late model 2/S engine on a flow bench then draw with it hard enough to produce the flow that 4/S coefficients dictate to make the power that th test configure is confirmed to make, the reeds hardly open.   Even if you draw on it harder to make say 50% more flow than 4/S coefficients state which might compensate for flow lost out the exhaust port and thus the relatively fuel mileage that 2/S engines yield, the reeds still barely open.   However reeds and cages  always show witness and wear marks which clearly show that the reeds have been contacting their stops at pulsed deflections many times greater than are seen on the steady state flow bench even at flow rates double of what 4/S coefficients indicate.  
                                            >

                                            Just been talking about this recently. The flow bench is just a simulation. Think on this.

                                            1 m3 of air weighs 1.2 kg. Now take your intake charge with fuel and run it at Mach 0.5.

                                            The reeds hitting the stops now seems possible.

                                            Cheers Wallace
                                          • hans_o_man
                                            Did you measure the squish, means, the clearance between squishband of the head to piston-dome? If you rise it to more than 1,5mm, the engine probably will
                                            Message 21 of 24 , Jun 26, 2013
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                                              Did you measure the squish, means, the clearance between squishband of the head to piston-dome? If you rise it to more than 1,5mm, the engine probably will knock.
                                              Useful literature is Alexander Graham Bells two stroke tuning book.
                                              You may contact me per email about it...

                                              Cheers,
                                              Hans
                                              --- In mc-engine@yahoogroups.com, Ken Augustine <kineticanalysis@...> wrote:
                                              >
                                              > Have you tried using decent gasoline?   Even using 100 low lead avgas can make the difference you need.   Ken A
                                              >
                                              >
                                              >
                                              >
                                              > Interesting.  Thanks for this.  Published compression for this engine was 11.4 but it would knock until I lowered to about 10.5.  I'm down to about 9 now and the power feels soft but it still pops.  Interestingly, the new generation engine is published at around 9 also.  Although..... that knock happened in the same exact type of conditions as this pop.  (The pop was also there when it knocked.)
                                              > >
                                              > >
                                              > >
                                              > >On Mon, Jun 24, 2013 at 11:47 PM, hans_o_man <der_hiha@...> wrote:
                                              > >
                                              > >
                                              > >> 
                                              > >>Hi there,
                                              > >>I made following experience: I increased compression ratio on my '86 TY250-Yamaha(trials) by welding and reshaping the combustion chamber to a more "modern" dome-shape. Before cr was around 11:1, now it's about 13:1. That's what modern competition trialsbikes have (so far I could find out..) Now it's popping badly into the exhaust, what makes slow riding with a bit more than idle-gas problematic . I'm still fiddling around with the carb adjustment, but it's difficult to find jets for the 26mm teikei. Possibly I have to change the slide-cut, but for easier bike-handling simplest would be to remount the old low compression head..
                                              > >>So my 5¢: dont't think about primary compression. I belive you'll find the answer in the secondary ...
                                              > >>
                                              > >>Cheers,
                                              > >>Hans
                                              > >>
                                              > >>
                                              > >
                                              > >
                                              > >
                                              > >
                                              >
                                            • Dan McManus
                                              I have the tuning book. Most of the assemblies have been with a 0.055 squish but it is now up to about .0.082 as I m trying lower compression to see what it
                                              Message 22 of 24 , Jun 27, 2013
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                                                I have the tuning book.  Most of the assemblies have been with a 0.055" squish but it is now up to about .0.082 as I'm trying lower compression to see what it does for the popping.  The challenge here is that reducing squish requires removing material in the non squish area to maintain compression.  I don't know how far I can go before hitting the water passage in the head.  I have all ready made it about 0.125" deeper.  Based on all of the input, I have more things to try.  Thanks for everyone's input.
                                                1. Try best race gas
                                                2. smaller slide cutaway (I finally found one and have it on order.)
                                                3. Reduce squish even further - This may require making a head with removable chamber so various chambers shapes/ sizes can be tried easily without cutting into water passage.
                                                4. Bring primary compression back to OEM spec.  - This is a lot of work and I don't even know if the crank has stuffers.  Most listers do not think this could be the cause.  Plus I ride it every weekend so it means down time.
                                                5. I would also would like to try a different head pipe but have not found one.  I wonder if there could be a small pocket or protrusion causing some weird aerodynamic/ pressure situation.  (When blocking the air intake in a CA model NT650 exhaust port, the exhaust started to produce a chirping sound like a bird.  The hole had to be plugged down close to the exhaust port to get rid of the chirp.)

                                                Thanks,


                                                Dan


                                                On Wed, Jun 26, 2013 at 10:29 PM, hans_o_man <der_hiha@...> wrote:
                                                 

                                                Did you measure the squish, means, the clearance between squishband of the head to piston-dome? If you rise it to more than 1,5mm, the engine probably will knock.
                                                Useful literature is Alexander Graham Bells two stroke tuning book.
                                                You may contact me per email about it...

                                                Cheers,
                                                Hans


                                                --- In mc-engine@yahoogroups.com, Ken Augustine <kineticanalysis@...> wrote:
                                                >
                                                > Have you tried using decent gasoline?   Even using 100 low lead avgas can make the difference you need.   Ken A
                                                >
                                                >
                                                >
                                                >
                                                > Interesting.  Thanks for this.  Published compression for this engine was 11.4 but it would knock until I lowered to about 10.5.  I'm down to about 9 now and the power feels soft but it still pops.  Interestingly, the new generation engine is published at around 9 also.  Although..... that knock happened in the same exact type of conditions as this pop.  (The pop was also there when it knocked.)
                                                > >
                                                > >
                                                > >
                                                > >On Mon, Jun 24, 2013 at 11:47 PM, hans_o_man <der_hiha@...> wrote:
                                                > >
                                                > >
                                                > >> 
                                                > >>Hi there,
                                                > >>I made following experience: I increased compression ratio on my '86 TY250-Yamaha(trials) by welding and reshaping the combustion chamber to a more "modern" dome-shape. Before cr was around 11:1, now it's about 13:1. That's what modern competition trialsbikes have (so far I could find out..) Now it's popping badly into the exhaust, what makes slow riding with a bit more than idle-gas problematic . I'm still fiddling around with the carb adjustment, but it's difficult to find jets for the 26mm teikei. Possibly I have to change the slide-cut, but for easier bike-handling simplest would be to remount the old low compression head..
                                                > >>So my 5¢: dont't think about primary compression. I belive you'll find the answer in the secondary ...
                                                > >>
                                                > >>Cheers,
                                                > >>Hans
                                                > >>
                                                > >>
                                                > >
                                                > >
                                                > >
                                                > >
                                                >


                                              • Duane Forth
                                                On Thu, Jun 27, 2013 at 8:31 AM, Dan McManus wrote: The challenge here is that reducing ... Just spit-balling here... could the piston
                                                Message 23 of 24 , Jun 27, 2013
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                                                  On Thu, Jun 27, 2013 at 8:31 AM, Dan McManus <dan.mcmanus5@...> wrote:
                                                  The challenge here is that reducing
                                                  > squish requires removing material in the non squish area to maintain
                                                  > compression. I don't know how far I can go before hitting the water passage
                                                  > in the head.

                                                  Just spit-balling here... could the piston crown be dished a bit to
                                                  reduce compression if you tighten the squish up some more?
                                                • hans_o_man
                                                  ... Keep us informed. I m very interested... :-) ... Smaller cutaway makes mixture rich. I feel, you d need a bigger cutaway. (but it s only a feeling ..) ...
                                                  Message 24 of 24 , Jun 27, 2013
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                                                    > on all of the input, I have more things to try. Thanks for everyone's
                                                    > input.
                                                    Keep us informed. I'm very interested... :-)
                                                    > 2. smaller slide cutaway (I finally found one and have it on order.)

                                                    Smaller cutaway makes mixture rich. I feel, you'd need a bigger cutaway. (but it's only a feeling ..)

                                                    > this could be the cause. Plus I ride it every weekend so it means down
                                                    > time.
                                                    Get a spare engine. Best for systematic adjustments/tuning.

                                                    Cheers and success,
                                                    Hans
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