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  • #16
    only way you could find out would be to dyno the motor before and after to see if HP increased

    But then they may have changed the jets also

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    • #17
      Having fooled around for decades with racing 2 strokes, I can assure you, almost anything you do makes minute (at best) improvement, some work can actually step you backwards, it's very much trial and error, you say they "decked" the head, might give you about a 0.01HP increase (or not) you "ported" it, might give the same increase, or set you back more than that amount, most motors are pretty well refined off the production line, so, without major engineering, you might or might not get more power, or use less fuel, either case, it will be almost immeasurable.

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      • #18
        from what I understand on my C40, I could replace the tuner and re-jet the carbs to turn it into a C50.
        my guess is the tuner allows more air flow out so more air gets in and the jets let more fuel into the motor.

        a previous owner of my motor drilled the jets bigger, it took me about a year to finally figure out why I had a high speed miss and ended up replacing the jets to solve it

        strange part of the results you use to be able to see in the performance Bulletins between the 40 and 50 they showed the same prop on the same boat set up.
        I would think a 25% incease in HP would call for a different prop
        Yamaha site does not show the old 2 stroke bulletins anymore for some reason.

        Op might look up what the differences are between his 90 and a 115 to see what all is different.
        I take it those are the same block

        from what I remember about porting and polishing automotive motors, it allowed larger carbs to ram larger amounts of air and fuel into the motor as long as you put headers on reducing the back pressure. But the polishing also reduced the turbulence of the incoming air/fuel so it did not mix as well.
        no one cared if it was all burnt properly and the loss of fuel economy
        Last edited by 99yam40; 1 week ago.

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        • #19
          90HP is an in line 3 cylinder, 115HP is a V4

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          • #20
            Originally posted by ausnoelm View Post
            90HP is an in line 3 cylinder, 115HP is a V4
            well that makes me way off on that thought
            apparently I was not even thinking
            Last edited by 99yam40; 6 days ago.

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            • #21
              Iím going to try the Hydretech kit on my 90hp 2-stroke. Carbon fiber reeds, bigger jets, redesigned head, redesigned exhaust tuner.

              The block is now rebored at a machine shop because the bottom cylinder was out of round by 0.06mm, and wanted perfect bores for the kit. Piston rings were all beyond service limit by 0.05mm (0.65mm end gap) Iím putting Wiseco pistons in it.

              Wish me luck

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              • #22
                what do they say it should dyno out at

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                • #23
                  Should add 10-25hp according to their webpage. I guess it depends on if your motor is in good condition or not.

                  http://yamaha-rebuild.com/Yamaha%20High%20Perf.htm

                  I know redesigned heads on motocross engines can easily add 10%+ hp with squishband creating turbulence in the combustion chamber.
                  Last edited by holmen78; 6 days ago. Reason: Added info on motocross engines.

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                  • #24
                    If you call Hydrotec they'll steer you closer to 15hp gain with everything, but much better acceleration. That's very similar to what I'm doing. I agree that most all cylinder heads up until recently can stand to be improved. OEM's have to compromise performance, cost and manufacturability.

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                    • #25
                      Originally posted by holmen78 View Post
                      Should add 10-25hp according to their webpage. I guess it depends on if your motor is in good condition or not.

                      http://yamaha-rebuild.com/Yamaha%20High%20Perf.htm

                      I know redesigned heads on motocross engines can easily add 10%+ hp with squishband creating turbulence in the combustion chamber.
                      looks like the 90 would go up 10 the 75 and 80 would increase more. probably going up to 100 also

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                      • #26
                        Originally posted by 99yam40 View Post

                        looks like the 90 would go up 10 the 75 and 80 would increase more. probably going up to 100 also
                        That makes sence if the port timings are the same on all variants. If the port timings arent the same then I think the 90hp will gain more hp than the smaller hp motors. I know ignition timing is 2 degrees more advanced on the 90hp, and probably different carbs as well.

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                        • #27
                          Originally posted by holmen78 View Post
                          Should add 10-25hp according to their webpage. I guess it depends on if your motor is in good condition or not.

                          http://yamaha-rebuild.com/Yamaha%20High%20Perf.htm

                          I know redesigned heads on motocross engines can easily add 10%+ hp with squishband creating turbulence in the combustion chamber.
                          It would want to be a pretty fancy head to "easily" get a 10% increase, or the original head is so bad in the first place, which would be rare.

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                          • #28
                            Originally posted by ausnoelm View Post
                            It would want to be a pretty fancy head to "easily" get a 10% increase, or the original head is so bad in the first place, which would be rare.
                            Well I donít know much about this, just reading dyno tests with modified heads. This is a KTM300SX with modified head and ignition tweak. Stock vs. modified.

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