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  1. #1

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    Powervalve timing & function

    First minor rebuild. Replaced cylinder & piston. Turn shaft, pistons moving fine. Getting ready to hook powervalves back up. Noticed that they all move simultaneously, whereas pistons , of course, are sequential. My weak little mind thinks that each powervalve should move in concert with it's piston. What is the actual function of the powervalve? Is that the only change that bumped the 1200 motors from a claimed 130 hp to 150 hp? What am I missing?


  2. #2
    Yamaha Mechanic TunedFX140's Avatar
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    ok from what i can figure is that the power valves change the exhaust port timing. When they are in the down position the port timing is effectively lower in the cylinder so the burning exhaust gases push on the piston longer which should create more torque in the low to midrange. When the powervalves open (draw up) it creates shorter port timing wich i nice in the higher revs to create more of a scavenging/cylinder filling effect drawing in more fuel and air from the crankcase compressing more of this mixture creating more top end power.

    See right after the plug fires and the piston begins its trip downward and the power valves are open this allows the expanding exhaust gasses to start off into the pipe which creates a slight vacume in the cylinder, then the intake ports are uncovered this vac. in the cylinder helps draw in air and fuel in combination with the piston pushing down on the mix below helping force it into the cylinder. Now you have achieved a cross flow across the top of the piston as both intake and exhaust ports are open so some of the fuel is actually drawn into the exhaust pipe. This is where the tripple pipe helps as it is 3 individual tuned chambers the exhaust pulst that started all this hits the narrow end of the pipe reversing on itself helping push some of the fuel and air that was drawn past the piston back into the cylinder as the piston has already covered the ports for the intake it is now pushing more fuel and air in to be compressed, if the tuned pipe is timed right it will let very minimal already burnt exhaust back in to contaminate the mix, now you just got a bigger bang out of that cylinder so now there is more pressure in the exhaust to create more scavenging and draw in even more air and fuel next time. Its really not all that complicated once you figure it out. Tuned pipes and powervalves to me ar kinda like a turbocharger it is effectively forcing more fuel and air in the cylinder to be ignited creating a bigger bang, creating more go fast.

    But for the simple answer yes all the powervalves are supposed to move at the same time as. I dont think i would want to work on one that had individually electronicly controlled powervalves lol besides Y.I.E.C.P.V exceedes yamahas 4 letter abbreviation standard lol

    This may help some http://science.howstuffworks.com/tra...wo-stroke1.htm

  3. #3
    Happily Self-Employed WFO's Avatar
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    Quote Originally Posted by TunedFX140 View Post
    ok from what i can figure is that the power valves change the exhaust port timing. When they are in the down position the port timing is effectively lower in the cylinder so the burning exhaust gases push on the piston longer which should create more torque in the low to midrange. When the powervalves open (draw up) it creates shorter port timing wich i nice in the higher revs to create more of a scavenging/cylinder filling effect drawing in more fuel and air from the crankcase compressing more of this mixture creating more top end power.

    See right after the plug fires and the piston begins its trip downward and the power valves are open this allows the expanding exhaust gasses to start off into the pipe which creates a slight vacume in the cylinder, then the intake ports are uncovered this vac. in the cylinder helps draw in air and fuel in combination with the piston pushing down on the mix below helping force it into the cylinder. Now you have achieved a cross flow across the top of the piston as both intake and exhaust ports are open so some of the fuel is actually drawn into the exhaust pipe. This is where the tripple pipe helps as it is 3 individual tuned chambers the exhaust pulst that started all this hits the narrow end of the pipe reversing on itself helping push some of the fuel and air that was drawn past the piston back into the cylinder as the piston has already covered the ports for the intake it is now pushing more fuel and air in to be compressed, if the tuned pipe is timed right it will let very minimal already burnt exhaust back in to contaminate the mix, now you just got a bigger bang out of that cylinder so now there is more pressure in the exhaust to create more scavenging and draw in even more air and fuel next time. Its really not all that complicated once you figure it out. Tuned pipes and powervalves to me ar kinda like a turbocharger it is effectively forcing more fuel and air in the cylinder to be ignited creating a bigger bang, creating more go fast.

    But for the simple answer yes all the powervalves are supposed to move at the same time as. I dont think i would want to work on one that had individually electronicly controlled powervalves lol besides Y.I.E.C.P.V exceedes yamahas 4 letter abbreviation standard lol

    This may help some http://science.howstuffworks.com/tra...wo-stroke1.htm

  4. #4
    wow, That was well put! +1

  5. #5
    Yamaha Mechanic TunedFX140's Avatar
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    really think so? Dang i only been working on these things for about 2 months now. I love the smell of 2 smoke in the morning but everyone runs the cheapest oil they can find so i get to fix them alot. Its sad because i used to really like 2 strokes, but i have a nice pile of wrist pins building up maybe i will make some wind chimes out of them or something lol.

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