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The rebuild was successful thanks to OBT

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This topic contains 6 replies, has 0 voices, and was last updated by  Roy 13 years, 8 months ago.

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  • August 21, 2011 at 2:09 pm #100371

    Roy
    Member

    The bike is back together but a big thanks must go to all that gave me advice especially White Rocket for sending me the KTM bible and Axel for his circlip advice.

    In the end I did pull the powervalve apart and clean it. Wouldn’t have been able to do it without the bible. I didn’t really understand how to fix dimension Z until I saw that and I don’t think any written advice I got here would have got me through. Pictures do say 1000 words.

    Checked the reeds. all good.

    Had a little trouble getting the torx screws out that hold the powervalve locking plates in. Ended having to replace one of those that was a little worse for wear by the time it was extracted.

    Putting the piston circlips in really challenged my patience. Watched axels thread. Still couldn’t do it and was calling myself every name under the sun. Settled down and the first one finally went in when I manned up and gave it a proper go. If I could have gotten hold of whoever designed that extra rim on the outside of the clip slot he would not have any testicles left by now. Then I still had to get the one done on the bike.

    Thought I would put the piston in so that the clip that needed inserting was on the kickstarter side as it had more room. Luckily for me I then remembered that the piston was meant to go in one way only and of course I had it sitting the wrong way. This si why I like to read as much info beforehand as I can so these thinks stay in my head during times of frustration. So pin out piston turned around and pin back in. To my surprise the clip went in a little easier this time. I reckon 9 or 10 more and I might have it almost down pat. No scratches on the piston though so still happy.

    Then for the head. Started with the 0.5mm gasket. Had to get my wife to hold the bore whilst I held the rings and slipped it into place. If someone can do all that by themselves then I take my hat off to them. Of course the ruler test went tap-tap. So off again and on again with 0.6mm, again tap-tap. I thought I must have stepped on a chinamans grave in a previous life. Was getting a little concerned now as my only options left were 0.7mm and then 0.9mm. The wife patience was wearing a little thin as well by the third go. Anyway 0.7mm on and no tap-tap. I’d hit the jackpot.

    Put the rest together and then thought I’d better wait till the next morning to give the gasket goo in the exhaust time to settle. So of course, all night my little brain was going through everything I had done step by step and thankfully if came up with no further mishaps.

    First thing Sunday out I go and second kick it burst to life. Sounded a lot better than it did before and no funny unexpected noises so all good. Let it warm up for a couple of minutes and then gave my daughter a ride up and down the back lane in celabration.

    I’m no speed freak in the mechanics stakes though. Seemed like it took me all day. At least it’s all done and it did feel good when she fired up.

    Thanks again to all that gave me advice. It certainly did help and I would have been stuck without it.

    August 21, 2011 at 2:14 pm #207480

    Roy
    Member

    Following my mechanical investigations I now have a few questions on how things work.

    1) What do the reeds do? My assumption is they let fuel through when there is a vacuum created in the cylinder and prevent backflow when there is a positive pressure in the cylinder.

    2) The science of the powervalve. I gather when the powervalve is open it lets the exhaust back into the cylinder better to give you that little bit better air volume for a more powerful compression. What causes the powervalve to come open. Is it just a revs thing overcoming some sort of centrifugal force? What do the two little openings that come open either side of the main exhaust do to help. I know they are closed when the powervalve opening is minimised and open when it opens but how much difference do they make and how?

    Thanks

    August 21, 2011 at 3:59 pm #207483

    Bruce Curtis
    Member
    Bundyroy wrote:
    Following my mechanical investigations I now have a few questions on how things work.

    1) What do the reeds do? My assumption is they let fuel through when there is a vacuum created in the cylinder and prevent backflow when there is a positive pressure in the cylinder.

    2) The science of the powervalve. I gather when the powervalve is open it lets the exhaust back into the cylinder better to give you that little bit better air volume for a more powerful compression. What causes the powervalve to come open. Is it just a revs thing overcoming some sort of centrifugal force? What do the two little openings that come open either side of the main exhaust do to help. I know they are closed when the powervalve opening is minimised and open when it opens but how much difference do they make and how?

    Thanks

    First up, congrats Roy ona job well done.

    Secondly, not far off in the reeds stakes, they were one of the huge improvements for 2T when boyeson developed them, they are a viable alternative to disc-valves and the 2Ts never looked back once they became commonplace.

    Ok P/Valve is generally on dirtbikes centrifugal forces higher revs=opens more, the side-valves are auxillary and do make a big difference at topend rev flow, they are a little step forward too, the trick is to not have the valves open too quickly and to match the valve opening time to the inherent power-curve the engine is designed for.
    The best and I mean the best have always been servo-motor controlled by an ECU, but on a dirtbike this could and has led to failures when there is moisture ingress.
    so go out, ride enjoy and do it again in around 80-110 hours time

    August 21, 2011 at 6:29 pm #207481

    Aaron Wilde
    Member

    They were trialing rotary type valves for use in 4T formula 1 engines but it failed. I am no expert but rotax engines had rotary valves deeeerrrr. But they also had reed valves like on my 950 2T jet ski 1999. As for the power valves. I was under the impression that they gave more back pressure to exhaust gasses to give them more power at low revs??? But opened up at higher rpm to let it breath and let the expansion chamber do its stuff. ?? My RGV 250m had two or three stages the powervalve would pull back as it went up the rev range. If this is not true I know you will tell me :laugh: Look up Wikipedia 2t there is heaps of stuff there.

    Oh and well done by the way on the rebuild. I will send my WR around for you to have a go at fixing it too. :laugh: :silly:

    August 21, 2011 at 7:03 pm #207482

    Alex
    Member

    Champion effort Roy. You can now charge to do everyone elses! I told you that manual would save some explaining.
    I was a little worried about you setting the Z dimension but if you don’t hear the piston hitting the PV flap then your all good! :)
    It still takes me most of the day, I check everything 10 times over as I don’t do them very often (had a nasty 4T for a while – good riddance)
    Yep the circlip in piston dance is best described with words best left unsaid. Why in the F*&^*&^*&^ would they put that extra rim/step/level in I don’t know.
    0.7 eh? That’s a 0.5 and a 0.2 then as the next size is a 0.75. Happy to see my trick helped. It’s a prick of a job getting the piston to exactly TDC as I said so rocking it like that is a sure fire way.
    As for the torx screws consider yourself lucky they phased out the phillips head screws before it. They chewed out twice as easy. If you don’t have one already the cheap impact driver/socket combo is good to get that sort of stuff out. If you have the extra dollars (highly recommended) you would buy the cordless impact drill that plays nicer and is good for a million other uses.

    August 21, 2011 at 7:12 pm #207490

    Alex
    Member

    Yep as Bruce said, reeds stop the charge from coming back out the inlet during the transfer part of the stroke on it’s way to where the action is. Good riddance to the daisy wheel valves of old.
    Lifting the exhaust ports higher in the cylinder results at better power at higher RPM but compromises lower RPM power (becomes a peaky bike). So why not have the best of both worlds and have the ports change depending on RPM – win win.
    My WR200 here has a stepper motor PV and as Bruce said they have a habit of dying and being hard to replace/expensive.

    August 21, 2011 at 7:40 pm #207492

    Roy
    Member

    One more thing that had me a bit cofuffled.

    When you take the exhaust flange of the front of the cylinder there a couple of holes with o-rings on them on each side in line with the pivot point of the power valve flap. They open up to a gallery that does a loop around the flange and back into the head on each side of the exhaust. I gather these are water galleries that keep the pivot cool. The other option I thought of is they are oil galleries to keep it lubed but then it would only get splash oil and I didn’t think that would work too well. Hope this description sounds better to you than it does to me.

    PS. Alex I was also worried about the z dimension thing until I read the book. Wouldn’t have even attempted it if I hadn’t got the book.

    PS. Aaron, bring it round mate, I’ll give anything a go, especially if it’s not mine. From memory the yamaha manual was pretty good as long as you needed everything in five different languages.

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