Yamaha DT 125 DERESTRICTING
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It’s over 57mm I think that it needs reprofiling ...
Make sure the power valve isn’t 180 degrees out. As long as it is fully open when the engine is off it’s ok. That and the cables are right. You should notice a huge increase in performance. Let us know how you get on...
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Best way of checking the valve is open when it should be, is to rev the bike over 7k. Then turn the ignition off (while it's above 7k). Then check the position of the valve. From what I've heard on here, different years operate the cleaning cycle etc of the valve differently.
If it's revving out fine in fifth. It might just be down to a not 100% healthy engine not having the power to pull 6th gear well. -
So when it is closed (when the bike is off) it looks like this...
https://www.youtube.com/watch?v=kephUMfdNIo&ab_channel=oupzySo the sound comes on higher RPMs when the valve is open I guess... Can I adjust it with cables on the side of the cylinder so there is less chance of hitting the piston or it is only possible with the cylinder off?
BTW sorry for my bad English... -
@Lukas55 removing the exhaust is the best way to check it 100%. You could always mark the power valve pulley and housing, with some paint or similar, while you have the exhaust off and the valves in the fully open position. That'll save you having to keep removing the exhaust to check. Just make sure you turn the engine off with the key switch once it's revved above 7k rpm.
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It is not supposed to be closed down like that. It is supposed to be as open as it can be. It only closes (less than that as well!!) when it does a cleaning cycle when you turn the key, and when the engine is running at low revs...
You can see the pulley move clockwise to close the valve when it cycles ant then when you run the engine at lower revs ...
Again OPEN when the engine is not running...
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You need to remove the pulley/cables from the end of the power valve, then rotate it anti-clockwise about a third of a turn (until you can see as much piston as possible) then replace the pulley/cables.
Check the the pulley moves clockwise, then back, when you turn the ignition key. If it moves anti-clockwise you have the cables back to front...
Please do this... -
@Lukas55 Lukas55 the piston shouldn't hit the valve what ever position it's in. The piston normally won't normally touch the valve unless your barrel has been re bored 57mm or more. And then you need to grind the valve down to create clearance.
In that pic you've just posted the plastic pulley is how it's supposed to be with the valve completely open. If you look up the exhuast port it should be more or less level all the way to the piston. If it is like that with the pulley in that position that parts set correct.
Then it's a case of making sure the valves in the correct position for what ever revs the engines doing. -
I was looking at the utube clip, not a pic. You can see the vertical seam where the 2 halves of the power valve fit together. It’s almost 2/3 to 3/4 closed!
Yes, move the power valve that way! (anti-clockwise) The pulley should fit snug back onto the diamond of the power valve after without adjusting the cables too much. You should be able to run your finger from the piston to the exhaust port outer edge without feeling any lips or hard edges. It should be smooth...
