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dt125 running worse after new top end


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I recently did a top end rebuild on the bike but I'm not very happy with how it's running since the new top end, it's worse than before the rebuild. I'm not sure if my power valve is set up correctly, how can I check this and get it set up perfectly?

Ways the bike is different since new top end:

  • It doesn't start as well. When you first start it you have to keep the revs above 3k for the first 30 seconds or so otherwise it will die straight away.
  • There's not as much top end power, not sure if power valve issue or if the bike needs to be re-jetted. Is it common for 2t's to need jetting adjustments after top end rebuild?
  • There is some popping in the low end when decelerating. non-existent before the rebuild.
  • Bike revs very sluggish below 4-5k rpm. If it give it a really quick blip, I can blip and close the throttle and bike barely even reacts, you have to hold the throttle slightly and the revs picks up.
  • Just doesn't seem to 'pull' as much as before. Acceleration is definitely worse.

What should I do from here, Is it worth taking it to get the carb properly set up? How can I check the pv, the gap/hole in the top of the pulley is already aligned with the hole behind it? I saw this thread, my pv rotates clockwise when I rev the bike, Is this wrong?

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Your PV rotation is correct, from the top of my head anyway. The way to check it is feel inside the exhaust port, you shouldn't be able to feel a step where the PV is with the pulley aligned

Sent from my E6553 using Tapatalk

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to check PV turn bike on and PV will set to neutral , take off left side cove place 4mm drill bit in the pulley slot and the hole behind then adjust the cables so it returns to this position when ignition is switched on and off 

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Nothing to do with the powervalve, air is getting in somewhere.

You need to check for air leaks, may only be small. Popping on deceleration is something a stroker  does when their is air going where it shouldn't.

Check your cylinder base, carb and reedblock joints, did you remove and clean (and profile slightly) the powervalve, you certainly SHOULD have. oe gaskets used? Could even be the plug not sealing properly.

The pv will have ZERO affect on the bike starting, no matter what position its in. If its healthy and set up right it should be possible for it to be started (carefully) by hand, I certainly can with my TDR250.

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28 minutes ago, Cynic said:

Nothing to do with the powervalve, air is getting in somewhere.

You need to check for air leaks, may only be small. Popping on deceleration is something a stroker  does when their is air going where it shouldn't.

Check your cylinder base, carb and reedblock joints, did you remove and clean (and profile slightly) the powervalve, you certainly SHOULD have. oe gaskets used? Could even be the plug not sealing properly.

The pv will have ZERO affect on the bike starting, no matter what position its in. If its healthy and set up right it should be possible for it to be started (carefully) by hand, I certainly can with my TDR250.

I totally removed the powervalve, disassembled it and cleaned it. I even replaced the o rings and oil seal. I used genuine yamaha base and head gaskets. I didn't touch the carb or intake at all during the rebuild. New plug installed properly as well. Any other suggestions where to check?

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25 minutes ago, Airhead said:

looking back at that old thread you linked I'd say the pv should rotate anti-clockwise with increasing engine speed, thus open up the port

 

BTW what shape is the end of the pv where the pulley attaches to it?

It's basically a rectangle, here's a pic I found...

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Ahh right so with that one it's either right or a full 180 degrees out...didn't know about the punch mark indicator.

 

anyway...what about it turning the wrong way?

does your model have that wire that must be grounded out to perform properly?  well is it?

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6 minutes ago, Airhead said:

Ahh right so with that one it's either right or a full 180 degrees out...didn't know about the punch mark indicator.

 

anyway...what about it turning the wrong way?

does your model have that wire that must be grounded out to perform properly?  well is it?

I've also just seen that, I'll check mine is the right way up tomorrow.

I'm not sure... How can I check?

 

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Just realised you were talking about the wire behind the speedo. My bike does have the wire coming from the speedo that has to be grounded to get better performance.

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55 minutes ago, Airhead said:

yes indeed, something to check then, along with the pv orientation and the cables not too tight...that's all I can suggest

Will check all tomorrow, cheers

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12 hours ago, YAMAHA21 said:

I totally removed the powervalve, disassembled it and cleaned it. I even replaced the o rings and oil seal. I used genuine yamaha base and head gaskets. I didn't touch the carb or intake at all during the rebuild. New plug installed properly as well. Any other suggestions where to check?

Nice one. People generally ignore pv seals even though its like having a leaky exhaust.

This may sound daft but. Have you tried using your old plug? 

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2 hours ago, Cynic said:

This may sound daft but. Have you tried using your old plug? 

I haven't tried the old one but I've had 2 new plugs in it since the rebuild and neither changed the way it was running.

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Ok.

Step back and think.

Something is causing this and it isnt the pv. Once upon a time 125's were sold with the pv not having a controler. Never affected their starting.

I still feel there is some sort of leak. Either air, fuel or oil going or not going where it should.

Got good 2stroke feed? Good fuel supply?

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2 hours ago, Cynic said:

Ok.

Step back and think.

Something is causing this and it isnt the pv. Once upon a time 125's were sold with the pv not having a controler. Never affected their starting.

I still feel there is some sort of leak. Either air, fuel or oil going or not going where it should.

Got good 2stroke feed? Good fuel supply?

If there was a leak at the head or base, would there be crap running out? because the head and base are clean. Also turns out my pv was upside down lol, just fixed that and the bike revs better, but it still had the starting issue so the bad things I listed in the first post aren't all caused by one problem. I'm going try and find a propane torch so I can test for air leaks.

2 Stroke feed is good, I bled it straight after "the accident". Pretty sure the fuel supply is good too.

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Depends. Not uncommon for air leaks not to show as the air is sucked in.

Does it start better when is warm?

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42 minutes ago, Cynic said:

Depends. Not uncommon for air leaks not to show as the air is sucked in.

Does it start better when is warm?

Yes, it's just the first 30 seconds when the bike is cold that it will die if you let the revs drop below 3k.

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Sounds like something interfering with the choke. 

Try giving the bike 4 or 5 kicks with the ig off before starting the bike.

Does it have a vacume fuel tap?

 

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