Help with 2000 yz426

Yamaha YZ426F 2000

36 replies to this topic
  • weekend woods racer

Posted March 10, 2016 - 07:03 PM

#1

Please help
I replaced the top end in the 426. Im trying to learn to do work myself.! Now that I put it together it is not running properly. It started up with the choke on but backfires and cracks with throttle and dies if you try to let it idle. my rubber boot between the carb and cylinder seamed old so I replaced it, but no change. Cleaned the carb while I had it off, no change. I tried to re time it today and still no luck. When I replaced the piston, rings ect ... I turned the engine over to make sure I had the piston and rings seated properly. The cams were not in when I did this/ is this my problem? Wrong tdc or something backwards? While replacing top end I removed the valves and replaced the valve guide seals and cleaned the valves (that were extremely dirty, especially with build up at the bottom of the neck).

  • 0bigsilver6

Posted March 10, 2016 - 07:39 PM

#2

TDC is TDC so you can eliminate that. Did you by any chance turn the pilot screw at all on the carb? Are all the valves in spec? Sounds like it's probably a lean condition, but I'm not expert. Greyracer is the man with yamahas, so I'm sure he will check in shortly. But I will also try to help with what I can.

  • damnbiker94

Posted March 10, 2016 - 08:02 PM

#3

Not sure what you mean that tdc is tdc. He needs to confirm that he is truly at tdc using the correct mark on the crank when timing the cams, so no he shouldn't rule that out.

Did the engine run fine before teardown?

  • 0bigsilver6

Posted March 10, 2016 - 08:18 PM

#4

Not sure what you mean that tdc is tdc. He needs to confirm that he is truly at tdc using the correct mark on the crank when timing the cams, so no he shouldn't rule that out.

Did the engine run fine before teardown?


I just meant that if the top dead center mark is lined up, then it's at top dead center. He was concerned because he turned the motor over without the cams in and he thought maybe he set the timing at the WRONG TDC. But that's not the case, hence my statement, TDC is TDC. You are correct though, I was just assuming he was setting timing with the ending at TDC. Hope this makes sense.

  • weekend woods racer

Posted March 10, 2016 - 08:19 PM

#5

Yes the engine ran fine before the rebuild, but it smoked. That was the reason for the top end rebuild, to change the valve guide seals. And btw no more smoke. I messed with the screw on the bottom of the carb but put it back to 2 1/2 turns. ImageUploadedByThumper Talk1457669944.329586.jpg ImageUploadedByThumper Talk1457669966.296932.jpg

  • weekend woods racer

Posted March 10, 2016 - 08:24 PM

#6

The pictures are from the first time. I tried to time it again today and I guess it is still messed up. I need to rule out that I have tdc correct so I can work on checking the valves. Would cleaning the valves cause me the need to re chim? Is there usually need to re chim after a top end rebuild?

  • 0bigsilver6

Posted March 10, 2016 - 08:24 PM

#7

Yes the engine ran fine before the rebuild, but it smoked. That was the reason for the top end rebuild, to change the valve guide seals. And btw no more smoke. I messed with the screw on the bottom of the carb but put it back to 2 1/2 turns. attachicon.gifImageUploadedByThumper Talk1457669944.329586.jpgattachicon.gifImageUploadedByThumper Talk1457669966.296932.jpg


I would turn that intake cam one tooth counter clockwise. Not sure that that would cause your backfiring and such though.

  • 0bigsilver6

Posted March 10, 2016 - 08:26 PM

#8

The pictures are from the first time. I tried to time it again today and I guess it is still messed up. I need to rule out that I have tdc correct so I can work on checking the valves. Would cleaning the valves cause me the need to re chim? Is there usually need to re chim after a top end rebuild?

Easiest way to know if the valves need reshimming is to check them with some feeler gauges.

This sounds more like a carb issue to me though. Is it popping on acceleration or deceleration?

Edited by 0BigBlue7, March 10, 2016 - 08:27 PM.


  • weekend woods racer

Posted March 10, 2016 - 08:42 PM

#9

It sputters, cracks and backfires. It ran with almost zero backfire before. It's like the motor is stumbling over itself.

  • damnbiker94

Posted March 10, 2016 - 08:55 PM

#10

Improper valve timing can cause the issues you are having. So can vacuum leaks, clogged jets, improper carb settings, loose exhaust, and more. Download the service manual if you dont already have one and confirm that you have coreectly timed the cams. Based on it running well before the teardown, this is where i recommend starting.

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  • 0bigsilver6

Posted March 10, 2016 - 09:03 PM

#11

Improper valve timing can cause the issues you are having. So can vacuum leaks, clogged jets, improper carb settings, loose exhaust, and more. Download the service manual if you dont already have one and confirm that you have coreectly timed the cams. Based on it running well before the teardown, this is where i recommend starting.

This. Start with ensuring the cams are timed properly, and the valves are in spec. Then maybe start looking at the carb. I know you said you cleaned it, which means you took it apart, so maybe there is an air leak on it somewhere. But, let's start with what he said. Timing.

If the timing checks out for sure, and the valves are in spec, then look at the carb. If the engine pops after a rev and the motor is decelerating, that indicates a lean condition, pilot screw needing adjustment, air leak somewhere, or even exhaust leak as said above.

And I believe, I read this somewhere, if it pops as the engine revs up, that indicates a rich condition. Meaning too much fuel needs more air. This may not be correct, someone correct me if I'm wrong. I'm trying to learn as well!

Edited by 0BigBlue7, March 10, 2016 - 09:13 PM.


  • weekend woods racer

Posted March 10, 2016 - 09:15 PM

#12

ImageUploadedByThumper Talk1457673340.425139.jpg
Did I mess this up?

  • 0bigsilver6

Posted March 10, 2016 - 09:34 PM

#13

attachicon.gifImageUploadedByThumper Talk1457673340.425139.jpg
Did I mess this up?

No. It should be fine. All this is saying is ensure the motor stays at TDC while you time the cams. Otherwise the timing will be off. The only damage that could have happened I think, is if the piston pushed on a valve while the cam was pushing the valve down, in which case, probably bent valves. But if you kept the piston at TDC while you timed it, you're fine.

Edited by 0BigBlue7, March 10, 2016 - 09:36 PM.


  • grayracer513

Posted March 11, 2016 - 07:40 AM

#14

Your timing is fine.  The most likely thing is that the pilot jet clogged with dried fuel varnish while it sat around waiting for the work to be done.  It doesn't take much.  a thin dried film can reduce the jet's effective size by 30% pretty easily.

 

Read:  http://www.thumperta...oving-the-carb/

 

The other possibility is that you assembled the vacuum release plate onto the slide upside down.  It's a very common mistake, as it actually looks like it should go that way, but the "square end" with the hole near the edge goes down (see #10 in the pic).  While you're looking at it, be sure it's not cracked or chipped, or severely worn at the corners. 

Attached Thumbnails

  • slide.png


  • weekend woods racer

Posted March 11, 2016 - 08:01 AM

#15

My throttle screw on my carb was torqued down so tight, I could not get that slide out so I did not clean that part of the carb because I didn't want to mess up the rubber seal. But I did clean all the jets.

  • weekend woods racer

Posted March 11, 2016 - 10:34 AM

#16

Ok I'm back into the bike now. After my first attemp before I got onto thumper talk, I tried to mess with the timing and this is what it looks like now, I think I messed it up.ImageUploadedByThumper Talk1457721038.920503.jpg ImageUploadedByThumper Talk1457721113.308657.jpg

But it is also at tdc when the cams look like this

ImageUploadedByThumper Talk1457721190.647442.jpg

Please help I want to ride!!!!!!!

  • grayracer513

Posted March 11, 2016 - 11:09 AM

#17

In the first picture in the above post, your your timing is correct. Both cams appear slightly retarded, which probably means you should replace the cam chain.

 

The camshafts turn at half engine speed, so on TDC of the compression stroke, it will look like the first picture, and at TDC of the exhaust stroke, it will look like the second one.



  • 0bigsilver6

Posted March 11, 2016 - 11:10 AM

#18

Are the photos with the cam chain tensioner on?

Edited by 0BigBlue7, March 11, 2016 - 11:12 AM.


  • weekend woods racer

Posted March 11, 2016 - 11:16 AM

#19

In the first picture in the above post, your your timing is correct. Both cams appear slightly retarded, which probably means you should replace the cam chain.

The camshafts turn at half engine speed, so on TDC of the compression stroke, it will look like the first picture, and at TDC of the exhaust stroke, it will look like the second one.


Do I have to be at tdc on the compression stroke, when I time cams?
Yes. The tensioner is in

  • 0bigsilver6

Posted March 11, 2016 - 11:19 AM

#20

Do I have to be at tdc on the compression stroke, when I time cams?
Yes. The tensioner is in


Yes. I think you can rule out the timing now. He said it looks fine. Just plan on replacing the cam chain very soon. You should probably start looking back to the carb.





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