YZ426F No Start

Yamaha YZ426F

33 replies to this topic
  • YZ426Supermoto

Posted October 03, 2015 - 05:02 PM

#1

About a month ago, I dropped an exhaust valve. I then bought a WR426 head, brand new Wiseco Piston, Intake and Exhaust HotCams, and a new spark plug.

Just to clear it up, it is a YZ426 bottom end on a WR426 head and WR426 valves. Valves were professionally shimmed. The only thing from the WR426 is the head and valves, rest is either new or from the original YZ426.

The engine is now built, torqued to spec, and kicks over easily. Problem is, it will not start. I have spark, fuel, and air. Carb was not touched, other than taken off and put back on. It will backfire every so often, and it is extremely loud, never heard my bike do this before, but it shows I have combustion.

There's some theory's I have, such as the pilot jet in the carb, since it sat for a month it could of got clogged. If you have any suggestions, please let me know. Thank you guys, I'll be in touch!

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  • Kawabuggy

Posted October 03, 2015 - 05:25 PM

#2

If it is backfiring through the exhaust, your cam timing is off. I would double, & triple check the cam timing. I see you have 2 marks on each cam. I have not seen cams marked like that before. A Honda cam has 2 marks opposite each other, or 180 degrees apart, and both marks have to be parallel with the head to be timed correctly.  Like this: https://www.google.c...444008314912136



  • YZ426Supermoto

Posted October 03, 2015 - 05:39 PM

#3

It was very interesting to see. This is the second time I've checked the valves. The flywheel has an "I" marking that lines up with the timing mark, but that is not dead on. The piston would always move up or down, only way I got it to stay there was using a 14mm and ratchet holding the marking by using the gravity of the wrench. Its not too far off when the position moves. wC5xlVU.jpg

  • YZ426Supermoto

Posted October 03, 2015 - 08:56 PM

#4

Also seems to be a gurgling noise coming through the airbox when I slowly kick it,

  • sixOHfour

Posted October 03, 2015 - 09:33 PM

#5

It looks like you are off on the intake cam timing in the pic. When I did my old Yamaha's top end my timing marks for the intake cam were pretty much flush with the head and lined up pretty much exactly like your exhaust cam. 



  • billyandgus

Posted October 03, 2015 - 11:21 PM

#6

Is the chain stretched?

  • YZ426Supermoto

Posted October 04, 2015 - 12:01 AM

#7

I can assure you I'm dead on. The cam chain is loose because the tensioner is out? Its a brand new chain. Even if the timing in 1 tooth off, it should still start. Its not 180 off, as that's impossible since I set TDC.

  • mlcompound

Posted October 04, 2015 - 03:43 AM

#8

I would have to check the manual but I would assume the timing marks should be left up up right. Currently they are left up left up. Did you verify the timing via the manual?

  • mlcompound

Posted October 04, 2015 - 03:50 AM

#9

Should look like this.

Attached Thumbnails

  • 1443959394565.jpg


  • billyandgus

Posted October 04, 2015 - 04:29 AM

#10

I am under the impression that the bike will fire if it was 180 out.
But not run correctly.
The timing should be set on the firing stroke not just tdc.
So after the inlet valve is activated that is the firing stroke
Then tdc should be set.
Only my opinion I may be incorrect.

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  • grayracer513

Posted October 04, 2015 - 08:46 AM

#11

The timing should be set on the firing stroke not just tdc.
 

 

Every stroke is a firing stroke, so it makes no difference as far as cam timing is concerned.  One TDC is like any other. 



  • mlcompound

Posted October 04, 2015 - 09:47 AM

#12

Every stroke is a firing stroke, so it makes no difference as far as cam timing is concerned. One TDC is like any other.


Not true at all. Just because the spark plug fires does not mean it is the "power" stroke. Why are there so many people talking about running it out of time. Just stop right now and get it right. Rotate till the intake valves open and close, then hit tdc, then set cam time. The flywheel mark should line up at this point. Also make sure it is like the pic I posted, left up up right.

  • grayracer513

Posted October 04, 2015 - 03:29 PM

#13

Not true at all. Just because the spark plug fires does not mean it is the "power" stroke. 

 

Where did I say both strokes were power strokes?  The fact is that the engine creates a spark on every revolution, so in terms of setting the cam timing, TDC is TDC and that's all there is to it.  The cams determine which one is compression/power and which is exhaust/intake.  

 

The idea that anyone could make the mistake of choosing the wrong stroke to time it on has no foundation whatsoever. 



  • YZ426Supermoto

Posted October 04, 2015 - 03:54 PM

#14

Right. The engine cannot be 180 off. I have tried ether directly into the carb, but that had no effect on it. I've been told these engines do not like ether. I believe the timing is set on it.

This particular bike does have a TrailTech speedo, RPM graph, and temp sensor. The RPM graph does show the bike has spark, and I've verified it is a bright blue spark. It has a WR426 stator to power the lights, and when kicked the bike gets power, considering the TrailTech flashes bright and the headlight flashes.

Fuel is questionable, even though I know its getting fuel (since it did backfire) I believe its got fuel. Also I know the carbs bowl is full.

Air is simple. Airbox and filter is on.

I'm not too sure what else it could be. Everything is there, and I understand that you guys would believe timing is off since all 3 properties to run this engine are there. I would like to double check the float and pilot jet in the carb sometime, just to rule that out. If that's not it, I'm completely clueless.

Edited by YZ426Supermoto, October 04, 2015 - 03:55 PM.


  • mlcompound

Posted October 04, 2015 - 04:11 PM

#15

Where did I say both strokes were power strokes? The fact is that the engine creates a spark on every revolution, so in terms of setting the cam timing, TDC is TDC and that's all there is to it. The cams determine which one is compression/power and which is exhaust/intake.

The idea that anyone could make the mistake of choosing the wrong stroke to time it on has no foundation whatsoever.


Does the flywheel timing mark hit the case mark every top dead center or not? If so, then no biggie. If not, then it must be set on that particular tdc. I still see the cams are not correct based on the photo.

  • YZ426Supermoto

Posted October 04, 2015 - 04:58 PM

#16

Does the flywheel timing mark hit the case mark every top dead center or not? If so, then no biggie. If not, then it must be set on that particular tdc. I still see the cams are not correct based on the photo.

No sir, the only way to get it to line up with the timing mark was to have a 14mm socket and ratchet hold it there by using gravity. The marking never lined up on a stroke.

  • mlcompound

Posted October 04, 2015 - 05:49 PM

#17

The marks need to be left and right, not left left.

Based on your picture the exhaust and intake are opening at the same time.

Attached Thumbnails

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Edited by mlcompound, October 04, 2015 - 05:50 PM.


  • YZ426Supermoto

Posted October 04, 2015 - 06:13 PM

#18

The marks need to be left and right, not left left.

Based on your picture the exhaust and intake are opening at the same time.

The only reason you see it like that is because the chain is loose. Once tightened the intake moves up a little bit. If you REALLY zoom into that picture you'll see the timing mark.

Here is good information to clear things up.
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  • mlcompound

Posted October 04, 2015 - 06:16 PM

#19

OK. In this pic look at the timing marks on the cam sprockets. They are left up left up.

Attached Thumbnails

  • 1444011398808.jpg


  • mlcompound

Posted October 04, 2015 - 06:20 PM

#20

If the marks are 90 out on the hotcams sprocket, then pushing on the chain will roll it to the right, now it would appear to be 1 tooth out.





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