White/Blue Smoke on throttle - 07 450fw

Yamaha YZ450F 2007

43 replies to this topic
  • fldrifter

Posted December 17, 2013 - 10:08 AM


Unfortunately all you can do to check the bottom end is to spin the crank by hand and listen/feel for binding and roughness and checking side to side play.
Excessive side to side play can let the rod tilt. Measure it and compare it to the spec in your manual. If its close to max replace the crank and both side bearings. It is a very important measurement. Of course ANY up/down play and it needs replaced ASAP.
Also you can look around the clutch basket, kick starter gear area for anything that would bind up, but that's all you can do.
After that you have to replace the crank and bearings. Also wise to replace all the transmission bearings too while your in there



When you say spin by hand, using a socket on the crank case side?  or grab the piston rod and move it through the motion? 


I took the trans case cover off, nothing appears to be broken/chipped, warped in there.  The only thing that looked weird to me was the spring for the kick lever, but I'm not sure how that normally looks to compare it.  (edit: looking at photos of a kick starter spring and sprocket online, my spring is clearly bent out of shape.  So I will order a new one, but I don't think this is what caused it to bind.)


This is my first time working on a dirt bike motor, I'm a car guy. 

Edited by fldrifter, December 17, 2013 - 11:21 AM.

  • grayracer513

Posted December 17, 2013 - 11:58 AM


The pictures you provided don't tell me what I want to know, but here's my take on the matter.  Have the cylinder measured by someone who can do it accurately.  If it's showing more than .002" out of straight/round anywhere, get a new one, or have that one reconditioned.  If it's OK, hone it using a Brush Research ball hone the right size (95mm) in 240 grit silicon carbide or aluminum oxide, nothing more aggressive.


The piston is suspect.  If there is any evidence that the ring lands (the area around the ring grooves) have made contact with the bore walls, it should be replaced.  If the ring grooves appear to have any visible wear on their upper and lower surfaces, replace it. If you can't decide, replace it.


Verify the fit of the valves in their guides and replace the valve seals.  That should do it for you.


On the crank, vertical play in the rod bearing is necessary.  Keeping it from being excessive is also necessary.  The manual specifies side clearance of .15-.45mm (.006-.018").  Side clearance is measured with a feeler gauge, or a dial gauge, and the term means how much clearance there is between the connecting rod big end and the crank.  To measure, slide the rod as far to one side as it will go and check the clearance with a feeler, or move it to one side, zero a dial, and push it back to the other side. 


Vertical rod play is measured by seeing how much the rod can be tilted.  Refer to the attached picture from the manual.  Set up a dial indicator to be able to measure the tilt of the rod from left to right.  As the rod is tilted, the rod should not be allowed to slide across the crank pin in either direction, just tilted.   The clearance in the rod bearing is necessarily much smaller than the amount actually measured as tilt at the top of the rod, but that clearance is what allows the tilt to occur, and the limit shown (.4-1.0mm, or .020-.040") is the only accurately repeatable method of measuring it without a sophisticated jig.

Attached Thumbnails

  • crank.png

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  • fldrifter

Posted December 17, 2013 - 01:00 PM


thanks for the reply.  I took those photos with my iphone, I can pull out my camera and take higher quality / better lit photos of something, just let me know which item you want to see a better view of. 


I did not look in depth at the piston, but everything when I glanced over it looked fine. The cylinder as well had no marks anywhere inside when hitting it with a flash light. 

  • fldrifter

Posted December 22, 2013 - 01:17 PM


Bikes back together, fired right up, and runs great. I shut it down to do a final check on the oil level, went to start it again and the kick lever seized up. This is after about 10 kicks through the day. This time the crank is not bound up and has solid compression.

I'm going to take the kick gear out and look at it. I think in my crash i must have screwed something up on it. I never had any issues before I went down off a jump, which the bike landed on the right side. I noticed when the trans cover was off before that I could push the kick spline rod in and out a bit. This time with it locked up I can take the cover off and see what has happened.

I'm guessing either - primary gear is screwy, kick gear, or I bent the spline rod.

It wants to allow movement only certain ways in the spline rotation.

Edited by fldrifter, December 22, 2013 - 01:19 PM.

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