2008 YZ450f Exhaust Smoke


23 replies to this topic
  • markitos020

Posted June 17, 2013 - 07:50 AM

#1

Hi, I recently bought a 2008 yz450f, I test rode it at night so I could not notice that the bike was smoking. When I turned the engine on the next day I noticed there is white/ bluish smoke coming out the exhaust. This happens only when I blimp the throttle If its on idle it wont smoke nor running at a steady pace. If I do engine brake like going down a large hill when I accelerate the bike smokes alot and then stops smoking, which made me think its valve seal problem. Bike runs great, turns on the first kick cold or hot.

So my problem is that I took the engine apart yesterday since i ordered a new piston, rings, gasket set and valve seals and noticed that the cylinder has a small scuff at the top, I would like to know if the cylinder is damage and needs to be replated or if this is normal and it can still be used after honing. The small white spot feels a little rough, not sift like the rest of the cylinder. My second question is that I noticed some side play on the connecting rod, I measured it and it has 0.011 inch of play is this within spec since i could not find the specs? I will give you a link to a video I took as well as soome pictures. Also is the color on the cranckshaft normal due to the heat treatment or is this color signs of overheat? In the pictures you can see that there is some oil on the right and left intake vavles. Exhaust valve and exhaust port was full of oil as well.

Pictures: https://www.dropbox....0ajh/KYJinJc8DR

Video: https://www.youtube....h?v=0nMwoypdlmM

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Mark

  • grayracer513

Posted June 17, 2013 - 10:20 AM

#2

As to the crank, the manual lists the side clearance as .006-.018", with a service limit of .020", so you are fine there. And, the color you see is the result of heat used during assembly, and is normal. You need to measure the rod bearing clearance as shown in the manual, also. If you don't have one, look here:

http://www.yamaha-mo...uals/index.aspx

http://www.yamahaown...ook.com.au/?r=0

The center intake valve always looks cleaner than the other two because that's where the bulk of the fuel goes. The piston is junk, and most of your oil most likely went past the rings. The cylinder doesn't look too bad in the pictures, but there's only one right way to know. Hone it lightly with a "bottle brush" type hone and have it measured by a machinist. If it's more than .002" out of being a perfect cylindrical bore, plate it or replace it.

The most recent YZ450 I rebuilt for anyone was overheated to the point where the metal between the exhaust valve seats just reached the melt point, and it lost the head gasket in the process. The way that the seal failure of the gasket was oriented internally allowed the engine's lube system to pump oil into the cylinder, so there was oil all through everything. If one of the head bolts seemed to be unusually loose, you may have had the same trouble.

  • markitos020

Posted June 17, 2013 - 12:38 PM

#3

So the white mark on the top of the cylinder does not imply that the plating is damaged? All bolts where tight but one of them did come out with dirty it looked like black grease or maybe some oil. I already orderd the piston kit, I will take the cilinder to a machine shop. My main concern was the scuff mark.

  • grayracer513

Posted June 17, 2013 - 01:55 PM

#4

It could, or it could be aluminum transferred from the piston. Either way, measuring the bore with a reliable dial bore gauge will tell you absolutely.

The left rear head bolt bore doubles as the top end oil feed passage. That bolt should come out oily.

  • markitos020

Posted June 20, 2013 - 09:37 AM

#5

Hi just wanted to do an update, I honed the cylinbder and the white scuff mark is now bigger and you can clearly see deep scratches, I went ahead and bought a new original cylinder for 270 shipped since replating it would cost almost the same and I prefer getting a new one. When everything gets here I will get back with the results after a test ride, Thanks

  • markitos020

Posted June 25, 2013 - 03:11 AM

#6

Hi, all the parts came in yesterday so I started to assemble everything. I had basically everything nailed down but I'm having two questions. FIrst when i first took the engine apart i noticed that it was off by around one tooth on the crankshaft, I aligned the cam marks but the crankshaft mark was moved to the left to the point it was barley visible. So yesterday i was setting the timing and a did notice that the intake cam alignment mark is a little low but if i rotate it one tooth to the left then its way to high, I took some pictures and want to make sure timing is ok as I set it now I rotated the engine multiple times and everything was rotating normal with no binding. The second question is the gasket kit included an exhaust gasket but when i removed the header I dont see any gasket there and when i installed the new gasket it sits flush with the exhaust port is this the way it goes or do i have an old exhaust gasket still in the head?
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Edited by markitos020, June 25, 2013 - 03:24 AM.


  • markitos020

Posted July 07, 2013 - 04:38 PM

#7

Today I was able to do my first ride on trails after rebuild. I gave it proper break-in time as well as changed the oil after break-in. The situation is the following after If i turn off the bike and try to restart it within a few seconds it starts right away, but if i let the bike sit for around a minute or more it takes way to many kicks to start. Before rebuild bike would start right away no matter what, could this be a jetting issue due to using a wiseco piston instead of an oem piston? Another thing that I did notice is I did leave the TPS accidentally unplugged could this also contribute to hard starting?

  • 123devinzz1

Posted July 08, 2013 - 06:56 AM

#8

Today I was able to do my first ride on trails after rebuild. I gave it proper break-in time as well as changed the oil after break-in. The situation is the following after If i turn off the bike and try to restart it within a few seconds it starts right away, but if i let the bike sit for around a minute or more it takes way to many kicks to start. Before rebuild bike would start right away no matter what, could this be a jetting issue due to using a wiseco piston instead of an oem piston? Another thing that I did notice is I did leave the TPS accidentally unplugged could this also contribute to hard starting?


wont affect jetting and tps shouldnt affect anything (alot of guys keep it unhooked) How many kicks does it take? sometimes these big 4 strokes can be stubborn

  • markitos020

Posted July 16, 2013 - 02:59 AM

#9

It takes around 8-10 kicks but what i do notice is that it only does it when the bike is really hot. I ran it with the tps plugged in and it does the same I also removed the outerwear that I installed on the air filter and it did help turns on easier around 5 kicks but still not as before. So I dont understand what could be wrong i had a kx450 2007 and it never took more that 3 proper kicks to start. I do think jetting on low rpm might be off since i have to open the idle screw 4-4 1/2 turns out to get the hot start to work properly (bike revs up when pulled while bike is on correct me if im wrong ) since bike would shut hesistate and shut off when pulled and also the idle would stay high a for a few seconds after blimping the throttle... so im guessing its runing lean. I think I will take it to a dyno and get it checked properly. If I accidentally changed the valves out of their original location lets say intake valve 1 with valve 3 could i have a sealing problem? Bike does run great its turning it on when hot where the problem is. I will be checking valve clearance as well since i shimmed it while the head was out maybe I went too small on clearance.

  • 123devinzz1

Posted July 16, 2013 - 07:26 AM

#10

It takes around 8-10 kicks but what i do notice is that it only does it when the bike is really hot. I ran it with the tps plugged in and it does the same I also removed the outerwear that I installed on the air filter and it did help turns on easier around 5 kicks but still not as before. So I dont understand what could be wrong i had a kx450 2007 and it never took more that 3 proper kicks to start. I do think jetting on low rpm might be off since i have to open the idle screw 4-4 1/2 turns out to get the hot start to work properly (bike revs up when pulled while bike is on correct me if im wrong ) since bike would shut hesistate and shut off when pulled and also the idle would stay high a for a few seconds after blimping the throttle... so im guessing its runing lean. I think I will take it to a dyno and get it checked properly. If I accidentally changed the valves out of their original location lets say intake valve 1 with valve 3 could i have a sealing problem? Bike does run great its turning it on when hot where the problem is. I will be checking valve clearance as well since i shimmed it while the head was out maybe I went too small on clearance.


hmmm. whats your fuel screw at? Try turning it over with your hand and on the compression stroke listen. If the intake valves are leaking you should be able to hear some air escape into the intake/airbox. You can also try a leakdown test. Mine starts first kick every time cold or hot since i tuned it (took a long time but now ive got a feel for how the bike is running)

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  • markitos020

Posted July 16, 2013 - 01:11 PM

#11

My fuel screw has to be ser at 4 and a half turns open... i did turn the engine by hand and i dont hear any air coming out at least with the engine cold, i will have to try it with the engine hot i will be cheking valve clearance and will remove the header and check for leaks on exhaust as well.. mine used to turn on perfectly before the rebuild....

  • 123devinzz1

Posted July 16, 2013 - 01:25 PM

#12

My fuel screw has to be ser at 4 and a half turns open... i did turn the engine by hand and i dont hear any air coming out at least with the engine cold, i will have to try it with the engine hot i will be cheking valve clearance and will remove the header and check for leaks on exhaust as well.. mine used to turn on perfectly before the rebuild....


If the bike is stock then 2 turns out on the fuel screw is recommended in the manual and mine is 2 1/4 out atm. But if you say it started fine before then im not really sure what your problem is. post if you find anything else wrong.

  • markitos020

Posted July 16, 2013 - 03:06 PM

#13

The problem is at 2 turns out it backfires like crazy and if the bike is turned on and i pull the hot start it starts to hesitate and shuts off... is this normal? At least on the kawi when i pulled the hot start it would rev up... on this bike it takes 4 turns out to stop the bike from backfire and for the hot start to work properly....

  • 123devinzz1

Posted July 16, 2013 - 03:51 PM

#14

The problem is at 2 turns out it backfires like crazy and if the bike is turned on and i pull the hot start it starts to hesitate and shuts off... is this normal? At least on the kawi when i pulled the hot start it would rev up... on this bike it takes 4 turns out to stop the bike from backfire and for the hot start to work properly....


Yes it should rev up if working properly. Just to clarify.. This is a stock bike and the jetting has never been changed/touched? Noticed you made a video of the crankshaft. A video of this "hesitation" and backfire could help also.

  • 123devinzz1

Posted July 16, 2013 - 04:05 PM

#15

your pilot jet may be clogged.If not go up a size. Shouldnt have to go more than 3 turns out on the fuel screw.

  • markitos020

Posted July 16, 2013 - 04:59 PM

#16

About the jetting i cant assure its stock i am going to check the jet numbers but the engine is stock with stock header and pipe as well. I will take a video backfire only happens when riding on decel not on aceleration.

  • markitos020

Posted July 16, 2013 - 05:11 PM

#17

I also remeber right now that i did change the mixture screw for one of those easy adjust ones but i got a cheap one off eb@y for 8 bucks and it did come with the tip a little bent and i straightened it eyeballing it... im trying to revert every single change i did to the bike after rebuild... just one at a time so i can identify which one was the problem.

  • markitos020

Posted July 16, 2013 - 05:17 PM

#18

I have another question do these bikes shoot 1 spark per rotation like some modern cars do or do they shoot 1 per 2 revolutions? I would like to know since i installed a hour meter and it has a tach as well and i want to configure it properly, I already took the video its uploading will post as soon as its done

  • markitos020

Posted July 16, 2013 - 07:36 PM

#19

This is the video,I will post more info tomorrow when I check valve clearance and all the jets, I have to find my jet removal tools.

http://www.youtube.com/watch?v=1dNN2Oc1GrI

  • jojo1224

Posted July 16, 2013 - 08:41 PM

#20

Yes these bikes fire once per revolution, like a 2 stroke. The cam timing doesnt look to bad, did you replace the timing chain while you had it apart? Also check your intake valve clearances (it is most likely the intake valve closest to the timing chain). Judging by the wear of the plating on the cylinder and the dirty looking airfilter and airbox that valve clearance is probably tight.





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