Engine died... valve or timing chain or ??


38 replies to this topic
  • biznet1

Posted April 06, 2005 - 12:36 PM

#21

So? Turn it around once.
:naughty:

As I've said many times, I'm no expert. But, if he has his mark on the crank lined up, shouldn't the cams be all the way over?

  • grayracer513

Posted April 06, 2005 - 12:44 PM

#22

you definitely need a new jug. take a pick of the crank gear when you get a chance.

Right. The cylinder and piston are toast. It's usually cheaper to buy a new one than to bore and plate the old. Maybe it's time for that big bore kit you always wanted. The top of the piston looks OK, and the combustion chamber doesn't look particularly bad, but suggests it may have been pretty hot when it ran last. Check the crank sprocket, like the man said, and the cam caps and the head where the cams ride for scoring. Have the head checked out for valve and seat condition.

I've seen bluing like that on healthy cranks. Not a worry by itself if the rod is not discolored and feels tight. If the rod checks out badly, be sure you inspect the oil passage leading from the filter well to the crank, and the seal that carries oil from the engine cover to the crankshaft itself. Check the oil pump for wear, too.

This looks like pretty classic piston seizure. Too much heat, not enough oil, or a chunk wedged between piston and cylinder. Or a combination.

  • grayracer513

Posted April 06, 2005 - 12:48 PM

#23

As I've said many times, I'm no expert. But, if he has his mark on the crank lined up, shouldn't the cams be all the way over?

Remember that a four-stroke takes two full revolutions of the crank to complete the cycle of intake, compression, power and exhaust. The cams determine which of the two TDC's it is at a given moment, and they turn at half the speed of the crank. So, if the cam is exactly 180 out of time on one TDC, rotating forward to the next one will make it exactly in time.
:naughty:

  • biznet1

Posted April 06, 2005 - 12:51 PM

#24

Remember that a four-stroke takes two full revolutions of the crank to complete the cycle of intake, compression, power and exhaust. The cams determine which of the two TDC's it is at a given moment, and they turn at half the speed of the crank. So, if the cam is exactly 180 out of time on one TDC, rotating forward to the next one will make it exactly in time.
:naughty:

AhhHaah! Common sense is a beautiful thing.

  • MotoGoalie

Posted April 06, 2005 - 01:08 PM

#25

1.Out riding yesterday the motor died and made a noise similiar to the drive chain coming off the sprockets and binding up, ...

2 .... removed the screen from the frame oil reservoir. I took it out to clean it for the first time and it was trashed from the previous owner and wasn't filtering anything, so I removed it.

:naughty:


1. You siezed the piston, judging from the sound of how it went and doubly sure after viewing your pics. Were you WFO when it siezed? What were the conditions when the bike stopped? Was the wheel still rolling or did you pull in the clutch by that time?

2. As gray already suggested. you may have plugged oil passages and this created a condition that was leading to your piston siezure, crank blueing, and misery in general.

My guess is your oil passages and pump are in need of an inspection in addition to EVERYTHING else, one reason may be because the inline screen is either buggured up by the previous guy and then disintergrated into your oil lines or it allowed a peice of something into a passage and clogged it up .

Big bummer but I really do appreciate letting us in on it. It's helping me learn too.

  • Guest_BrandonV_*

Posted April 06, 2005 - 02:05 PM

#26

Were you WFO when it siezed? What were the conditions when the bike stopped? Was the wheel still rolling or did you pull in the clutch by that time?

I wasn't even at half throttle when it happened, I was just starting from a dead stop and hadn't even shifted yet. Not sure what you mean by conditions when it stopped. I pulled the clutch and braked as soon as I knew something wasn't right.


Heat damage, huh? I just flushed the cooling system the day before and was running a mixture of 70% water, 30% Maxima cool-aid and a few caps of water wetter. It has no freeze protection, but I thought it would be a good choice for the Cali desert.



This sucks! I just invested about $1500 on chassis & suspension and then this happened. Should've bought new I guess......

  • Ga426owner

Posted April 06, 2005 - 02:14 PM

#27

I wasn't even at half throttle when it happened, I was just starting from a dead stop and hadn't even shifted yet. Not sure what you mean by conditions when it stopped. I pulled the clutch and braked as soon as I knew something wasn't right.


Heat damage, huh? I just flushed the cooling system the day before and was running a mixture of 70% water, 30% Maxima cool-aid and a few caps of water wetter. It has no freeze protection, but I thought it would be a good choice for the Cali desert.



This sucks! I just invested about $1500 on chassis & suspension and then this happened. Should've bought new I guess......



Look at this way - it will practically be a new bike when fixed now,,,,, :naughty:

  • mkporn

Posted April 06, 2005 - 02:54 PM

#28

Right. The cylinder and piston are toast. It's usually cheaper to buy a new one than to bore and plate the old. Maybe it's time for that big bore kit you always wanted. The top of the piston looks OK, and the combustion chamber doesn't look particularly bad, but suggests it may have been pretty hot when it ran last. Check the crank sprocket, like the man said, and the cam caps and the head where the cams ride for scoring. Have the head checked out for valve and seat condition.

I've seen bluing like that on healthy cranks. Not a worry by itself if the rod is not discolored and feels tight. If the rod checks out badly, be sure you inspect the oil passage leading from the filter well to the crank, and the seal that carries oil from the engine cover to the crankshaft itself. Check the oil pump for wear, too.

This looks like pretty classic piston seizure. Too much heat, not enough oil, or a chunk wedged between piston and cylinder. Or a combination.



I have done a couple dozen of these motors from 98 up and I have seen the crank blueing on just about every one of them.... Definitly a siezure.....

  • Guest_BrandonV_*

Posted April 06, 2005 - 03:52 PM

#29

Should I worry about the crank or not? I'd really rather not split the cases unless necessary.
Grayracer, you commented on a big bore kit. Would this be a core exchange or rebore this one? Are the bb kits good bang for the buck on pump gas, stock carb, cams, etc...?
If so, how do I it reliably and at least expense?

Thanks!

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  • mkporn

Posted April 07, 2005 - 06:17 AM

#30

Got any metal floating around in the case? How does the crank feel? Any tight spots? Does it roll over freely? Any up and down slop in the rod?

  • Guest_BrandonV_*

Posted April 07, 2005 - 08:18 AM

#31

The oil was clean with no shavings, which is good cause it only had 30 miuntes use on it. I'm not sure how a crank should feel..... It rotates fine, no binding or scraping sounds but is kinda hard with a clunky piston dangling up top. I'll pull the piston and try again. The rod felt tight with no play.

To the best of my knowledge here's what I need to do:
-new cylinder and top end kit (piston, rod, bearings, etc...)
-have the head checked for valve wear
-check the timing chain gear on the crank
-check the crank for excessive wear
-replace the timing chain & tensioner

In my first pics showing the cam timing it looked as if I was on the exhaust stroke, if thats the case wouldn't my timing still be off even if I we're to rotate the assembly to TDC of the correct stroke?
Wouldn't my cams be at exactly 180 deg. from where they sit now?
Or is that a conclusion that cannot be made now and since the cams are out I'll never know?
I'd like to do the decomp cam mod, but am fearing to put in a new cam without knowing if my timing truly jumped or not and if it did, why?


Whats my best route for the cylinder? 2mm bore on this jug and oversise Wiseco? Might as well get a little extra kick out of it while I can.


Thanks!

  • YZ250F_Rider

Posted April 07, 2005 - 08:38 AM

#32

I would check the cam chain, tensioner, and slider too. I wouldnt make a judgement call on the timing until you determine if the crank gear is not worn out.

the 2mm overbore is still amam legal if you ever want to do some harescrambles or whatever, go for a higher compression piston to give it a good kick in the pants. The autodeomp cam is a good thing regardless of why it failed.

  • grayracer513

Posted April 07, 2005 - 10:54 AM

#33

Your crank is probably OK.

Your assessment of what needs to be done is about right.

On your timing, it would be really hard to tell from a picture of the cams on the exhaust stroke what it would look like on the compression stroke. The center timing mark wasn't visible because it was presumably straight down, and the other two marks on either gear are in slightly different positions relative to the center mark. That is they are both at exactly 90 degrees away from the center, so trying to gauge the timing on the wrong stroke is hard to do. I guess you'll just never know, as you said.

On the cylinder/piston, OEM stuff will be the least expensive, and you know exactly what it will run like when it goes on. OTOH, a big bore kit and a little more compression is one of the best ways to bump performance, because both will improve power all across the board. And, it probably won't be too tough to adjust for in terms of jetting, etc., either. And the cam mod is worth worth the trouble, big time.

  • rbm33

Posted April 07, 2005 - 01:20 PM

#34

it sounds like your assesment is correct. i woud recomend going with a factory yamaha cylinder. they arent that expensive anyway. but, as grayracer said earlier i would determine without a doubt what caused the problem. lack of oil to the cylinder? to lean? broken ring?

  • sanpedro426

Posted April 07, 2005 - 01:54 PM

#35

Hey Brandon, sorry to hear about the motor. The wear on the cylinder looks weird being in one spot only.What does the piston look like in the same spot?Is there anything stuck in the ring groves?At any rate i would go with stock stuff because youd know exactly what to expect as far as jetting and such. From what i saw in the first photos, it looked to me like the timing wasoff one tooth on both cams which would suggest to me that it jumped time at the bottom sprocket.Or it is only off a very little due to stretch of the chain.I seem to recall when we swapped bikes that day, it was low on compression. JMO.Did you notice a difference when you kicked mine over.

  • Guest_BrandonV_*

Posted April 07, 2005 - 02:35 PM

#36

All I noticed about your bike was the suspension :naughty:
I haven't really had a chance to go over it as thouroughly as I like since its stored at the in-laws. I'll have a look at the piston where it trahsed the cylinder and see what happened.

I dont think the timing jumped, I took that pic when it was TDC of the exhaust stroke. And thats why I work in the telco field and not mechanical........


I checked the jetting and was running a 158 main, 40 PJ, clip in the middle (4th) and about 2 turns out. I had just installed an FMF PB and Q silencer the day prior and run the bike w/out the airbox lid. I didnt realize that main was so small, that probably didnt help......

How do I verify the oiling system is working properly?

  • moto_xxx

Posted January 20, 2006 - 12:02 PM

#37

The blueing on the crank is normal...you will only find the blueing on one part of the crank on all these yzs. That part of the crank is heated during manafacturing to push some sort of pin in the crank. I did research on mine and this checks out. Mine looked like that when I rebuilt my top end. Even a brand new yamaha crank has that blueing!

  • moto_xxx

Posted January 24, 2006 - 10:02 AM

#38

The oiling system can be checked by cracking open one of the external oil lines on the right side of the motor while it is running. If oil seeps out you are good to go.

  • Butta

Posted January 25, 2006 - 09:06 AM

#39

Holy old post resurrection, Batman!!!!





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