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xr600 valves hitting piston


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My 1989 xr600r began to act up after riding slowly on some tight trails, the bike died and when I restarted it, it would not rev past about a quarter throttle without sputtering, misfiring and backfiring. Thinking that something may have clogged up a jet in the carburetur I cleaned the carburetur out. It still ran the same after that so I was thinking that the stator was begining to go bad because I read that the stock stator can wear down from sitting in hot oil, so I put a ricky stator 200w in and it did not make a difference. I was thinking that carbon could have built up on the head from riding at low rpms and I think the pilot jet might be rich. I took off the head and found that there are small dents in the piston from the valves. I am thinking this is from carbon build up or the timing could have been off. I am leaning more toward carbon build up because when it started acting up I was riding slowly and was not putting a heavy load on the engine. Before I took the engine apart the bike idled fine when cold but as soon as I turned the throttle it starts misfiring. It is very difficult to start when hot. Also it feels like it locks up alot harder at TDC when trying to start, could the compression have increased? This is the first time I have taken an engine apart so I wanted to see is anyone has had a similar problem that could give me advise. I attached pictures of the piston and valves. The valves look like they are caked in carbon. I tryed to scrap the carbon off one but it was really packed on.

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So.....either the timing chain is/was stretched so had that the valves hit the piston.or the timing jumped..

 

the only other possibility is a shagged lower rod bearing..and one usual quick way to tell that is pull the downtube oil screen,if you find brass/copper lookin bits then you have a bad connecting rod.....the brass pieces are the rod thrust bearings flying apart..

 

Did you clean the piston????.....was the outer edge of the top of the piston clean,,if so that tells you that the rings were passing oil..

 

B

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Probably did just what my L did when I first bought it. riding fast on fire roads it was fine. When I got in the tight trails it started knocking and stalled. Checked the oil and it was low. Let it cool down and rode it home. When your rings are worn the oil level can drop rapidly and your first indicator will be the valves begin ticking because they start to stick in the guides. That gives the false indication that cam timing has jumped when it's really just the valves are moving so slowly that the piston is helping them to close. It doesn't always show up till you go slow and there isn't enough air flow to cool the head down. At speed it's more likely to seize the piston. I was lucky enough that they didn't get bent. You need to check yours and rering the piston in the least. Your camchain is most likely fine as is the piston if there is no sticking of the top ring from the valves crushing the crown down on them. It's always easy to tell with 600/650s when you get those little intake valve dents like that. The other RFVCs don't show that same problem in the same way as they don't have to get rid of as much heat from the smaller bores.

Edited by valvesrule
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Brianhare, shouldn't  he check bent valves with puddling mineral spirits on valves/seats, with head upside down, looking for leak thru?

 

Yes.......and NO..... :devil:

 

Yes it will show if the valves leak.......it won`t necessarily show you if they are bent...they could be straight and unbent and just pitted and leaking..

 

What would i do as a quick check...take the valves out and spin them in a drill and look for wooble.....

 

Or..put some valve lapping compound on the cleaned valves and seats ..give them a quick lap and see if the lapping compound is not cleaning all the way arround the valve face..pitted valves will show cleaning all the way arround...bent valves will only show a spot being cleaned..

 

B

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Brianhare, did you say 600s are known for the rod to crank bronze side to side shims/bearing/bushings wearing out? You've seen many? & why do you think this is? more so than 650s? maybe due to most xr600s are run harder or raced in dirt/tracks?

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Never seen an XR650L rod go bad......doesn`t mean it can`t and never happened,but i bet it`s rare,,

 

Seen quite few XR600R rod failures.....wouldn`t say it was super common though

 

The XR600R has more compression in stock form..

 

The XR600R lower rod bearing goes bad,eats the thrust washers and BAM... :jawdrop:

 

Not sure why....but i heard the XR650L lower bigend bearing had more rollers...so it stands up better

 

B

Edited by brianhare
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Thanks, who else would have knowledge about this? Cyclewizard still around? Maybe bearings of a 650 can be put on a 600? or are they different journal dimensions? Maybe I should start a new thread?

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The L rod has a smaller diameter journal pin than the 600 is my understanding. Perhaps the rolling elements are larger diameter but I doubt there are more of them. I haven't had the 2 apart for actual comparison but from what I've gathered that seems to be the difference. I'm fairly certain CW installs 600 rods on his stroker L cranks because they are actually better than the L rods. He uses shorter piston compression heights to compensate for the longer stroke and rod. There's plenty of room for the longer 600 rod, offset and oversize bore the crank halves for a 600 pin to get 84mm stroke and the compression height almost takes care of itself. I'm hoping one day to build one of my own that way after I get my mill setup. A longer rod is always better than a short rod regardless of stroke too. I don't think L rods are better than Rs in any way at all. They just in most situations are running lower compression and rpm than Rs. It's more the circumstances of operation than any strength or durability issue.

Edited by valvesrule
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I did check the oil screen and there were not any copper or steel pieces. I haven't had a chance to get my piston and cylinder measured. Valvesrule, how did you fix yours, new top end? Could my valves have been hitting the piston from sticking to the guides cause the timing to jump when the engine got hot from riding tight trails? Because the bike was running fine and then it shut off and when I got it restarted it would not run right.

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Have you disassembeled the engine?  If it is still together be sure to check the cam timing before taking it all the way apart.  It could be a worn chain that is too long, but it may have skipped a tooth.  If it skipped it's due to a failed cam chain tensioner.  I've had three fail on me.  There is plastic in holding the moving parts and too much heat causes the plastic to fail leading to a tensioner that moves both ways instead of only one way like Honda intended.

 

I have a few pistons with the valve impact dings.

Edited by cleonard
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I did check the oil screen and there were not any copper or steel pieces. I haven't had a chance to get my piston and cylinder measured. Valvesrule, how did you fix yours, new top end? Could my valves have been hitting the piston from sticking to the guides cause the timing to jump when the engine got hot from riding tight trails? Because the bike was running fine and then it shut off and when I got it restarted it would not run right.

Your pics show a lot of wet, black oil on the head and valves. I would check your ring end gaps. Mine were way out of tolerance. My valves were fine, not bent or leaking. I honed the cylinder, checked to make sure ring gap was at the minimum tolerance, replaced the piston with a Wiseco 9-1 and the cam with a Hotcam. It hasn't missed a beat or burned a drop of oil since then. If you didn't check the cam timing before you tore down there's no way to say if it jumped or not. If the valves were sticking badly enough to hit the piston they could just as easily stuck enough to jump the camchain whether or not it is stretched or worn. Since the intakes are still open ABDC if they are sticking they can stay partially open till TDC at which point the piston will power assist them shut. Usually the bike will barely be running at this point anyway since a great deal of he compression is being pumped back out thru the carb instead of trapped in the cylinder to burn. At this point you need to check to see if the intake valves are bent. The bike won't run right if they are and is lucky to run at all if that's the case.

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