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Old 06-16-2012, 06:41 PM   #1
domains OP
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Motor almost jumped timing 180degrees

Hi...XR650L Before dissembling motor I checked compression and got a reading 30... 60... and one time 90... So thought I bent valves. Took it apart and did a leak test with head upside down and it seems fine. I was driving real slow in first gear when it jumped and it almost started with 170degree jump.
Do i have to take valves off or the leak test is enough? Think my compression tester is messed up, its those rubber tips you push instead of threads. When I push on it maybe the tip closes, works fine on my skidoo but ATV plug's smaller.

domains screwed with this post 06-17-2012 at 12:03 PM
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Old 06-16-2012, 08:13 PM   #2
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Did you make sure to hold the throttle WFO during the test? If carbs, must insure any vacuum slide is open too.
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Old 06-16-2012, 08:45 PM   #3
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Might take them off to be sure... need to borrow tool
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Old 06-17-2012, 01:46 AM   #4
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Maybe this mysterious motorcycle has cam shaft decompressors to get those low psi readings.
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Old 06-17-2012, 10:13 AM   #5
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My cam chain tensioner was shot from the start making it jump with new chain and sprockets.
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Old 06-17-2012, 12:01 PM   #6
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Took them off and they don't look bent while turning them with your fingers but with drill you feel a little wobble.

Tried my old oem valves to compare (Cheap 80$ valves on ebay)... maybe i'm better off with stock only 12000 miles on them.
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Old 06-17-2012, 12:45 PM   #7
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Your working on what? A john deere tractor???
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Old 06-17-2012, 01:34 PM   #8
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Now that you edited your original post. Them XR650L have a decompressor.
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Old 06-17-2012, 01:45 PM   #9
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I had The valve cover off when checking compression.

I noticed the rubber of idler was chewed up but never thought it could make the bike jump timing.... people say they make noises when worn out but i heard nothing, bike was running great.

It jumped a few times because I noticed power loss... thought maybe it was my imagination :)
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Old 06-17-2012, 01:49 PM   #10
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Did you put the bolt back in the cam chain tensioner shaft when you installed the new cam chain???
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Old 06-17-2012, 02:22 PM   #11
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mcma111... Yes bolt is/was there.

Now that I think of it I never heard it made sounds but sometimes when it stalled it made a loud metal to metal sound.

The shaft of the idler has marks on it aswell.

I bought a used xr600 off ebay shaft with idler. Next day I saw a new one off ebay 50$ but no shaft. I bet the dealer wants like 150 for it.

Bine a week still waiting for it :(
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Old 06-17-2012, 03:47 PM   #12
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Old 06-17-2012, 05:35 PM   #13
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Quote:
Originally Posted by itsatdm View Post
How do you know your bike was out of time? I assume you had the bike on its timing marks. You do know that the crank makes 2 revs for every one for the cam, correct? Just because the flywheel is on its mark does not mean the cam is on its mark. The cam lobes point point fore and aft, not towards the center, when all is lined up.

Probably the bike utilizes a wasted spark, meaning a spark on every crank revolution. Even if the timing is out 180, it may sound like it would like to run, but it won't.

It his highly unlikely you just skipped 180 degrees without doing some valve damage and it should be very visible.

You do not mess with a cam tensioner unless the bike is on its marks correctly. The chain will have varying levels of slackness as it goes through its strokes. On the mark with the piston TDC is where it has the most and when you adjust the tensioner.

No leaking of fluids or marks on the piston, make me think no damage, just improperly knowing TDC on compression and where the cam lobes should be pointing...

Put it back together correctly. As for checking compression with an auto compression release, most have weights connect with a coil spring. Take the spring off, open your throttle and try again.

Yes, I also wondered how an engine could jump 180. A tooth or 2 but 180 I don't think so.
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