No compression on 2008 YZ450F


10 replies to this topic
  • sgarrick

Posted September 12, 2011 - 04:47 PM

#1

I have absolutely no compression when kicking my bike. Started first kick, compression was stout and ran perfect last weekend. All I did was wash it between then and now. I have 39 hours a this new piston and cylinder and 6 hours on new cam chain. I pulled valve cover and only found center intake valve out-of adjustment ( .16), but would this cause compression to be so weak? Spark plug Looks perfect also, nice tan color. I can easily move kicker by hand and can see piston moving freely through spark plug hole, no strange noises at all either just no compression at all.

  • ChuckZilla

Posted September 12, 2011 - 04:59 PM

#2

When you had the cam cover off did you check to see if the decompression actuator is sticking or does it move freely?

  • grayracer513

Posted September 12, 2011 - 05:00 PM

#3

If you can see through the spark plug hole, I'd say that's your problem, but assuming you know that...

The first thing to check would have been the cam timing. Following that, when you found the center intake had increased clearance, which is abnormal, the next thing would have been to tap on the lifter a few times to bounce the valve shut. It could have been held open by carbon or something on the stem or seat. This will mean you'll have to reshim the valve again if that fixes it.

If neither of these has an effect, you need to have a quick leak down test done to see where the compression is escaping.

  • sgarrick

Posted September 12, 2011 - 05:13 PM

#4

Decompression actuator moves freely just checked it. Timing marks all line up and chain is nice and tight. I'll re-shim the center intake valve and see what happens. This bike ran perfect last weekend at the track and all I did was shut it off and take it home. Thanks for the help

  • alwho44

Posted September 12, 2011 - 05:30 PM

#5

2nd on the leakdown test. easiest way to find where the cylinder is leaking
good luck

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  • grayracer513

Posted September 12, 2011 - 06:11 PM

#6

If you haven't reshimmed, don't. Not until after you see if the valve can be bounced the rest of the way closed.

  • sgarrick

Posted September 12, 2011 - 06:35 PM

#7

Good call grayracer, I bounced the valve as you suggested and valve is sticking open. As I rotate the cam and watch the bucket, it will stay all the way down flush and then slowly pop back up. A couple of times the bucket stayed down for a minute before popping back up. I put a magnet on the bucket and it moves freely . All the other buckets pop up and down normally. Looks like I'm doing a valve job. I've got 140 hours on this bike and this is my first issue, not bad. I still love my YZ!

  • grayracer513

Posted September 12, 2011 - 09:16 PM

#8

So, then, let's try something.

Get a bottle of a name brand injector cleaning additive like Wynn's or STP. Put some of it in a small squirt or spray bottle, or a small syringe. Remove the carb, and open the intake valves. You'll see the ridge on the valve stem where the carbon has formed. Spray a bit of FI cleaner (or brush it on) onto the valve and leave it for a few hours, possibly refreshing it from time to time.

After a while, dab a bit of engine oil on it, then see if it still sticks. With luck, you can get it going again. Use the label on the FI cleaner to determine an appropriate amount to add into the fuel tank and run some through the next couple of tanks.

You'll probably be able to dodge the valve job for another 100 hours.

  • blakers213

Posted September 13, 2011 - 04:42 AM

#9

Grayracer, Will you move next door to me?

  • motomanwr250

Posted September 13, 2011 - 12:32 PM

#10

I had the same problem. It turned out to be a stuck valve on my 2008 yz450f.

  • sgarrick

Posted September 14, 2011 - 06:07 PM

#11

I tried what grayracer suggested, but no luck. I pulled head today and found that if I push valve down far enough it stays stuck open. It does not appear to be bent and head is in perfect shape and surprisingly really clean as is the piston. There does'nt appear to be any crap or build-up around valve or head and stem looks shiny clean when valve is stuck open. I'm guessing valve guide? Anyway, going to do a complete valve job now that I have head off. Thanks for all the suggestions. I'm new to this forum and really appreciate the help.





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