07'450 43 hours - little to no compression


10 replies to this topic
  • lftdjeep426

Posted July 08, 2009 - 05:25 PM

#1

I have a 07' yz450f with 43 hours on it and it hasn't been wanting to run.

Here is the other thread I origionally posted with my trial by error so to speak:
http://www.thumperta...ad.php?t=799910

From that time, I have completely switched the complete electrical system off of a running 07' 450 and still nothing. I kept kicking and trying to get it to fire and after about 10 minutes of kicking I pulled the plug again and it was bone dry. I pulled the carb and sprayed carb cleaner into the intake and it wouldn't even fire. This leads me to believe that I didn't have enough compression to suck the carb cleaner into the combustion chamber. I did a compression check and after about 10 kicks at WOT it built up about 35 psi. It should be over 100. I checked my gauge on an old honda xr80 and it built 105 lbs after 4 kicks.

I have a hard time believing that I have bad piston and rings at only 43 hours so this leads me to the valves. I have never ripped into a 4 stroke so I am a little leary. I did check the valves and the intake were within spec but the exhaust valves were loose if anything. I'm guessing that either there might be a problem with the auto-decomp or one of the valves aren't seating.

My plan is to remove the head on Friday and pour mineral spirits into the exhaust port and see if it bleeds past the exhaust valves and do the same to the intake. I figure that since I have it that far I should put a new set of rings in as long as the cylinder and piston are within spec (take those to the shop and have them gapped). If anyone has any pointers or any other ideas please post. If anyone knows where there is a good "how to" remove the head/cylinder and everything accompanying. Are there videos for this? Is there any special tools needed for anything I'm about to do.

Thanks guys ahead of time:thumbsup:

  • desert-rat660

Posted July 08, 2009 - 05:35 PM

#2

is the cam timing off?

  • lftdjeep426

Posted July 08, 2009 - 05:51 PM

#3

I don't think so. The engine has never been into until me. The valve cover had never been removed until me and when I rotate it to TDC the marks on the cam gear line up outside to outside level with the head. I'm assuming that means that it hasn't skipped timing. I had a 250f Kawi that spun the gear on the cam shaft but they were pressed on and these are bolted on so I don't see how that could be the case. :worthy:

  • desert-rat660

Posted July 08, 2009 - 06:15 PM

#4

i believe the stock cam gears are press fitted on the cams. did u install a set of hot cams?

  • lftdjeep426

Posted July 08, 2009 - 06:43 PM

#5

no but there is a 10 or 12mm bolt on the side of the cam gear I belive. I don't have it apart but I am almost certain that they were bolted on. I thought all of the newer 450's (any color) were bolted on because of the torque they have and the possibility of slipping the cam gear on the shaft. reguardless..... I still have to find the problem. I don't think it is out because I wouldn't have been able to slide the feeler gauge under the lobes if it had slipped time. I'm not100% sure with this statement either but if it were to slip time or spin the cam gear it would tighten the valve before they got loose and when I checked them the exhaust were loose if anything. :worthy:

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  • grayracer513

Posted July 08, 2009 - 08:07 PM

#6

Check the cam timing. None of the YZF engines of any size have cam sprockets bolted to anything. They're pressed on, and rarely ever slip. The cam chain can develop kinks and cause skipped timing, as can a partial seizure of the cams.

Any cam with a sprocket bolted in place is an aftermarket unit intended to allow indexing the cams.

  • lftdjeep426

Posted July 08, 2009 - 08:23 PM

#7

How do I check the cam timing other than make sure the index markes are aligned with the cylinder head with the crank marks lined up?

  • grayracer513

Posted July 09, 2009 - 05:34 AM

#8

If you suspect a cam of slipping in its sprocket, line up the timing marks to start with. Then check the position of the cam lobes from the right side. They should point out and up at about 10:00 and 2:00 from your viewpoint, and pretty much mirror each other.

If you have sprockets that allow adjustable timing, the process is more complicated, and involves setting up a degree wheel and a dial indicator.

  • gdbarr1976

Posted July 09, 2009 - 06:33 AM

#9

You said in your preveous post that it started after back firing but did start and you rode it down the block before it quit? Did it run ok after it did start and for that 1/2 block? If it did I would say it is not your cam timing or even fuel. I would guess electrical. The compression puzzles me if you say it ran fine right before it quit. Then ran fine after you restarted it and rode 1/2 down the block? Did you change out your coil? As in the spark plug cap. Mine went out and had very similiar symtoms. Ran fine for about 5min. Then would quit.

  • lftdjeep426

Posted July 09, 2009 - 08:28 AM

#10

You said in your preveous post that it started after back firing but did start and you rode it down the block before it quit? Did it run ok after it did start and for that 1/2 block? If it did I would say it is not your cam timing or even fuel. I would guess electrical. The compression puzzles me if you say it ran fine right before it quit. Then ran fine after you restarted it and rode 1/2 down the block? Did you change out your coil? As in the spark plug cap. Mine went out and had very similiar symtoms. Ran fine for about 5min. Then would quit.


Those were my exact thoughts also. I ended up swapping out the stator and pickup and the coil from a running 07' 450 and it wouldn't fire. I even took the CDI box out and tried a different one. The only thing I didn't swap that was electrical was the wiring harness.

It never back fired it would just start up in the morning and I ran it down the street and then it would die. I know the compression is really low though unless there is a proper/improper way to test the compression.

"Insert the gauge where the spark plug goes, hold the throttle wide open and kick several times to see what kind of pressure builds on the gauge."

Again it should be well over 120psi and it built 35 psi after more than 10 kicks.
:worthy:

  • grayracer513

Posted July 09, 2009 - 09:02 AM

#11

I know the compression is really low though unless there is a proper/improper way to test the compression.

"Insert the gauge where the spark plug goes, hold the throttle wide open and kick several times to see what kind of pressure builds on the gauge."

Again it should be well over 120psi and it built 35 psi after more than 10 kicks.
:worthy:

Again, compression tests on engines with auto decompression are reasonably useless, and the correct way to test combustion chamber sealing is with a leak down test.

It is interesting that your cranking compression is that low, but all it really does is indicate that you should test the engine. It should be noted that an engine which has low compression because of cam timing will pass this test near TDC and on the power stroke, but show a leaking exhaust valve too near top BTDC on the compression stroke.

In either case, a leak down test will not only accurately evaluate your engine's condition, it will pinpoint the problem(s), if any.





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