Automatic decompression troubles?

6 replies to this topic
  • JSJ17

Posted July 19, 2005 - 09:39 PM


I have a 2003 YZ450. I change the oil and clean the air filter religiously. I recently had the top end re-done, and everything seemed fine until I hit the 10 hour mark on the engine timer, now I am having problems with it again. On my last ride, it suddenly flamed out on me (stalled out on flat ground while I was giving it about 1/3 throttle with practically no load on the engine). When I tried to start it, I noticed that there seemed to be no compression for about six or eight kicks, then suddenly it was back to normal. I couldn't get it to re-light, so I decided to change the sparkplug (which looked fine). Again, it would kick through a bunch of times with no compression, then suddenly there was a TON of compression (I could barely move the kickstart). Then it would go back to normal (3 or 4 kicks with light compression, then normal TDC again).

After messing with it for a while I finally got it started. Initially it ran ok, but on the way back in it lost power twice, wanting to stall out again. Since I was idling back in fourth I had some momentum going, and it would come back and run fine again for a while. When I got home, it was VERY hard to start, and I noticed that it would occasionally try to die out even while it was sitting on the stand, with no load on the engine. I completely cleaned out the carb (found a little gunk in one of the jets and thought "Aha!"), bolted it back together, and went riding again. It started up immediately, again seemed to run fine, but 5 minutes into the ride it started the same "die out/come back" crap. Unfortunately, I didn't make it all the way in before it completely flamed out on me again. After kicking for a while, I gave up and managed to get towed back in. :applause:

So, now I am thinking it might have something to do with the automatic decompression? Has anyone else out there had something similar happen? I'm thinking about tearing into it to check things out ... any tips/suggestions?

Thanks !!

  • MNellis

Posted July 19, 2005 - 10:26 PM


I think your thoughts are on target. It sure sounds like you've got a malfunction with the auto-decomp mechanism on the exhaust cam. It's easy enough to pull out and check.

Good luck and let us know what you find.

  • grayracer513

Posted July 20, 2005 - 07:07 AM


When you say you had the top end re-done, what does that mean? Did you only do the rings and piston, or was it a valve job?

I ask because it sounds a bit like a valve sticking on a carbon build up on the stem.

Even when the decompression device is active, as it always is at starting speeds, the bike should have about 90-100 psi of compression. It is a compression reducer, not really a compression release. It is frankly more likely to fail into the inactive mode, and drive the compression up to normal levels. You could also have a tight valve lifter, possibly as a result of overtorquing the cam caps.

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  • JSJ17

Posted July 20, 2005 - 02:55 PM


The exhaust valves on it were burnt, so I had them replaced. Since they were in there anyway, I had them replace the pistion and rings, too. I had roughly 20 hours on the bike (bought it brand new) when it started making snapping and shredding noises whenever it was under a heavy load (low rpm through to high rpm, not at all like a lean decel pop). The valves were in spec, and I spent a couple of months chasing down false leads. I finally gave up and took it in to the shop and they showed me the damage. Practically a brand new bike with major engine damage and they never could explain the cause, even though they grilled me about maintenance and setup. Apparently, I had done everything right, but they said that they'd seen engines with 2 years on them that weren't as bad as mine. The whole thing ended up costing me $950 ... :eek:

Anyway, if it doesn't end up being the automatic decompression, I'm concerned that they may have put something back together wrong. You said that it could be a tight valve lifter -- wouldnt that show up right away (remember, it didn't start acting up until 10 hrs after the rebuild)? Any other ideas on what the problem could be? :applause:

  • 02YZ426

Posted July 20, 2005 - 06:30 PM


I would bring it back to the shop and let them look at it.. maybe they did screw up.

  • BC3

Posted May 19, 2007 - 12:57 PM


I just posted about the exact same situation.....I had the head repaired because of a seized intake cam bearing (was lack of clearance from factory) and the bike ran perfect....for about 10 hours if it's doing the same thing
yours is doing...I guess I'm going to take it apart and see what's up....


Posted May 20, 2007 - 06:10 PM


I have had this same problem on both my 04 and my 05 450. I could never figure out what the problem was. My 05 has no more than 50 hours on the motor. They both seemed to have plenty of power once they got started, but it always scared me a little bit that the compression would mysteriously just disappear. My 04 was that way for a year before I sold it, it never seemed to cause any noticable problems. I would like to get to the bottom of this problem though?!?!?

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