Kick through YZ400F TDC W/out decompression lever, Please Help!

6 replies to this topic
  • Jeff22

Posted March 19, 2013 - 10:14 PM


So the other day I had my yz400f apart, nothing too major, just changing a tire and the spark plug. Then I went to start it and it would kick past TDC fairly easily without use of the decompression lever. I kept slowly kicking it because I was in disbelief, and it slowly started to get some compression. I was weary, but decided to see if it would start anyways. So I kicked it over and it started first kick (like usual). I let it idle for about 15 seconds or so, then I shut the engine off and began slowly kicking it over again. This time it had more compression and I could feel when it hit TDC, but I could still kick through it without touching the decompression lever. I started it up once again and let it idle for a little longer so the oil could warm up before I changed it. Once I killed it again and began to kick it over, and it stoped at TDC. However when I put all my weight on the kick starter and push down a fair amount it pushes through. Before it was taken apart, it seemed to be running fine, mind you I'm not sure if I've ever put all of my weight on the kick start at TDC. I torqued the spark plug to spec, and that was the only thing I touched that would have an affect on the compression. I have read around and found that there is only 3 things that can cause this, the decompression, valves, and piston rings. I have yet to check any because my friend and I were going to check them sometime soon. The decompression lever seems to be operating properly and smoothly, from the outside at least. I have just recently got the bike and only gone on a few rides. I had no compression troubles when I was buying the bike, nor on my first few rides. So as it sits, if I put all my weight into it I can push it through TDC without using the decompression lever, and seems to be running fine other than that. Where should I be start on this? And would I be okay to go riding with it how it is?


  • etuke

Posted March 20, 2013 - 02:56 AM


the plug you took out did it have any carbon build up?A piece may have broken off and is now lodge between the valve and the seat.

  • Jeff22

Posted March 20, 2013 - 09:37 AM


The old plug was very sooty when I pulled it out.

Edited by Jeff22, March 21, 2013 - 08:11 PM.

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  • Jeff22

Posted April 04, 2013 - 08:42 AM



  • DGXR

Posted April 12, 2013 - 09:38 AM


I would either adjust the compression release way loose or just disconnect it, and see if the compression returns. It may be hanging up on something, perhaps just coinciding with the timing of your spark plug replacement. (Engines can start and run with low compression, ask me how I know.) Otherwise, I'd put in another plug -- clean the old one or get another new one -- and see what happens. You should always keep a backup spark plug anyway. Good luck.

Edited by DGXR, April 12, 2013 - 09:41 AM.

  • grayracer513

Posted April 12, 2013 - 11:17 AM


Compression has apparently already returned. This will happen occasionally, and it's nothing to worry about. The rings dry up and leak a little, or the valves were open when you parked it last and the seats rusted a little, whatever. Kicking through a few times to wet the cylinder usually improves things, and after you run it, it all seems fine. Pretty common.

  • Jeff22

Posted April 12, 2013 - 11:24 AM


Thanks guys, much appreciated.

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