04 YZ 450 will not start

2 replies to this topic
  • mvandyke

Posted October 26, 2013 - 06:30 PM


I'm new to the forum, and new to 4 strokes in general. I have always been a 2 stroke fan, but I picked up an 04 450 with a few nice mods for $800. The bike did not run, and the seller said that he thought it had jumped time. He said that he was riding the bike, shut if off normally, and it would not start again, crank and crank and crank, nothing. I have spent all day researching and tearing the bike down. This is what I found: Line the crank up at TDC( I couldn't find a line, but I found a dot on the flywheel through the timing mark hole, and using a screwdriver verified that the piston was at TDC.) The Intake side cam was 1 tooth off and the exhaust cam was 2 teeth off. The timing marks I used I got from a picture on google that showed the cams had a divot at the 12 o'clock and 3 o'clock position and I lined those up flush with the head surface. The chain was tight, the cam gears have no sign of slippage, breakage etc. I reassembled the motor, checked the carb, and it was clean cranked until I was blue in the face, still no signs of life. Spark was fine, everything was there, Just wouldn't fire up. Roll started it down a hill, it would run, but it killed out like it hadn't been ran in years. The plug when pulled back out was black and wetttttttt. Which leads me to believe it is in the carb.


As for the carb, The seller said it had the Accelerator pump mod, Done with an O-ring? I don't even know what an accelerator pump is, lol and it appears to have no hot start, and the choke will pull out, but will not stay pulled out. I know most of these bikes had a remote hot start on the handlebars, but this one doesn't have it. He said that he installed a JD Jet kit in the carb, and bumped the main up to a 175. What is the stock jetting or at least a baseline setting as far as air screw etc?


I am lost at this point! I know a motor has to have air, fuel and spark, but a thumper is a whole new ball game for me, could a valve be warped? Could the valves need shimmed? It is in the carb? Any help to get me out of this jam would be greatly appreciated? I would like to know how the carb is supposed to go back together properly. Thanks in advance!

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  • BBrown626

Posted October 28, 2013 - 06:32 PM


any luck with this?

If you look at the outer edge of the flywheel there should be a | mark rather than a dot I think. Download the manual from a link in the FAQ if you don't have one.

If too much fuel, maybe their is an issue with the carb? If extra wet not sure what that would be.

Does it feel like it has good compression when kicking it?

Popping the top and looking at the valves is fairly easy and will only cost you a gasket set for the head and cylinder. There could be some valve damage. At this point you don't have anything to lose by taking a look.

  • grayracer513

Posted October 29, 2013 - 07:03 AM


1) Use a fresh spark plug.  You can have one that will show spark laying on the head, but will have enough conductive junk on it that the compression pressure will "blow it out" in the engine, causing it to short as if fouled.


2) Fresh gas.  Modern pump gasoline can go stale in a remarkably short time under the wrong conditions and become nearly non-combustible.


3) If the flywheel was replaced by a weighted one, or had a weight added, it will cover the timing marks.  You should verify that the flywheel woodruff key is intact and properly in place.  The only way to achieve this is to remove the flywheel and look.  You'll need a $15 puller. 


4) Don't guess about the valve clearance, check it. 


5) Almost all of the starting troubles you'll run into relate to either cam timing, which you say you've checked, and problems with the carburetor.  Commonly, the pilot jet will partially clog with varnish if left to sit a while and dry out.  It only takes .002" to reduce a #45 pilot to a #35, and that would be so lean it wouldn't stay running even if it did start.  Check the pilot adjusting screw.  Be sure that it's still there, that it has a healthy O-ring on it, and that it's out about 1 3/4 turns from gently seated.  Might check the vacuum release plate on the slide to be sure it's not cracked, broken, or installed upside down.


Manuals are in Common Threads (scroll down to near the bottom).  Baseline jetting is listed there as well as HERE.


Attached is the stack up of parts you should find under the pilot screw, and the correct orientation of the vacuum release plate.

Attached Thumbnails

  • screw.png
  • slide.png

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