extremely frustrated with 450!

33 replies to this topic
  • corndogg

Posted June 06, 2013 - 08:05 AM


hey, i do have a topic open about this already but i figured i would start a new one. i just finished rebuilding the top end on a 04 yz 450. new piston, new rings, and new timing chain now. the bike would start last night after getting pulled behind a truck and popping the clutch. but would not kick over at all just ocassionaly backfire. checked the timing about 15 times and all seemed good as well as my valve clearances. last night we checked the compression but was only getting about 15 psi so we assumed one of the rings were not on properly. when we pulled the head and cylinder all rings were properly inplace and not broken ( brand new) so next i did a leak text on the head and valves and all was good.

what could possibly be going on? if forwhatever reason my timing was off could it be giving me a low compression reading :s ( if its off its only a tooth or two)

any help would be appreciated as i almost traded the bike for a case of beer last night


  • Family Man

Posted June 06, 2013 - 09:06 AM


If the bike wont start after a rebuild dont pull/ pop start it. It should start easy if its timed right and the valves are in spec. I bet its not timed right. One more thing if you have it apart is hone the cylinder with a ball hone, helps the rings seat.

  • woods-rider

Posted June 06, 2013 - 09:24 AM


Not sure what year you have, but I believe you can't do an accurate compression test on models with automatic decompression which might explain the low pressure reading you got.

  • corndogg

Posted June 06, 2013 - 09:39 AM


its a 04 yz 450. even with a auto decompression it should have more than 20 psi when kicking it over should it not? and yes it does have auto decomp. and for the timing, im almost positive it is correct i will take a picture of it when i get it back together but my marks line up. one over the casing on the outside and the top one around 12 oclock. and both cam lobes are pointed outwords.

  • corndogg

Posted June 06, 2013 - 09:48 AM


wow i feel like a idiot hahah damn auto decomp. how did i not think of that. but i would still think i should be getting more than 15-20 psi. guess i still got a lotta work to do on this thing.

  • thefickler

Posted June 06, 2013 - 10:28 AM


A picture of where your cams are at when it is "timed" would be helpful.

Are you sure you have all the simple things verified? Gas turned on, adequate flow to the carb? Spark plug gapped correctly, coil firmly seated on the plug? Are you sure you're getting a good spark?

How are you attempting to stat the bike? Could you be flooding it before it ever fires?

What were your leakdown psi numbers? What was your input pressure and your "leak" side pressure on the gauge?

  • corndogg

Posted June 06, 2013 - 10:56 AM


carb was cleaned right before i tryed to start it, float is not sticking. and the plug is getting fuel when i pull it. plug is gapped. and gas is turned on. i just pulled the cap an plug put it agaist the frame an kicked an there was lots of spark. dont have the numbers from the leak test on the valves buddy did it at his place because i dont have the equip an he said all was good and hes a mechanic as well. im sure its going to end up being something simple that im overlooking and its driving me to drink haha

  • corndogg

Posted June 06, 2013 - 10:58 AM


for now im just kicking it over. i did a bump start on it and it fired after the 6th one but only for a moment and wouldnt go again. done with bump starting just going to stick with kicking it from now on after a member on here said.

for now im just kicking it over. i did a bump start on it and it fired after the 6th one but only for a moment and wouldnt go again. done with bump starting just going to stick with kicking it from now on after a member on here said

  • thefickler

Posted June 06, 2013 - 11:02 AM


Post a pic of your cams at TDC when you can so we can see how they look.

  • corndogg

Posted June 06, 2013 - 11:51 AM


allright will do, thanks for the help!! appreciated.

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  • corndogg

Posted June 06, 2013 - 12:58 PM


i know that i wont get a proper compression reading now because of the auto decomp but anyone have any idea what it would normally read on a properly running bike with auto decomp? 15 psi cant be right even with that feature.

  • KTM_Pat

Posted June 06, 2013 - 02:25 PM


Did you remove the buckets to the valves when you rebuilt your top end? Common issue is people remove the buckets and the shim falls out then they put them back in the wrong location when rebuilding. You could have a valve not closing all the way. Check your clearances and make sure your not zeroed out on your cam. Common mistake I've seen. Hope its that simple for ya.

  • YZPaGuy

Posted June 06, 2013 - 05:29 PM


Rag in the air box or intake on the motor or exhaust?

All electrical connectors tight?

Throttle adjusted correctly?

I always go to the easy stuff, it usually something stupid I overlooked when I have problems

  • corndogg

Posted June 07, 2013 - 04:51 AM


here are some pictures of my timing i have set up right now. double checked for any shims not in proper place and clearances last night.
cam1.JPG cam3.JPG cam4.JPG

the first image is the first set up i had and the second is the way it sits right now. the picture of the lobes is for the second option. which looks better? the first image has 13 pins between 12, and the second only has 12.

Edited by corndogg, June 07, 2013 - 04:51 AM.

  • corndogg

Posted June 07, 2013 - 04:52 AM


and im 100% sure its at tdc i double checked with a screwdriver in the plug hole with the alignment marks.

  • thefickler

Posted June 07, 2013 - 05:34 AM


What chain did you go with? It seems like something is off with your chain. Are you sure its for your 450?

I just replaced mine, as it was stretched pretty bad. I should have replaced it after I bought the bike last year. I attached a pic of what my chain looked like before I took it off. It was stretched pretty good,

Attached Thumbnails

  • P5060307.JPG

  • DC_Excitement

Posted June 07, 2013 - 05:52 AM


it looks like one of them may have spun?

  • corndogg

Posted June 07, 2013 - 06:04 AM


in the picture is the old chain, the new chain was the wrong size, not very happy with the yamaha dealer about it. and a new one is a 2 week back order right now.

  • KTM_Pat

Posted June 07, 2013 - 06:05 AM


the first picture is correct.

i "think" you have a valve problem... here an easy test for ya...

Take off the carb to expose the intake, Take off the stator cover and put socketed wrench(rachet) on your crank. Put your hand over the intake and turn the crank manually with your wrench. you will feel the intake suction on your hand on the downward stroke(1). Then on the upward swing of the stroke(compression stroke (2)) you should feel "very little" to "no" back pressure on your hand and you will feel the pressure build on your wrench as your doing the compression stroke. Now very cafefully while you build pressure look for the TDC mark, when you hit your TDC and continue down on the exhaust stroke(3) you will hear the release of air into your exhaust. (should not be feeling any air flow on your intake with your hand) then on your upward (final stroke (4)) to clear the exhaust near TDC both exhaust and intake will open together for about a degree cause of airflow dynamics so don't be worried about that. Then whole process repeats.

In my experience people make the mistake of valve not seating properly, Buckets not seated right or binding, shims in wrong place, bad valve seats, etc... The test i gave ya should just make sure your sequence of operation is correct. If the test fails you have an issue with the head. If not you can look other places.

  • KTM_Pat

Posted June 07, 2013 - 06:08 AM


in the picture is the old chain, the new chain was the wrong size, not very happy with the yamaha dealer about it. and a new one is a 2 week back order right now.

An old chain can give ya issues for sure but no to piont where you can start it all. And not have good compression, in most cases. Unless the chain is about to snap.

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