Timing problem


7 replies to this topic
  • jasonlion54

Posted May 22, 2011 - 04:58 PM

#1

I'm sorry to post this because I know that the topic has been covered, but I just can't figure out what I'm doing wrong. I put my valve train back together and the timing is off. Here's the background:

I had a heat problem that caused the head to warp a bit and the head gasket to fail, so I had the head resurfaced and replaced the gasket. I reassembled everything and now it kicks over way too easy (no compression). I put a plastic bag over the muffler and when I turn the engine it's actually sucking air in the muffler, so the timing is drastically off.

As I understand it, the timing is determined by only three things, the timing marks on each cam and the flywheel. All of these are lined up properly. I made a picture to clarify how everything is lined up:

Posted Image

There are thirteen pins on the timing chain between the 12 o'clock marks on each cam.


The spark plug is tight, all the head bolts are tight, and the auto-decompression system seems to be working properly.

I feel like I must be missing something really obvious because the timing is so wrong. Please help!

  • jasen 2007

Posted May 22, 2011 - 05:09 PM

#2

Just for some clarity, what are you working on? I just did 06 that way, I used the basically the same reference points. Mine runs great.

  • QuietFlight

Posted May 22, 2011 - 05:28 PM

#3

Get a long skinny allen wrench and set it in the spark plug hole. Verify TDC by rocking the crank bolt back and forth noting the movement of the allen, use a breaker so the engine doesn't roll on you. Set the timing like you have done and give it another whirl.

  • jasonlion54

Posted May 22, 2011 - 05:56 PM

#4

Sorry, I forgot to mention that this is an 08 YZ450.

I did verify TDC using a screwdriver and it matches the reference mark on the flywheel.

Thanks for the suggestions. I'm wondering if there is some other unrelated problem, like a stuck exhaust valve or something like that? Everything on the head checked out visually when I put it together.

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  • grayracer513

Posted May 22, 2011 - 07:44 PM

#5

As I understand it, the timing is determined by only three things, the timing marks on each cam and the flywheel.

Assuming that the cams have not slipped in their sprockets, yes.

There are thirteen pins on the timing chain between the 12 o'clock marks on each cam.

On a YZ450 or a YZ426, yes.

I put a plastic bag over the muffler and when I turn the engine it's actually sucking air in the muffler, so the timing is drastically off.

Not necessarily. The exhaust valve opens around 115 degrees BBDC on the power stroke, and at cranking speeds that can create a vacuum in the exhaust. That should be followed by a larger outward flow of air, though.

With the marks as you have them drawn, the cam lobes should be pointed outward and up slightly. Double check to be certain the cams are not one tooth retarded: Rotate the crank backward about ten degrees, then rotate first the exhaust and then the intake cam backward until the chain is drawn taut, then turn the crank slowly forward to TDC again and check the marks.

Also check the valve clearances. One possibility is a shim tilted in its pocket.

  • jasonlion54

Posted May 22, 2011 - 08:26 PM

#6

With the marks as you have them drawn, the cam lobes should be pointed outward and up slightly. Double check to be certain the cams are not one tooth retarded: Rotate the crank backward about ten degrees, then rotate first the exhaust and then the intake cam backward until the chain is drawn taut, then turn the crank slowly forward to TDC again and check the marks.


Thanks, Gray.

When you say backwards, do you mean counterclockwise or opposite the running direction?

The chain is already taut since the tensioner is installed. Should I undo it to allow the cams to rotate independently?

I will check the valve buckets, but how do I know if the cam sprockets have slipped?

  • grayracer513

Posted May 22, 2011 - 08:53 PM

#7

When you say backwards, do you mean counterclockwise or opposite the running direction?

Opposite the way it runs.

The chain is already taut since the tensioner is installed. Should I undo it to allow the cams to rotate independently?

No, that advice assumes the tensioner is still out of place.

I will check the valve buckets, but how do I know if the cam sprockets have slipped?

If the clearance is OK, there's no need to concern yourself with the shims being tilted.

If the cams slip, it's usually because they seize in the head. If there was no evidence of that, they probably didn't. Normally, they look like this when the timing marks are aligned:

Posted Image

  • jasonlion54

Posted May 24, 2011 - 07:13 AM

#8

Also check the valve clearances. One possibility is a shim tilted in its pocket.


That was it! Fired right up! Thanks!:smirk::smirk:





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