Lack of compression?


7 replies to this topic
  • elgin repair

Posted April 29, 2011 - 11:57 AM

#1

I'm a small engine guy but trying to get a 99 YZ400 together for my son. It was bought tore apart needing a timing chain. Its all back together now but won't start. The starting procedure states to push the kick start to the extreme resistance position. There isn't one. I can , with modest effort kick the engine over and over. There is a different "feel" when the compression release is on (though missing the cable and lever for it grrr!) but not a bunch easier. As best I can I've followed the Clymer manual, it states to install the cams at TDC identified when the "I" mark in the flywheel is in the side case sight hole. This one has what looks like the word "HI" written on it. no other markings. Used the I, would that be right? Otherwise the cams are installed with the lobes opposite each other, the 2 dot markings on exhaust cam gear facing out and down (or up). Overall condition of the piston, cylinder and valves seemed quite good. Any ideas?

  • grayracer513

Posted April 29, 2011 - 12:21 PM

#2

The I is the correct mark. Each of the stock cams has three timing marks on it: a dimple with an E, another across from it with an I, and a single dimple in between.

Verify that the I on the flywheel is actually TDC by probing for the piston with a long thin screwdriver through the plug hole. Then, with the engine at TDC, align the exhaust cam so that the E mark is level with the top surface of the head. Be sure that the front run of chain is drawn up slack free when you check. Then align the intake cam so the I aligns with the top surface of the head, being sure that the front and top runs of chain are free of slack.

Posted Image

  • clutchkowalski

Posted April 29, 2011 - 06:37 PM

#3

You say it's missing the cable and lever for the compression release and the fact that it kicks over easily makes me think someone has already converted it to auto decompression maybe?

  • grayracer513

Posted April 29, 2011 - 07:34 PM

#4

The way to tell that is if it has a flyweight assembly on the exhaust cam sprocket.

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  • elgin repair

Posted May 01, 2011 - 06:01 AM

#5

I'll tear into it again this morning and have a look. It would be nice if it was already an auto decompression. Thanks for the fast responses.

  • elgin repair

Posted May 01, 2011 - 09:49 AM

#6

When the "I" on the flywheel is between the two marks on the engine cover access hole and the piston is at the top of its stroke this is TDC right? This is the orientation of the cams, both gears and lobes.
Posted Image
http://i1129.photobu...pair/gears1.jpg
Also this is the compression release (missing the cable and lever) doesn't look like an auto anything.
http://i1129.photobu...ompression1.jpg
The piston and bore looked in very good shape and the valves moved freely and were in good shape.
Any idea why there is no need for the compression release as it kicks over relatively easily but won't start (doesn't even sound like its trying to fire.)
Getting frustrated, don't know what else to do?

  • sledtrash

Posted May 01, 2011 - 05:52 PM

#7

I'm a small engine guy but trying to get a 99 YZ400 together for my son. It was bought tore apart needing a timing chain. Its all back together now but won't start. The starting procedure states to push the kick start to the extreme resistance position. There isn't one. I can , with modest effort kick the engine over and over. There is a different "feel" when the compression release is on (though missing the cable and lever for it grrr!) but not a bunch easier. As best I can I've followed the Clymer manual, it states to install the cams at TDC identified when the "I" mark in the flywheel is in the side case sight hole. This one has what looks like the word "HI" written on it. no other markings. Used the I, would that be right? Otherwise the cams are installed with the lobes opposite each other, the 2 dot markings on exhaust cam gear facing out and down (or up). Overall condition of the piston, cylinder and valves seemed quite good. Any ideas?


http://www.yamahaown...ook.com.au/?r=0 I think that you can download a factory manual here for it.

  • sledtrash

Posted May 01, 2011 - 05:54 PM

#8

Here is a link to where you can download the factory manual.

http://www.yamahaown...ook.com.au/?r=0





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