compression after cam 426 cam swap


4 replies to this topic
  • e30M3

Posted January 18, 2008 - 09:41 PM

#1

When I first got my 426 I road it a bit and it felt strong. I put in the 450 cam per the instructions and it still runs great, if not better.

My question is when I start the bike it is SO easy it almost feels like there is no compression. I never gave it much thought since it has always run fine but recently I rode an '03 450 and when I started it I could feel a lot more engine compression than my bike. Is it normal for my '02 426 to kick over so easy? I really don't have any experience with any other 4 stokes so I have nothing to compare it to.

  • grayracer513

Posted January 18, 2008 - 10:18 PM

#2

Once the engine is equipped with auto decompression, cranking compression is not indicative of the engine's condition in any meaningful way. Get a leak down test done. That will tell you accurately what shape it's in.

Remember that AD is not effective at all at any speed higher than about 600 rpm.

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  • e30M3

Posted January 19, 2008 - 07:44 AM

#3

Thanks Gray,

I had read that if the timing wasn't right it could make it harder to start (something having to do with the AD not engagin all the way), wasn't sure if the opposite could happen causing the AD to engage too much.

The bike appeared to be in good shape when I bought it and does seem to run strong (especially compared to the '03 450 I rode) so I don't think I will mess with a leak down test just yet.

  • KJ790

Posted January 19, 2008 - 08:10 AM

#4

The automatic decompresser eliminates compression when you are slowly turning it over. This is why it feels like there is little compression, because there really is little compression.

  • grayracer513

Posted January 20, 2008 - 06:45 PM

#5

Thanks Gray,

I had read that if the timing wasn't right it could make it harder to start (something having to do with the AD not engagin all the way), wasn't sure if the opposite could happen causing the AD to engage too much.


The automatic decompresser eliminates compression when you are slowly turning it over. This is why it feels like there is little compression, because there really is little compression.

What the AD system actually does is reduce compression by holding the exhaust valve off its seat during the first half or so of the compression stroke. The valve is reseated at roughly 50-60 degrees BTDC, and from that point, the engine compresses the air/fuel charge normally and fires.

Because of this, it does not eliminate compression at any speed. However, because it cannot be timed independently of the camshaft, incorrect cam timing will cause the AD to cause problems. If the cam is timed one tooth too far advanced, the AD pin will roll past the lifter too soon, and will not release enough of the compression stroke to even be noticeable, and the bike will be hard to kick over. If it's one tooth too retarded, then the AD pin will reseat the valve too late, and the engine may not develop enough compression to start at all.





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