sxViper Mountain Compression

[Automed]

I just checked the compression on my sled. it has only 450km and I know that the compression ratio is supposed to differrent on one cylinder to allow for identical carb jetting.

I got these readings (all tests were done twice with throtle wide open):

Mag cyclinder 125psi
Centre cylinder 125psi
PTO cylinder 115psi

Can anybody help clarify.

 

[Mtnviper]

Those readings look about right. Mag and center run 10 psi higher then the pto cylinder. If you ride the mountains (higher elevations) and use good premium fuel. You can pull 2 layers out of the head gasket, it raises the compression 5-10 psi. Throttle response is improved with a little more squeeze.

 

[Automed]

So it's the PTO side cylinder that they reduce the compression ratio on...

Here I thought it would be the center one...

Do you know why Yamaha did that? and how it was done?

 

[Mtnviper]

It's so you can run the same jetting all the way across. Probably to reduce warranty claims from installing the wrong jet in the wrong carb. The way they do it, is the PTO cylinder head chamber size is larger.