How do you know when its time to chance the pistion?

[2012yz450f]

2008 yamaha yz450f

I had the valves adjusted about 10 hours ago, runs awesome, but when i stall it up at mcnutt, sometimes when i kick start the bike, it feels like there isnt any compression but it would fire up.

 

 Next thing i know its got lots of compression, i am getting confused.  I came from the two stroke world, normally if it start to feel less compression, almost time to change the piston.  Would this apply to my bike as well?

your feedback would be much appreciate,

 

David

[grayracer513]

Intermittent lack of compression is more often due to valves being temporarily held off their seat by carbon deposits at cranking speeds.  

 

To be frank, the only way to know if your piston needs replacement is to look at it and measure it.  Ring condition can be tested by running a leak down test.  

[2012yz450f]

Intermittent lack of compression is more often due to valves being temporarily held off their seat by carbon deposits at cranking speeds.  

 

To be frank, the only way to know if your piston needs replacement is to look at it and measure it.  Ring condition can be tested by running a leak down test.  

 

Thank you for your input, and sounds like i should do the leak down test to be safe.  Does the valves wear out fast? 

[Rocket416]

My bike is a 2009 model and it does the same thing as you described when trying to start it. I always thought it was the automatic compression release and it was nothing to worry about. Let us know what you find out.

Edited October 8, 2014 by Rocket416

[grayracer513]

  Does the valves wear out fast? 

 

No.

[2012yz450f]

My bike is a 2009 model and it does the same thing as you described when trying to start it. I always thought it was the automatic compression release and it was nothing to worry about. Let us know what you find out.

Oh is that right? if that is the case then i shouldn't have to worry about it.    I will ask Yamaha dealers and see what they say.