Compression on 650cc Yamaha

[gr8rg8r]

I am restoring a 1992 Boston Whaler Rage jet boat that has the 650cc 50hp Yamaha engine. The compression is 130 and 142.<br /><br />I thought compression should be within 10% of each, which this is, but I don't know what the acceptable compression should be.<br /><br />Is this acceptable?<br /><br />Thanks in advance.<br /><br />Richard

 

[gr8rg8r]

Re: Compression on 650cc Yamaha

As a followup, I am told the compression in a new engine should be 150. I am going to apply de-carb to see if the compression goes up a little from 130 and 142.

 

[BiXLL]

Re: Compression on 650cc Yamaha

150 seems a bit high for a stock yamaha motor. I have a performance head on mine and and have 155 and MUST use premium fuel. I believe stock compresion was more in the area of 115.

 

[Jeff Walkowiak]

Re: Compression on 650cc Yamaha

its fine don't mess with it its got time on it, after 12 years of use its great , don't fix what aint broke, or it may end up broke.