Yamaha 760 compressions

[Dave Danger]

I'm a bit out of my depth here, but searching for information.<br />An acquaintance that owns a yamaha wave runner powered by a 760 two cylinder called me a few days ago with an odd question. He'd had his engine in a local shop for some repairs, and had been advised that the compressions between the two cylinders were drastically different, requiring an overhaul to one of the cylinders. His confusion stems from a conversation he'd had some time earlier with another marine mechanic that said that the Yamaha 760 was designed with different compression ratios for the two cylinders... 6.8:1 for one cylinder and 7.2:1 for the other (He thinks those are the numbers he was told). His query to me was what the reasoning might be for that design, and had I ever heard of such a thing. I had to answer that I'd never heard of a varied compression engine of any sort. Nor could I think of a reason for it. Anyone here have any info on this?

 

[Ray Neudecker]

Re: Yamaha 760 compressions

It is not really that uncommon. The V-6 Special and Excell also had different port heights on 1 and 2 from the other four cylinders. We also do this on race motors.

 

[rodbolt]

Re: Yamaha 760 compressions

all the 760 motors I rebuilt usually had numbers within 5psi. good thing is its easy to do an inframe piston replacement on that motor or in the worst case they usually take less than an hour to set the motor on the workbench.

 

[Dave Danger]

Re: Yamaha 760 compressions

Ray,<br /> Bear with me here... What is the reason for a different ratio? I'm an aircraft mechanic with 30 years experience with piston engines and have no experience with an engine that would have varied compression ratios. This sounds like it throws all the standards... balance, fuel economy, etc out the window.<br />Give it to me in tech-speak if need be.. I can take it <br />Rodbolt, I'll take your word for the ease of access on this particular powerplant, but I'm still trying to convince myself whether or not this engine actually needs cylinder work. I'm assuming that a marine powerplant compression test would be taken similarly to an automotive engine... screw a gauge into the spark plug hole and spin the engine over, record the pressure and move to the next one. (Always has seemed a bit haphazard to me, having always used the differential style of compression testing, but that's probably comparing apples to oranges). If this particular engine actually has different compression ratios in the cylinder design, how do you reconcile the diferent compression test pressures you get ?? For that matter, will they be that much different?<br /><br />Dave