[robert graham]

My 1999 Yamaha 90C has compressions of 5.76:1 on cyl.#1, 5.67:1 on cyl.#2 and 5.86:1 on cyl.#3 as per the Yamaha book. I'm certain the bores and strokes are the same for each cylinder, so the only variable can be the port timing. Does anyone know why these compression ratios would be different? The only other Yamaha motors with different compression ratios are the 175 2.6L VMax, 150 2.6L VMax and the 200 2.6L VMax. All the othe Yamahas have the same compression ratios for all cylinders. Any ideas on this?

[99yam40]

Robert,
I have no idea why they would be different on the same motor, I could understand a difference in models or years, but not 3 different on one motor.
Did you look at the part #s for pistons and others to see if they are the same?If the bore and stroke are the same and the pistons are the same the only other things that would effect it would be the head chambers or like you said the porting. Maybe something to do with the way air flow moves through the motor.
It is kind of like, why does my C40 have a different size main jet in the middle carb? They have done something strange to need different amounts of fuel in different cylinders on a motor.
Did you get that info from your manual or a different Yamaha book.
Have you ever checked your compression to see if it different on the three cylinders?

next time you go out tinkering with you motor would you check the fuel flow from your prime start system like I talked about in one of my other threads and get back to me on that. I would appreciate it.

[robert graham]

Yep, Different jets and compression ratios in some motors. Maybe only a Yamaha engineer would know these things but it is curious. I'll check the prime start and let you know.