Main Jets Different Sizes???

[RagingMain]

Hey Guys
Dont know if this has ever been addressed before but
I was reading through the tech manual at work and it calls for different size mains
1&3 152.5
2&4 150

Just wondering if anyone knew why?
Thanks
Cheers

 

[Shuriken]

On my XS11 (inline-4), it was because the inner cylinders did not cool as well, so richer jets were used on those to help keep temps on the inner cylinders down.

 

[maleko89]

Same reason for this V4. Rear cylinders get pretty hot. If I remember right Yamaha only put different main sizes in '85 and '86.

 

[godsend1]

So on my 86 I should leave them at stock? Not run all 150's?

 

[maleko89]

I wouldn't worry about it. For some reason Yamaha changed it to be the same size all the way around and the engines are the same. It would probably more beneficial if you had the mixture screws on the back carbs out a little more than the fronts. Main jets have their biggest influence from 7K on up and you should be getting some air when moving that fast.
IMHO.