Why the different sized main jets?

[bgrubb7]

I've got an 81H, and I know that the outside 2 carbs come with 115s and the Inside 2 come with 120s

Why? How necessary is it to keep this staggered configuration?

It has been suggested to me in an earlier thread that when I rejet I should just make them all the same. Which makes obvious sense.

I was just wandering if this wasn't just done by Yamaha for emmisions purposes.

How many have rejetted and kept the staggered setup and how many have made them all the same? And if I make them all the same should I rejet from the smaller size or the larger size?

[prometheus578]

"That's waay cool!"

Inner cylinders have the larger jets so that they get more fuel and run cooler. The inner cylinders run hotter than the outer ones due to the fact that the outer ones have more cooling fin surface for heat transfer. Keep the larger jets in the middle

[bgrubb7]

Ya, that makes sense.

Then how come everyone with the Pre '80 carbs doesn't run out and stagger their Mains? Engines would still share the same temp characteristics, right?

Come to think of it wouldn't all air cooled inline 4s be the same?

[motoman]

If your at, or close to sea-level and using stock air-box(which is always best choice) which will allow some restriction creating a proper vacuum at any given rpm so carb diaphrams allow metering rod to rise out of main jet at correct rate, Run 117.5 mains all across and metering rod clip in third position(where it most likely already is). Ride it for a few days and check plugs for some color. If white and still looking new, drop needle clips one notch to hold them out of jets a bit more. Check plugs again, if the same, set clips back where they were and try 120 mains. Make sure also that floats are EXACTLY the same at 23-24mm before changing anythig!

[pggg]

The jets are staggered because the carbs are jetted too lean at the factory. Especially the U.S. models. In reality the heat difference is 2/10ths of bugger all. You'd think the bike will run smoother with each fuel setup the same. I'm picking most XS11's with staggered jets run as rough as guts.

[pggg]

PS, if the extra 2 or 3 degree temp is still causing loss of sleep, just bung a pair of BP7's in the middle 2. Now why didn't the Yamaha factory think of that huh?