Pistons...

[Travis]

01ps, it's great to hear some real world feedback on using a single carb on the XS400..

 

[lhaolpa]

XS650s have 360 degree cranks. Both pistons rise and fall together. One is on the intake stroke while the other is on the power stroke.

XS400s have 180 degree cranks. One piston rises as the other falls. One is on the intake stroke while the other is on either the compression stroke or the exhaust stroke depending which cylinder you call the first one.

How this all relates to 2 into 1 intakes for having a single carb...

On the XS650, the intake strokes are directly opposite of each other.

Intake Compression Power Exhaust
Power Exhaust Intake Compression

So the single carb is going see a normal amount of air being pulled through it at equal intervals, but now at every revolution of the crank instead of every other revolution like it would if there were 1 carb per cylinder. That's why you can almost get away with the same size carb with just slightly different jetting depending the new length of the intake tract (and some other things obviously, that are way outside of the scope of this explanation).

On the XS400, the intake strokes are right next to each other, one right after the other.

Intake Compression Power Exhaust
Exhaust Intake Compression Power

I'm not sure how that would work with a 2 into 1 intake setup with one carb. I don't know how one cylinder pulling in air one right after the other is going to affect things. Maybe someone can chime in about that. Is one carb going to get less fuel than the other one so the system won't work on a 180 degree engine? Or does it not matter that the intake strokes are one after the other? If there are problems, can that be overcome by intake design?

Maybe that's why one carb

has a bigger jet than the other? lha​

 

[Travis]

^ It does? Didn't know that.

None of that should matter though if you have two separate carbs. I don't see why one would need a larger jet if you have 2 carbs as they are basically independent of each other.

 

[drewpy]

^ It does? Didn't know that.

None of that should matter though if you have two separate carbs. I don't see why one would need a larger jet if you have 2 carbs as they are basically independent of each other.

it must be an idiosyncracy of the 400's, some have, some don't. they must be tested at the factory.