Pistons...

hardtailed77

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STUPID QUESTION ALERT.....

I know my KZ400 and KZ440 both have a "360 degree crank", meaning that as one piston falls, the other rises. In turn, the engine only sucks in enough fuel as say a 200cc bike (200cc being half of our 400cc).

Do our XS's do the same, or do both pistons rise and fall in the cylinders at the same time?

Sorry for the stupid question, just a wondering...
 
i can't answer your question but i 've one question for you.
What do you think about KZ between xs400?
tks
 
the pistons rise and fall together. Remember these are FOUR cycle motors so there's an intake stroke, a compression stroke, a power stroke and an exhaust (followed by intake)
Up, down, up, down... That's two turns of the crank or 720 degrees. The 360 degree firing order means that as one piston is coming up on compression the other is exhausting. As one is on the power stroke downward the other is also falling, intaking.
 
nope :shrug:, the xs400 isn´t a "parallel twin" its a "180 degree-crank engine", very easy to hear by the sound, if you listen carefully while the engine is on idle. Greets from Munich Hannes
 
i can't answer your question but i 've one question for you.
What do you think about KZ between xs400?
tks
I like them both really. The parts for both bikes are pretty hard to come by, and sort of expensive when you do find them. I'm more of a "bobber" guy, so the KZ frame is better in my opinion....it only has a single backbone, as the XS has a triple. The single backbone lends itself to universal hardtail kits really well. On the other hand, the KZ dual carbs are alot more finicky than the on the XS, and they have rubber diaphragms in them that are RIDICULOUSLY expensive to replace. I think overall, they are about the same on performance really, but I can't really judge because my XS has been a hardtail since before it ran for me.
 
the pistons rise and fall together. Remember these are FOUR cycle motors so there's an intake stroke, a compression stroke, a power stroke and an exhaust (followed by intake)
Up, down, up, down... That's two turns of the crank or 720 degrees. The 360 degree firing order means that as one piston is coming up on compression the other is exhausting. As one is on the power stroke downward the other is also falling, intaking.
Ok, so if they rise and fall together....

I want to switch to a single carb instead of duals, but I'm having trouble finding out which size carb to use. On my KZ, I just searched for a carb from around a 200cc bike. So for the XS, I guess I need a carb from a bike that's more in the 400cc range? Does that make sense?
 
From the Haynes manual....

Engine%20Text.JPG


As erna said it its a 180 deg crank which is not so common on a parallel twin.

The advice from the Kawi forums on carb size for 2-1 manifolds for 400cc bikes is normally to use a 34mm carb, with the Keihin PWK 34 being the favourite. The members of those forums love a 2-1 conversion and there are loads of posts on it.
 
From the Haynes manual....

Engine%20Text.JPG


As erna said it its a 180 deg crank which is not so common on a parallel twin.

The advice from the Kawi forums on carb size for 2-1 manifolds for 400cc bikes is normally to use a 34mm carb, with the Keihin PWK 34 being the favourite. The members of those forums love a 2-1 conversion and there are loads of posts on it.
Thanks Cad...I'm definitely aware of the posts in the Kaw forums. I have a KZ440 and a KZ400, so I frequent those forums daily! haha I knew that the choice carb for the KZ's would be a 34MM-35MM, but the KZ's only suck air/fuel into one cylinder at a time, so I wasn't sure that the same sized carb would work on the XS seeing as how both cylinders are fed at the same time. It seems that the XS would need a bigger carb in that aspect.

I just purchased (today actually!! lol) a Keihin PJ 36MM carb to try on the XS. They usually run around $75-$100 on Ebay, so I think I got a steal on mine at $40!! It even came with the throttle cable and twist throttle assembly! I think I can get it to work, and since the jets are readily available and cheap, it shouldn't break the bank. Only time will tell I guess.
 
the xs only sucks air in at one time too. you get combustion every 2nd stroke. alternating in each pot.

I used to have a Z400 (uk name) hated riding it, sorry!
 
the xs only sucks air in at one time too. you get combustion every 2nd stroke. alternating in each pot.

I used to have a Z400 (uk name) hated riding it, sorry!
Ok, that clears it up perfectly! Thanks again Drewps!!

And ship that KZ over to me, so I can rid you of all the misery! lol
 
Oh, and it doesn't matter whether your bike has a 180 OR 360 degree crank, they are BOTH Parallel twins, as this term describes an engine with two cylinders side by side.
 
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?
 
So I've been doing a bunch of reading on this. Seems most people say one cylinder is going to rob the charge from the other so the system isn't ideal. On the other hand, a guy with a GS450 (another parallel twin with 180 degree crank) seems to have made his run quite well on one carb and he thinks it is because the duration of the intakes do not overlap. :shrug: So who's going to be the first to try it out on their XS400?! :D
 
I tried it on my XS400 this past winter. I had it starting and idling decent, but out on the road is ran like crap. Gutless, lean, very poor. I blamed it on the length of my intake runner letting the fuel seperate from the air, but the more I read about the firing order it starts to make sense that a single car is not ideal on this bike. I used my stock carb and played around with the jets a lot, but never really found a successful combonation. The right cylinder seems to be the main problem, as I had the carb mounted on the left side of the engine with the intake warpped around the back. You can probably find it on here if you search.

I'd love to see this work as I really like the idea of a single carb, just for something different.
 
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