Why are the left and right jets different on carbs

[ksqrly]

So I noticed awhile back in my manual that the main jet sizes are different for the left and right. I have read that the left cylinder runs leaner so it needs a bigger jet, but why?
Also why is the difference for my bike #10 and the german version of my bike only has a difference of #2.5

 

[16VGTIDave]

Only DOHC engines have the jetting difference as far as I'm aware. As best as I can figure, it is due to the airbox design. The battery mount ends up shrouding the inlet to the right carb. Switching to pods and removing the airbox, or carving it open as I did, removes the restriction and allows both carbs to run the same jets.

 

[ksqrly]

Only DOHC engines have the jetting difference as far as I'm aware. As best as I can figure, it is due to the airbox design. The battery mount ends up shrouding the inlet to the right carb. Switching to pods and removing the airbox, or carving it open as I did, removes the restriction and allows both carbs to run the same jets.

Ok cool. So I can take from this that most twin carb bikes have the same jet sizes, as long as the flow is the same for both of them?

 

[Charles]

I thought it had to do with the phasing of the engine?

 

[ksqrly]

I thought it had to do with the phasing of the engine?

I have also heard this, but I have no idea. Anybody have some links to literature?

 

[heavymetalheavymetal]

my guess is because they run in series, the first carb might draw more fuel than the 2nd?

 

[bentwrench]

Once you switch to pods, there is no reason why they would be different.

 

[ksqrly]

Ok I will research this a bit more and let you know what I find. I am almost certain I read something like charles post, but not sure. It doesnt make sense to me that they use different jet sizes, but again I have no idea.

 

[16VGTIDave]

It is amazing what one can read on the Internet.

It is a 2 cylinder engine with separate carbs. Almost like 2 single cylinder engines running side by side. The only thing I've seen that would explain the jetting difference is the crappy air box. Which is eliminated when pods are installed. But I'll let you come to your own conclusions...

 

[Charles]

It's amazing what you can find on this very forum

http://www.xs400.com/forum/showthread.php?t=7986

I would imagine, depending on the position of the piston, perhaps due to phasing one side has less force pushing it down, which would mean less vacuum, which would cause one side to run a bit lean??

Hell, I dunno.

 

[ksqrly]

It's amazing what you can find on this very forum

http://www.xs400.com/forum/showthread.php?t=7986

I would imagine, depending on the position of the piston, perhaps due to phasing one side has less force pushing it down, which would mean less vacuum, which would cause one side to run a bit lean??

Hell, I dunno.

It wasnt that post that I read, but it was another two into one carb thread that said the same thing. I cant really find any scholarly evidence of it, but maybe its a myth. Does anyone have link that isnt a forum post. Its hard what to belive on some post because people say different things. UUUUUggghhh

 

[16VGTIDave]

Wait a second! The original question was NOT about single carb conversions or the postings of one member. The question was simply "why the jetting differences between the left and right carbs on a stock DOHC XS400 motor?".

My answer to that question is:
The stock air box shrouds the inlet to the right carb. Because of the reduced airflow, it has to be jetted leaner than the left carb. Switching to pod filters eliminates the need for jetting differences.

Now, lets try to not confuse the OP, or those who read this thread in 10 years...

Thanks,
Dave

 

[Charles]

Has nothing to do with single carb conversions. Dude says the phasing of the engine (again, has nothing to do with one or two carbs) causes the cylinder 1 to be lean. So basically, no matter what carb setup you use, pods or stock, cyl 1 will be lean. At least that's what he's claiming.

 

[ksqrly]

Has nothing to do with single carb conversions. Dude says the phasing of the engine (again, has nothing to do with one or two carbs) causes the cylinder 1 to be lean. So basically, no matter what carb setup you use, pods or stock, cyl 1 will be lean. At least that's what he's claiming.

Right, I just read the same thing on another two-into-one carb thread. It always comes up when people are doing 2-1 carbs, because they are worried about the different sides of the engine running at the same mixture through one carb.

So ya I understand that a single carb wouldnt change motor phasing, but that is usually the topic when I have read about this phasing difference. Am I making sense hahaha?

 

[16VGTIDave]

Has nothing to do with single carb conversions. Dude says the phasing of the engine (again, has nothing to do with one or two carbs) causes the cylinder 1 to be lean. So basically, no matter what carb setup you use, pods or stock, cyl 1 will be lean. At least that's what he's claiming.

Ah! Now I understand. Many get bent out of shape worrying about phasing, not realizing that as long as the crankshaft/camshaft synchronicity is maintained, it won't be an issue. Assuming the ignition pre-advance and post-dwell characteristics are correct, of course. And you know that the engineers in Japan were well aware of all this and designed our motors accordingly. How else would they have lasted so long?

So, not to worry!

 

[16VGTIDave]

And for the record, the preceding post of mine is pure gibberish. But it sounded good, didn't it!

My point is, don't trust one single source of information, or sources that all use the same data...

 

[Charles]

Then why should I trust you when you say phasing doesn't matter.

 

[16VGTIDave]

You shouldn't. But because you are promoting this nonsense called phasing, you should be able to explain how it does what it does and why we should be concerned.

Bring it...

 

[Charles]

Uh, phasing is a rather simple concept. It's how the crankshaft is set up to determine the rotation and the degrees at which each piston is in relation to the other. I'm not promoting anything, "phasing" is actual term. I'm simply trying to determine if it has anything to do with fueling as that one poster suggested. The XS400 is phased to 180 degrees. So when cylinder one is bdc, cylinder two is tdc. I guess I don't see why that would have anything to do with causing a lean condition. Like I said, simply discussing someone else's comment, chief!

 

[ksqrly]

Uh, phasing is a rather simple concept. It's how the crankshaft is set up to determine the rotation and the degrees at which each piston is in relation to the other. I'm not promoting anything, "phasing" is actual term. I'm simply trying to determine if it has anything to do with fueling as that one poster suggested. The XS400 is phased to 180 degrees. So when cylinder one is bdc, cylinder two is tdc. I guess I don't see why that would have anything to do with causing a lean condition. Like I said, simply discussing someone else's comment, chief!

So I have read the same thing in a couple of different posts, even on some xs650s. It doesnt make sense to me that the different cylinders would need different size jets, but maybe they do.

I read one post where I guy put a 2-1 on his 650, and said that he could never get it just right. He said one cylinder would be dead on and the other had to either be either to rich or to lean.

I believe the shape and air flow of the original box probably contribute to the main portion of the difference on my particular bike, but maybe phasing has something to do with it. Most bikes run the same size jets from what I have seen, so I think that is what Ill do, and go from there. After all jets arent that hard to change.