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Discussion Starter · #1 ·
..I've been kicking this idea for a while. Here is where it all started out...

I purchased a XS650 being fed off of twin 36mm round-slide mikunis. Too big in reality. And the 350 Honda has the factory jetted Keihin's(spelling?)

So I said, why not build manifolds for both of the bikes, and run each with a single 36?

Some may say, why would you run the 650 twin with one of the carbs it has two of already!? I spend some time on the JJ, and while I really just lurk, I do learn some things. I have seen many machines, mostly British, running a single 32-40mm carb with displacement upwards of 500cc, and be just fine. In fact many were built that way, with the associated cast manifold.

Few reasons why, as I'm sure some are wondering.

Simplicity. Single cable, single idle screw, single mixture screw, one set of jets to change, no chasing problems dependent on the carb. I also think one carb sticking out either side of the machine looks tits.

I have them already. I have the carbs, the XS and the Honda can both benefit from this. I'm about to move to a very cold state, and I need something to keep my hands busy during the winter.

Modern(new) carb bodies. Lets face it, the factory Keihin's aren't particularly the cats pajama's. I mean really, who designed circular float bowl gaskets to go onto a five sided float bowl? Some of the fuel passages can be corroded, and well, get to that point, and there is not much one can do. Parts for the Mikuni's are readily available, jets of all sizes, and full tunability. Jets, slides, springs, needles, anything and everything.

Jetting for conditions. Yup, the 350's jets are a little on the rich side. Am I going to change them? Probably not, and why? Well, lets think of the fun of rebalancing the carbs upon re-installation, resetting the cables to sync them. And that terrifying feeling I get when I feel like I'm going to strip every thread on the body when I tighten the screws down. Singular carb? Off, rejet, install, set idle. Go RIDE.

I'm sure some on here will be against the plan. And I'm hoping some will think it might not be a half bad idea.

Thoughts? Opinions? Discuss.

Steve
 

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I say why not??
Go for it.
I'm not sure how you'll come up with a manifold, though.
Unless you know someone who owns a foundry, or have your own machine shop.
 

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Discussion Starter · #3 ·
tbpmusic said:
I say why not??
Go for it.
I'm not sure how you'll come up with a manifold, though.
Unless you know someone who owns a foundry, or have your own machine shop.
I'm thinking tubular stainless would work fine. I'm going to get me some brew and stare at the bike for a little while tonight and see what I can scheme up. I might have an issue with clearance between the backbone and the manifolds, but we'll see...

Steve
 

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If you know any engineers, you may want to consult them.
I suspect there may be a LOT of variables involved, like making both "paths" to the cylinders the same length, all that sort of stuff.
 

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It's a piece of cake. On a site I used to frequent dedicated to the Kawasaki KZ400 & KZ750 twins, there were 4 or 5 guys that had done it and posted write-ups on how. Some were sophisticated, equal length deals. Others were less so, with the carb sticking out to the side. All claim to run great, once the jetting got sorted.

Go there and search. http://www.armbell.com/forum/index.php?mforum=kz400

I owned a Honda CM185T that had one carb. All the benefits mentioned - simplicity, ease of maintenance, etc. The Honda 250 Rebel and Nighthawk models have always been single-carb.





I say go for it.

Kirk
 

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kirkn said:
It's a piece of cake. On a site I used to frequent dedicated to the Kawasaki KZ400 & KZ750 twins, there were 4 or 5 guys that had done it and posted write-ups on how. Some were sophisticated, equal length deals. Others were less so, with the carb sticking out to the side. All claim to run great, once the jetting got sorted.

Go there and search. http://www.armbell.com/forum/index.php?mforum=kz400

I owned a Honda CM185T that had one carb. All the benefits mentioned - simplicity, ease of maintenance, etc. The Honda 250 Rebel and Nighthawk models have always been single-carb.






Never know til you try. ;) Besides, like you mentioned, there used to be quite a lot of bikes that only had one card. (a lot of the British bikes, up to the 700cc Royal Enfields, in fact, were that way!) Even the Indian/enfield Apache 700cc verticle twin had a single carb and that sucker would really roll on! :cool: ;)
I say go for it.

Kirk
 

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tbpmusic said:
I suspect there may be a LOT of variables involved,
Indeed..... The intake lengths must not only be the correct diameters and length (for the desired torque curve) the carb must be large (throat) enough to feed the added volume of the combined intake plus the "currently active" cylinder, but it must also be small enough NOT to flood the cylinder,......Jetted large to allow the necessary charge, but small enough that the manifold doesn't "condense" or accumulate excess fuel between intake openings
a 180 vs 360 crank will affect necessary carb size as well......
simply to have it run is not too hard to do, ....but to run well...there's LOTS to consider if optimising performance.....
BUT, have fun!.....It IS possible!....... Steve
 

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A lot depends on the performsance your expecting as well.
Torque should improve at lower rpm but power will drop off just about everywhere. You don't need as big a carb as you may think as your dealing with a pulsed airflow and not steady state flow so carb should actually be more efficient and jetting wont need to be so rich at low/mid rpm.
PJ
 

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Discussion Starter · #9 ·
Hey folks, besides the simple facts I know about designing an intake manifold with proper runner length..how much of a negative effect would there be with building basically a log type manifold and mounting the carb off to one side? There are a few automotive applications that I have seen this used in, with basically unequal length intake runners. I know this isn't ideal, but just for shake down/'hey lets just see if this will work' applications, I could swing this right? Thoughts?

Steve
 

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Yep, you can do it and it'll work fine with no issues. I've seen the results, as posted above.

I mean, you know going in that this isn't the 'optimum high performance' set-up, but there shouldn't be any running issues at all.

Keep us posted!

Kirk
 
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