Carb questions?

I'm looking into changing out the four carbs for one larger unit. Has anyone done this? In one of my recent chopper mags, the guy took an xj650 and used an S&S super carb, with a custom intake mani. The main reason I'm wanting to do so is for ease of tuning for a turbo setup. I'm just curious as to what size I would need? Is there a chart I can use to figure up the size?
Thanks in advance.

 

Interesting idea... but no sorry I know nothing about it

 

Well, I would figure that you add together all the sizes of each carb, and then get a carb that is the total size of all four, and make sure it flows the same cfm as all four combined, and you should be fine.

Maybe someone with more technical expertise can give you a better suggestion!

 

that would be great! If I knew the cfm All kidding aside, I really think it would be the way to go as far as reliability and tunability.

 

hahaha the nightmare of 4 carbs is part of the experience of owning an XJ.... without that its just a bike!

 

maximuschop, if you figure out how to so this, fill me in!

 

Well see, I actually plan to ride my bike rather than tune it all the time. I got enough headaches from my old cb, and 87 cbr. I just think this will be best as far as tuning for the turbo goes. Does anyone know the cfm?

 

>>...and make sure it flows the same cfm as all four combined, and you should be fine.<<

Not really. Each carb is only feeding one cylinder, and the single carb will also be only feeding one cylinder at a time. Making it 4 times larger than one carb would make it unable to run.

 

can anyone point me in the right direction?

 

I'm going to have to disagree, if only by virtue of the fact that older car engines were able to feed four, six, or even eight cylinders... using one carburetor.

Might be an interesting experiment, though.

 

The way I see it, you want to convert your Jag into a Ford Mavrick.

Sure that Jag is hard to tune but aint no Pinto either!

So as I see it you can go buy a Honda cb 450 and simply ride orrrr
you can learn to do what few others can do an REALLY RIDE!

1 CARB................... pretty funny!

 

I've seen a single carb on a twin, but not on a 4 cylinder...

Don't completely see the benifit, maybe you should look at adapting the turbo setup from a stock seca turbo, then maybe up the size of the turbo if you are looking for more power / more custom setup.

 

It's a long time since I was in a fluids lecture and none of my old textbooks are to hand (and I read the Wikipedia article on fluid dynamics and thought maybe the writer had swallowed my text books and that's why I couldn't find them :roll: ), but I vaguely recall that the airflow varies with the fourth power of the throttle diameter. In english, that would mean that double the diameter would allow a flow 2x2x2x2=16 times the flow.
If you want to quadruple the air flow of one carb, the fourth root of 4 is about 1.4 (1.4x1.4x1.4x1.4 is about 4), so if the carbs are 22mm (I can't remember if that's the size or not, I'm a techie, not a geek), then the larger size would be 22x1.4=31mm approximately. If the turbo carbs are a different size, then recalculate appropriately. (24mm=>34mm, 26mm=>37mm etc)

Assuming I've remembered my fluids okay, this will still only be a starting point as different carbs will have different flow parameters.

I suppose you could just compare it with engine sizes and engine speeds that others have used - a harley 1300 to 5 thousand (do they go that high ?) should draw a similar volume of air to a 650 at ten thousand.

One thing I can be sure of, though, is that a single carb would not be four times the size - a 4" throttle would be something to see though !

 

I would look into adapting but you still run into rebuilding/tuning issues. Wouldn't the bigger carbs foul out my little 550? Plus I have no idea where I could even find a turbo setup off of a 650 seca. I'm only going to be running 5-6 psi, so I'm not worried about gobs of power, just building something diff. from the norm.

 

Sorry - I thought you wanted to use a single carb. To have one carb doing the work of four, it will have to take quadruple the flow (four cylinders/carbs worh of air through one) - I was suggesting a way of calculating how much bigger the carb should be to make that change.

It would almost certainly be a way of detuning your engine, though, even assuming you found a way of equalising air flow through a manifold. Adding a boost to try and claw some power back sounds like a lot of effort to get only partial results.

Standard carbs don't need that much fettling - if there's no major problem just leave them and they'll work for years without any heartache. Certainly some fettling can help, but it's not essential.

 

I am, that was my initial question. But everyone seems against it. I know it can be done, I'm just trying to figure the type of carb I would need. If not, then I might try to find a 650 turbo setup.

 

To add some input, I'm interested in doing this also. Not so much to gat any more power. I just want to simplify the whole tuning issue and make things simple as possible (lsee moving parts, less to go wrong, etc.)
There is a site that make dual manifolds for 750 Hondas to run two carbs and they have great success with them. Here's the link:

http://www.cyclexchange.net/Honda%20Page.htm

So if you can go to two carbs, why not one? I would think the hardest part would be to fabricate the proper manifold (1-into-four) and find room to mount the carb so it works right. It would probably have tobe some type if sidedraft carb, as there is little room to fit manifolds and a downdraft carb behind the cylinders, under the frame rails on your bike. I'd assume there are some car carbs from say a Honda Accord, etc that may work. Infact, that's probably the best place to start. Go to a salvage yard and look at older Honda Accords. My '88 has fuel injection, so I can't do any comparisons with that. But the '87 and older (Civics also) have carbs and with some mods, the manifolds may even work if the spacing is close.
If you can run one carb in a 4 cylinder car, why not a bike? So you may lose some power, as long as it runs. You can always go forward from there. PD

 

VW ran a single bore down draft carb for the boxer motor. Datsun and Toyota and others ran dual side drafts for 4 and 6 cylinder inline motors in the 70's. The performance upgrades for the 6cyl at least were three side draft. Some of the big V-twin choppers run single or dual carb either down or side draft.

Tuning the intake runner length wouldnt be so simple. No one wants to go down in performance.


On the other hand, the recent ignition coil thread leads us in the best direction of fuel injection. no more corroded and stuck carbs Thats the direction I would go.

 

would the fuel injection off of a 600cc sportbike work? Then maybe I could use a power commander to fine tune the turbo setup with the fuel injection. Man I wish I could just swap in a 650 seca turbo setup, but I just can't find one.

 

well, if we think about it like this. A 650cc bike like ours redlining at 9500 rpms will max out (if 100%VE could be possible) at 218cfm, by my math. If this is true, each carb only flows a maximum of 54.5cfm.

Now, we take a common V-Twin like the newer H-D 88ci found in most of their bikes. At 6000 rpms, they could flow 305cfm (if 100%VE is possible).

Even the H-D 883cc Sportster only flows 187cfm at 6000 rpm(assuming the afore mentioned volumetric effeciency could be possible).

So, tell me folks........

Why could we not try to make an equal length intake manifold and place a single carb on it's end that could flow the proper amount of air. All that's needed after that is to make sure that the jets and adjustments are within limits for "proper" operation of the motor.




I'm not saying I advocate this type of modification. But I don't like people saying it can't be done without any good reasons or data backing it up.

 

Oh, you could do it.

You might ask yourself why Yamaha didn't though. By the 80's carburetors were about as advanced as they ever got. For that matter, why did most manufacturers use 1 carburetor per cylinder?

Some reasons they may have had:

1) Packaging - easier to fit four in there than a manifold and one larger carb.

2) Weight - may well weigh less than the alternative.

3) Performance - narrow carb throats work better than larger ones. That's why performance cars came from the factory with 2 and 4 bbl carburetors and the Dodge Dart came with a big 1 bbl. If you really wanted performance you replaced the factory get up with 3 small 2 bbls (a six pack).

4) Throttle response. Longer intake runs introduce a lag in engine responding to throttle changes. Less of an issue when you have a long stroke engine generating lots of torque. Big deal when you need to spin up a high revving short stroke engine to get your torque.

 

MiCarl,
I see things in a whole new light the way you explain it. It may be alot of trouble to fabricate things to work properly, but still interesting to actually see it done. Some things are best left to people who know what they're doing. And then there are some if us who can never leave things as they are. I always like to see it on paper first before attempting to make the plunge and going under the knife. This isn't exactly brain surgery, but then I don't want to make another Frankenstien either. PD

 

Back in the mid to late 70's, the aftermarket turbo kits used a 42mm mikuni sidedraft on the intake of the turbo which went to a log type intake. The exhaust plumbing was pretty simple as well. These kits usually producced about 4-6 lbs of boost and basically added at least 50% hp gain. Hope these images show. This is a kit for the CB750, which shares the same carb spacing with our XJ650's. cruzerjd

 

>>This is a kit for the CB750, which shares the same carb spacing with our XJ650's. cruzerjd<<

Okay....this is intersting to know....Gotta think on this for a while.

 

The formula is:

Cubic inch X R.P.M. / 3456 =cfm.

650cc = 39.52 cubic inches
X 9000 rpm =355680
355680 / 3456 = 102.9 cfm

 

My statement is based on the article in the March 1983 Cycle World on hopping up the Seca 650. Seems that the in chasing a carburetion issue, they looked at replacement carbs. The pre '79 Keihens had the almost perfect spacing and were used with good results. I would guess that while the intake might match, the exhaust ports would be different. Still gives some ideas for turbo'ing an XJ. cruzerjd

 

Thanks techlinetom. I will just have to see what I can find in the way of parts.