Single Carb Mod

[cywelchjr]

I thought I'd ask the same question I asked over on .org, as it has gotten a really good discussion going and I want to broaden the pool.

I had a thought today that there has been talk a couple of times about trying to put a single carb on our bikes, and the hangup has always been trying to fit 4 runners of equal length into the frame and still have room for the carb and the filter.

So I was thinking, self, I thought, why couldn't you do a pair of 2 into 1 manifolds, and then another 2 into 1 manifold to mount the carb onto. That should be able to be pretty short, and fit into the same space as the boots, carbs and airbox now, and still allow the carb and a filter.

So, what do folks think? Yes, I know, I have too much time on my hands, but that's gonna be that way for a while yet.

BTW, I think we've figured out that these engines at 9000 RPM flow 175 CFM so that would also give us the info to size a single carb if this turns out to be feasible. As well, from what I've been able to find, it appears that the BS34 carbs are supposed to flow about 110 CFM.

[Sol]

Interesting

I'm not planning on it but I like scheeming!
so as you describe it, it is 2 x 2into1's in parallel with another 2into1's in series making a "tree" and right into a horizontal type carb? The number of abrupt bends in the space would be a challenge. the shorter it gets the more it becomes a crosswise log with "T"s rather then "Y"'s. one might split the differance and do 2 x 2into1's in parallel with 2 carbs, shorter but the carb mounting spacing would be the hard part, without biasing the intake track. I saw somewhere around here someone who had 2 x 2into1's in parallel upstream of the stock carbs (for pods) that had a 45deg bend off the horizontal. if you could drop the intake track down lower like that and then have the 3rd 2into1 end with the carb connection vertical you <might> squeeze in a vertical carb. This might shorten the assembly but adds another level of complexity. Hum, How would one do it? I have not bothered to see if anyone has done this, but someone must have on some I4! I know there is a lot of talk about it for the old goldwings which have a layout that at first glance lends itself to this kind of thing, but those are totally different! one problem they have is fuel condensation since the run after the carb is longer (which would be the case on an I4 too). I think the forced induction guys just blow into 1 log intake and just deal with the imbalance, the pressure might mitigate that a bit for them. Good question! This is fun, ruminating on subjects I know nothing about! I'm sure I'm reinventing the wheel or making it square. I know of I4 cars with carbs that have manifolds that go 1 to 4 but they have more room to work with, and the tuners like to ditch it and give them 4 carbs! Thanks for the fun!

[Sol]

really getting into it...

Relocate the battery!

[soccer4m]

Single

I saw the post too on using a manifold 4 into 1, single carb,,,,sounds way too cool. I think the post mentioned a side mounted carb so it wouldnt interfere with the extended inlet manifold which would mount to the intakes. Or,,,an extended intake manifold, 4 into one with a redesigned air filter and removal of the stock filter system. It would remedy the one big problem on these great bikes,,,,having to have all carbs working in synch all the time and all the problems four carbs can bring. Having just one would be so simple and even better a single electronic fuel injector for all four cylinders. If someone could come up with a working 4 into one manifold and either a single carb or single injector that works,,,,the masses would DROOL uncontroloby,,,I know I would......ANY MECHANICAL GURUS THAT WANT TO DESIGN AND TEST THESE IDEAS ????????? WE NEED YOU.....................Mike in Sun DIEgo

[cywelchjr]

I asked because I'm toying with trying to try and put something together. I'm not sure a CV carb will work though, we may need to run something with an accelerator pump, as IIRC that's one way issues with responsiveness is handled at low RPM's in these situations. But I don't know quite yet, I'm still in the investigation stages. Then see if I can come up with a manifold setup that looks workable, and test fit it on my parts frame and engine. If it all fits and looks workable then test fit it to my bike (although I might put together the parts to make the parts frame run to test it, but maybe not, depends on how long it would take to test fit it.) Then test drive it and see how the driveability is. If I can come up with something that responds well, then we can work on seeing how it performs and see what we can do as far as tuning on it.

[Toolmaker Tim]

Volume and restriction

Seems to me that balancing the airflow by using different diameters would be the way to do this. If it was a 4-1 setup, you would have to restrict the flow to the 2 center intakes to get a balanced system. This may sound odd, but when the US Army attemped to make a short barrel carbine out of the M16, they ran into a timing problem. Because the gas port was too close to the bolt carrier, they couldn't get reliable function. What they did was increase the distance between the two by twisting the gas tube into a pigtail shape to increase the time interval for the gas to reach the bolt carrier.The pigtail shape was longer, so it increased the pulse time. It's going to take longer for the airflow to reach the outer two cylinders, so somehow we need to increase the length of time to the center ports to even things out. What are the advantages going to be over what we now have?

[cywelchjr]

Seems to me that balancing the airflow by using different diameters would be the way to do this. If it was a 4-1 setup, you would have to restrict the flow to the 2 center intakes to get a balanced system. This may sound odd, but when the US Army attemped to make a short barrel carbine out of the M16, they ran into a timing problem. Because the gas port was too close to the bolt carrier, they couldn't get reliable function. What they did was increase the distance between the two by twisting the gas tube into a pigtail shape to increase the time interval for the gas to reach the bolt carrier.The pigtail shape was longer, so it increased the pulse time. It's going to take longer for the airflow to reach the outer two cylinders, so somehow we need to increase the length of time to the center ports to even things out. What are the advantages going to be over what we now have?

That's why my thought was two 2 into 1's into a 2 into 1, which would give equal distance to all 4 intake ports from the carb. I have to do some layout work on it, this is something I just thought of on the way back to work from lunch today. But I'm beginning to think it might be able to be made to work. I don't know if it will work great, but I think it will be decent (I hope).

[crazy steve]

Well, it all depends on what you're trying to accomplish by doing this; if you're looking for more power, I can't see this getting you there...

Better fuel economy and ease of tuning? That I can see, but I think you'll take a real hit on top-end power, with it bleeding into the mid-range a bit. The problem will be uneven mixture distribution, and too many bends/corners in the intake tract will slow/impede flow. If you look at all the science that's gone into intake manifold design over the years, it's been all about having the straightest shot at the intake valve they can get. One of the reasons you've rarely seen multi-carb set-ups on anything other than performance cars (and motorcycles) is their relatively poor efficiency at lower engine speeds, while they shine at high rpm. Single-carb intakes are generally just the opposite; good down low, poor at high rpms, assuming decent design and a properly-sized carb.

What's the answer? Well, IMO I'd try a common-plenum type manifold, with long-as-possible individual runners into the plenum, then a single carb runner into that, located in the plenum center as close as you could get it; something like a 5.0 EFI Mustang intake. The big problem would be fuel drop-out (the fuel not staying atomized) as it makes the turns. I do think that this type of intake, if run 'dry' (direct port injection with EFI) could with a single throttle body offer real gains, but you're looking at a EFI conversion...

Carb size? Well, the 175 cfm number is correct if you get 100% volumetric efficiency, something rarely done. A more realistic number might be 90%, which lowers your cfm number to about 157, and even that may be too optimistic. Going too large lowers the vacuum signal through the carb at lower speeds and makes your low end really soggy (a lesson I learned the hard way..) and hurts mileage. A good choice may be the 40mm Harley Kehin CV carbs (certainly easy to find), as these have been proven to able to easily support up to 100 hp, a number a stock XS is very unlikely to exceed. If you're looking for something with an accelerator pump, the S&S Super E shorty would fit the bill. I will say though that from running both, the Super E can give a low-speed flat spot if throttle is applied sloppily. Total HP-wise, there's little difference. If you need larger, the Super G is the next size up, but it's not known for it's economy...

As an aside, the Harley CV is being used as a 'performance' upgrade on the Suzuki Samurai 4-banger, with only slight mods (rejet and a 'improved' float valve to allow a pressurized inlet) to replace the stock 2-barrel....

[yamahansolo]

Have you looked at a Turbo xs11 manifold? Seems like it would already be built.
Also I think you would lose alot of top end power with a single.
I have mentioned thus before. Research Tbilisi on 3 rdgen.org. I think a small throttle body injection system might work. Its 2 barrels and all you're Ned is tunercat and winaldl. You can get any old GM Ecu that is nit obd2. Like 87 to 95 models.the tbis can even be mounted sideways. You would also need a 11 to 15 psi fuel pump.

[dpotter58]

XS east this year i saw a V-Max with a Holly 650 mounted on it. the guy told me it was a little slow on take off but it really kicked on a roling twist of the throttle

[XSive Okie]

I think a small throttle body injection system might work. You would also need a 11 to 15 psi fuel pump.

Would the stock charging system handle all that? My bike barely has any headlamp output unless RPM's are up.

[crazy steve]

Would the stock charging system handle all that? My bike barely has any headlamp output unless RPM's are up.

That's been an issue with the few that have done this; everything electrically needs to be in tip-top shape...

[Toolmaker Tim]

Manifold

If one were to pick a spot where the carburator would be mounted, and fab the intake tubes to the outside cylinders from that point, you could then measure the length of those tubes. The next step would be to figure an arc that would reach the center pair of cylinders and also make the length of the center intake tubes the same as the outside. Are we talking a verticle mounted carb here or a horizontal mounted one, like a car?

[Toolmaker Tim]

Maybe crossing the center intakes would ad enough distance to make the lengths the same. You would have to figure out how to snake them around each other. Run one over the top and the other under that one. That way you could keep everything nice and compact.

[TADracer]

Something else to consider is the length of your intake tubes since the longer they are the better the chance that fuel/air will stack up when the intake valve closes causing a pressure wave in the runner. If this wave does not bounce back and hit the intake valve exactly when it opens it will cause a loss of top end power. That is why high RPM motors usually have short intake runners. I believe you will need to have some sort of plenum with a volume that is calculated for the efficiency of the motor and made in such a way that the flow stays swift enough to keep the fuel in an atomized state until it gets into the cylinder.