| Author | Message | |||
     Reepicheep |
I'm putzing with a 78 KZ-400 (a parallel twin), and I am going to have to pull the carbs for about the 11th million time, and I am wondering why the obsession with multiple carbs? Is it just a volumetric question? Is it that two smaller carbs can achieve more volume than one bigger carb? The blast uses a single big carb for the 500cc, and does fine for power. I wondered if there were other reasons for the huge extra complexity of running multiple carbs (which isn't just 2x the complexity, its like 3x the complexity, because now you have to sync them, and gang them for the throttle, etc.) | |||
| Hughlysses |
I think it's basically easier to control the fuel/air ratio over the range of possible flow rates with multiple smaller carburetors rather than a single large one. As a friend of mine on the Yahoo Ascot Group is fond of saying "Carburetor is a French word that means 'leave it alone'".  | |||
| Natexlh1000 |
With multiple carbs, the intake tracts can be made more straight. Look at the intake path of a harley. Goes in, takes a 90 degree bend, takes another 30 or so degree bend down. Pretty bad. Now imagine a big long inline-4 with all of those little hungry valves to feed. If you put the carb in the middle, the outer jugs have a longer path. I'm pretty sure the effective atomization would be altered by that. I think this would make it nearly impossible to get the mixture just right for all four. So make an intake manifold with curvy inner ports to make the lengths all the same, right? Now you're looking at a huge mess of plumbing in the middle of the bike. Can you imagine something like this living under the seat of a CB750?  Or you could get a big plenum type of thing but I think throttle response would suck bad as a result. You can see how much easier four identical carbs could become more attractive. For a parallel twin, I would wager that a single carb would be easier to work out though. Triumph had them for a long time like that, right? | |||
| Ratbuell |
...love me some leaning tower of power...nice! | |||
| Hughlysses |
Slant 6 with a Hyper-pack! Sweet! | |||
| Hootowl |
It's not the number of carbs per se, rather the number of venturies in the throttle body. That single carb has 4 venturies. The precision of metering is similar to 4 individual carbs, but you don't have to synch it. | |||
| Hootowl |
Doesn't really come into play except at low speed/low flow rates. A big venturi isn't as precise as several smaller ones at the same cfm. | |||
| Sifo |
Doesn't intake runner length have a lot to do with tuning? Short intake runners for high RPM power, longer runners for better response and torque at the lower revs, as I recall. Larger plenums, associated with single carb systems also affect the tuning. The typical UJM has a very good power to weight ratio, so they can be tuned with a heavy bias toward high RPM performance. I did have a buddy who had an older Gold wing. He said it was fairly common for owners to get fed up with the multiple carbs and do home-brew PVC manifolds. Not the best performance mod, but easy to maintain. | |||
| Fast1075 |
I had an old KZ-400 rat that was given to me. The carbs were rotted thru and thru. I had a 32mm Mikuni carb laying around. Made a manifold from copper fittings since I had some of those too (PVC would have worked fine too). A little jet fiddling and it ran fine for running around on. Don't have any pics, but here is the basic idea as bird's eye from above. Carb stuck out the left side. Used a foam filter sock.
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| Reepicheep |
I was thinking about getting a blast carb (cheap) and making my own manifold. But then I came to my senses.  Looks like the Mikuni VM30's are a common "swap over". And I bet I could get OK chinese knock off's of them on Ebay for $40 each. But I like CV carbs, if I can finally get these working. They've been nothing but trouble so far though, even after putting in the rebuild kit. Leaking needles, hard to hold a sync, and now one appears to have just stopped "carbing" all together. Thanks for the education. This was an informative thread. I think Buell made the right call to keep using one carb all the way up until fuel injection became practical. | |||
| Dwardo |
Multiple carbs cab both flow more and take better advantage of tuning the lengths of the intakes for better power within a given rpm range. They can also be easier to package. Back in the day, many British twins were available with either one carb or two (Triumph and BSA come to mind). In general, the two-carb versions made a little more power at the top end but the single carb bikes were easier to keep in tune. Use a set of multiple vacuum gauges to set them up. | |||
| Aesquire |
Mercury filled carb stick decorated the wall of my garage for years. Made synchronization easy after you pulled the carb rack & plumbed the hoses, then reinstalled. ( it was possible without pulling the carbs but cramped ) I had the model for six carbs but never used all six except once. On a Jaguar. It was made when the Honda CBX was hotness. But six bangers never really caught on. That's ok it worked fine on twins, triples & fours. My Suzuki 750 &1100 ran very smoothly & ran in the twelves & elevens if I did my part. I never did bracket race my Yamaha 750 triple. But by golly the carbs were sync'd. Unfortunately the carb stick got dropped & broke when I loaned it out. At least the mess wasn't in my garage. | |||
| Aesquire |
As pointed out accurately above. ... multiple carbs each have a stronger signal to each carb. More responsive & better fuel/air mix. At the cost of complexity & well, cost. I'm a big fan of the Slant Six. The stock intake manifold wasn't equal length like the beauty above. There was also a three Weber manifold but I've only seen it in person once. | |||
| Dwardo |
My late uncle had a 3-weber side draft setup for his Hudson Hornet race engine. It was beautiful. He needed money a few years ago so I helped broker a deal with somebody in Denmark (I think) who wanted it. They are pretty scarce now but you occasionally see one at a Hudson meet. | |||
| Reepicheep |
Syncing is easy. Which is good, because I have had to do the stupid job about 4 times now, and will need to do it a 4th time. I use about 20 feet of small diameter clear tubing, zip tied to a 48" dowel rod in a U shape, with two stroke oil in the lower 8 inches. It doesn't give you an absolute reading, but it lets you true any carb to any other carb with insane accuracy. You would watch a gnat's wing get ingested on that gauge. Total cost, $5, and if things go really wrong you just suck up the 2 stroke oil and your bike smells faster for the next 10 minutes. But one carb is a PITA. To carbs is worser. You couldn't pay me to do another rebuild on a 4 carb inline 4. (Well, OK, you could, but you wouldn't want to, based on what I would charge you). | |||
| Brother_in_buells |
These are CV carbs ,right? How are the rubber diaphragms? And leaking fuel past the needle is probably a worn or damaged seat. Maybe time for some carbs in better shape!? | |||
| Reepicheep |
The diaphragms were in good shape when I rebuilt the carbs the firs time (with new needles, I don't think the seats are easily replaceable in these carbs, but it's been a while). Carbs in better shape would definitely be warranted at this point. But I'm trying to avoid it. I bought this bike for $350 or something (in pieces), and have another $200 or so into it so far (with a lot of ebay / fabricated pieces). I *think* I'll get my money out of it when I sell it so far, but I don't think that will be the case if I put another $300 of carbs into it. It was a fun exercise rebuilding and rewiring (from scratch). I really did enjoy it. And it was nice to have an old simple bike to learn on. The main thing it taught me though, was that newer bikes are way nicer than older bikes, even nice old bikes. I should have taken all the time and energy I put into this one restoring an old M2 or S2. Same amount of work, head and shoulders better bike. At least I didn't make the mistake of dumping $1k into it, just to discover that old UJM's are heavy and uncomfortable and handle badly. It's a points system bike though, so at least I have something to ride after the EMP is set off. |
