single carb on a twin?

Hey, I've been reading through your great little forum here and learned alot from it. I owe you all a thanks, this place has helped alot with my little project bike.


Here's a question for ya.


I've read a little here about converting to a single carb. I would love to do this myself. I'm a big fan of simplicity, and not very fond of carburetors. The first thought to jump into my head was "I need a bigger carb", but now that I think about it, DO I? Since when one of my cylinders in intake phase the other in in exhaust, could I just use one of the Keihin carb's on my bike to feed both cylinders??? It would be very easy for me to build myself a little 2-into-1 intake manifold from stainless steel, and I could whip up a little air cleaner with ease too. If that would work, I'd be riding in a day or two! The old KZ440 I bought starts and run's great, but one of the carb's has a torn diaphragm that costs $157. I just cannot justify that cost right now, I have a 3/4 finished jet powered hydroplane project (all built from scratch) that I need to finish before I can spend my fun $$ on this bike.

Thanks fellers!

Years ago, I owned a BSA Cyclone (A50C) 500cc twin engine that I shoehorned into a B40 (previously 350cc engined) frame. This was a BSA factory prepared flat track race engine I bought from Gary Nixon. The Cyclone had 2 carbs, but most other 500cc BSA twins of the day had single carbs. I think they were called the Royal Star (A50R) and Wasp (A50W). Here's my suggestion - try contacting one of the BSA clubs (there are several on the Internet) and ask their members what size the stock carbs are on the stock Cyclone (twin carb) and on the single carbed Royal Star or Wasp. If they have the same size carbs, your theory is probably correct. Don't let them get confused and quote the carb size from a B50 as that was a 500cc single cylinder engine with one big carb. The one other issue to consider might be the fuel reservoir on the carb and if it would have enough capacity to feed 2 cylinders at wide open throttle. Ed

Hey Ed,
If you're capable of welding the aforementioned manifold, you're time is worth more that that. $157.00 for a diaphram seems like a bunch but it's you're pass into all the engineering that Ma Kaw has already done on your bike.

Trust is the answer, in this case.

KZCSI

Thanks Ed! I had also wondered if the feed to the bowl would be able to keep up. That's my only concern though. With a four stroke engine and 1 carb per cylinder the carb is only breathing 25% of the time, so double-duty shouldn't be a problem as long as the bowl doesn't run out.


KZCSI, TIG welding, and general fabrication has been a hobby for me for some time now. Something as simple as this little manifold would take less than 1 hour to build in my little shop. I wish my time was worth more that, I seem to have more time than money these days though, and for me most of the fun is in building things anyway. LOL, I have jet engines that have never done anything but annoy my neighbors and pushed the scale on my thrust measuring stand. Then they end up in a corner of the shop gathering dust.

First issue is carb size. Your 440 uses two Keihin CV carbs... 36 or 38mm I think. From a gas flow standpoint, I am not sure how to quantify what will be needed... like as in a formula but from an empirical standpoint, I do know that if you have a bike model that uses twin carbs on one version and a single carb on the other, the single carb model will use a somewhat larger carb than the twin carb version. For instance, the Vulcan 1500. The twin carb VN1500A used two 36mm Keihin CVK carbs while the single carb version; the Vulcan 1500D used a single 40mm Keihin CVK. If I were doing a conversion, coming from twin carbs, I would use a single carb from a single carb application to take advantage of the throttle mechanism that had been developed as well as the choke. If you swipe a carb off a rack of carbs, you will have to figure out how to work its choke butterfly/plunger and throttle.

Next issue is the manifolds. I can tell you that hard manifolds seldom hold up. I have seen this a lot on V-twins. When you bolt metal to the cylinder head and then attach to the carb, something will eventually give from vibration. I WOULD use something hard for the runners, like PVC which would be very easy to work with or steel as you mention but would put the ends into rubber sleeves for vibration control. Make sure these rubber bits are able to withstand a gasoline environment. It might be OK to use the rubber couplers from the hardware store to join an airbox to a carb but when you are in constant gas flow, I am not sure what will happen. Look for nitrile rubber which has great gasoline resistance. That is what is used to manufacture carb orings. I would have a rubber sleeve on both the cylinder head side and carb to venturi attach side to control vibration cracking. The manifold should have enough strength to support the single carb (which doesn't weigh much) and you should be good to go. I think perhaps making a Y pipe out of PVC and using rubber sleeves; perhaps from old pods would be how I would think it could be done easiest and using a carb out of a dirt bike where the throttle would attatch direct to that carb and there would be a way to actuate the choke so you didn't have fab too much.

Thanks, George. Good point about the throttle mechanism. I would MUCH rather have a mechanical slide carb, like on a dirt bike. Like I said before, I'm a big fan of simplicity. Simple = tough and reliable. I was planning on making the manifold slide into the existing rubbers that bolt to the cylinder head, and connecting the carb with a piece of petroleum resistant rubber hose. I think I'll give a carb from a 250cc dirt bike a try. I'll be sure to let you guys know how it turns out.