from four to two

Got a quick question, Im doing a custom intake on my kz1 and im only going to be using two of the bs34 mikunis. any body got a good idea the size of jets ill need to make sure this works correctly?

Jetting may be the least of your problems. The carb was designed to flow fuel and air for one cyl and now it needs to provide for two. Can it flow enough air for both? I would think a larger carb would be needed? Can you give us more details or pics of your design?

You can run the same carbs because you are not drawing to both cylinders at the same time but you will have to use one carb on cylinders 1&4 and the other on cylinders 2&3. Otherwise you will have asymmetrical air flow. The air flow will be 180* out of phase and this will supply more air to one cylinder than to the other if you use one carb on 1&2 and the other on 3&4. This will happen because one cylinder will speed up the air flow for the other but not in reverse. So Yes, to agree with Bluemeanie, jetting will be the least of your problems.

otakar wrote:

You can run the same carbs because you are not drawing to both cylinders at the same time but you will have to use one carb on cylinders 1&4 and the other on cylinders 2&3. Otherwise you will have asymmetrical air flow. The air flow will be 180* out of phase and this will supply more air to one cylinder than to the other if you use one carb on 1&2 and the other on 3&4. This will happen because one cylinder will speed up the air flow for the other but not in reverse. So Yes, to agree with Bluemeanie, jetting will be the least of your problems.

+1 - 2 carbs is not ideal, and makes for serious headaches.

Been there, done that.

Ive seen it done, and it worked wonderfully, I know there is a way to do, so. Considering big bore Harley's will run off one 32mm carb all day. I don't believe that will be my issue. And we all know how screwy Harley timing is hahaha

I am new to motorcycle engines, but Ive built many hot rods and street rods with all kinds of different intake setups and Ive never encountered anything of the sort.

I appreciate the advice and warnings but If i could get the original question answered, that too would be much appreciated.

Thank you.

Years ago (late 70's) they sold a 4 into 1 intake manifold, for z1's. It had a single flat slide carb, for all cylinders. I can't remember the name, but the carb was purple.....It was flat slide, with no float bowls.

Hmm Ill try to find something on that. thank you.

Doing it on a two cylinder is completely different than doing it on a four cylinder. When both cylinders are at TDC at the same time but fire alternately it is no problem. This is why I stated in my first post that if you use one carb on cylinders 1/4 and the other carb on 2/3 than it will work fine. But not on 1/2 and 3/4 That will not work because one is at TDC while the other is at BDC. ALL 4-cycle twins run both cylinders at TDC at the same time. This is why you can run a single carb. The only way you can do it on a 4 cylinder is if you run a cross manifold, connecting all four cylinders together. And even than you will not have precise metering to all four cylinders. If you make two manifolds of equal length connecting cylinders 1/4 and 2/3 than the metering will be equal and two carbs will work fine. Comparing a twin to a four is like comparing a hawk to a duck they are of equal size and they both have wings. But if you try to feed them the same way, at least one of them will dye.

LLC wrote:

Hmm Ill try to find something on that. thank you.

You might be able to find some old magazine ads, with that setup in it. I can find, a picture of the carb, at the library....Maybe it's on a CB750 honda.

I hear what you're saying, I see your point. Good analogy by the way haha. Ill look more into it. Unfortunately the new frame design will not permit me to make a manifold large enough to cross over. that would be a strange manifold either way.

That is how Jaguar used to do it on the straight six when they used to run 3 carbs. This is also why Kawasaki did not put three carbs on the 1300 because it did not work. They wound up mounting three two barrels. You might contemplate mounting two of those two barrels. That will give you two carbs but four individual fuel supplys.

As intriguing of an idea as it is. There is just no room for anything like that. Its either two or four. And so far it looks like I might be stuck with the four.

I would like to do a single carb off to the side of the engine. But i fear vacuum wont be great enough especially on 1 and 2.