single carb conversion

I have a 75 kz400 with bad stock carbs. looking to do a single carb setup but not sure which VM carb to get . would the VM34 be ok by itsself and what jet/setting should i start at?

www.kzrider.com/forum/3-carburetor/60799...on-a-76-kz400#779975


www.kzrider.com/forum/3-carburetor/60683...ds-left-kz400#769988

One way of looking at the size of carburetors is that the larger the venturi, the slower the air speed so a carburetor with a huge venturi won't have a lot of air speed until rpms are pretty high. This means more horsepower at the top of the RPM range. The problem is that down low, it creates huge bogs off idle. At one time my shop would road test carbs and I noted that on my stock KZ900 test bike that VM33s bogged excessively off idle. Once you feathered them off the idle circuit and got close to where the cams were cut to make power the bike screamed. Although it was fun to test since the bike felt like a rocket up on the top of the rpm range, down low, well... it was miserable. You can mitigate this bog some... just some... by using the smallest pilot air jets.

I think you would be better off rebuilding your current set and if you really want new carbs, find two VM24 or VM26 with the tops set so you can put in a Y throttle cable and pull both slides up at the same time. Generally, each will have a choke (enrichener actually) and you may find it cumbersome to choke both when starting but you can try choking only one to get the bike started I guess. These would require different carb holders (intake manifolds) and unless you are really clever connecting to the airbox with fabricated air box ducts, pod air filters and jetting for the same. It can be done. The complexity for someone who doesn't understand jetting and will likely not get a lot of useful first hand advice makes rebuild of your current set a more viable option. I have rebuilt dozens of KZ400 Keihin carbs and they are actually pretty good carbs.

Is this bike some sort of "bobber" conversion w/hardtail or similar? It seems we get these inquiries from guys that are building such bikes anyway. All I can tell you is that performing a mod such as this will kill performance and create a LOT of work compared to simply rebuilding a set of stock carbs.

Good luck and if you do the mod please post up enough info to create a tutorial for the next guy.

Another issue with using a single carb is the reduction in power. All else being equal, single carb twins make less horsepower than dual carb twins. The KZ400 doesn't have excessive power stock, so reducing the power doesn't seem like a good idea to me. Ed

These seem to come and go then the carb/manifold setup appear for sale or we don't hear from them anymore.
Steve

Burky23,
Contact this guy, he may have some useable carbs.

www.kzrider.com/forum/23-for-sale/608547...z400-part-out#784518

I tried rebuilding the stock carbs but it didn't work out so hot. I cant get over 40 mph and from what ive found the right cylinder isn't firing all the time. I wanted to do a single carb cause syncing a dual setup made me nervous. I haven't done it before. and I can fab up a 1 into 2 manifold pretty easy. and it seemed cheaper to convert to one carb. I could be all wrong though. the bike does have pod filters and straight pipes. I have welded up a "2 into 1" box like muffler that would tuck underneath in front of that back wheel to help with back pressure and noise.

Your problem may not be carb related.
Have you confirmed that the valve lash is within factoryspecification? A badly out of spec valve can create all sorts of problems, and it is an all too often neglected maintenance item.
You may also have an ignition problem. These items need to be in good shape before messing with the carbs. I think the easiest way is to turn to the maintenance section of the factory service manual, and perform every item in order.

kz400.com/WorkshopManFrameset-5.html

By the way, those carbs are notoriously difficult to set up for pods and open exhaust. There are several threads on this site about that very subject.

I've found that jetting the piston-slide Kz400 Keihin carbs pretty straight forward. Not simple, but at least not counter-intuitive. There are a lot of jets on those carbs and the air jets are easy to mix up, so make sure to take good notes of what goes where.

The most important thing on those carbs is to make sure the slides do not get hung up. You have to put a drop of motor oil (nothing heavier than 10W40) on the slide shaft and the around the outer edge of the slide piston. Then reach in with a finger and move the slide up and down as you slowly snug up the top covers. If the top covers do not get seated properly they cause the slides to stick. When you flip the carbs upside down you should only hear a single click as the slides top out. Then another click when upright the carbs and the slides drop back down. If you hear two distinct clicks, one is getting some drag.

Another issue is that the carbs will sometimes get worn out throttle bushings. They let air in. If you can't get new bushings, you will have a vacuum leak that is not easily fixed.

so with pod filters and straight pipes, what can I do with my stock carbs? I cant find anything for them. the "muffler" will be attached someday so that might help a little.