1981 KZ440 carb recommendation

I'm getting that good old GI Bill money this week and finally taking care of my carb issue on the old bike. The issue was the stock carbs were crap. I'm looking at carbs right now on Dimecitycycles.com and I don't know which one to get for the bike,
I don't need anything super high performance but I want to make sure I get some quality parts for the swap.
I was looking at the following carbs.
Mikuni VM32 pre-jetted for 400-650CC twins
Mikuni VM34 standard jetting
What's the major difference between the 32mm and the 34mm and which one will work better on the bike?
If you have any other recommendations for carbs please do tell. Also, what else should I get for the swap?

What carbs are the originals? 34mm? I'm assuming CV's of some sort on an 81. I would have thought a 32 none CV carb would be plenty for a 440, unless it had some major motor modifications. Remember you cannot directly compare sizes between CV carbs and traditional slide carbs. Considering the cylinder size, relative to a Z1 at 903cc, it's slightly smaller per cylinder on the 440, HP wise it's about the same per cc at 40 hp at similar rpm - similar state of tune. Z1's ran 28's, some like the 29's for performance, but folks using 32's and 34's usually have some serious modifications done. The KZ1000 was originally offered with VM 26's to improve slow running and idle issues.

My experience with carbs on many applications is that "going big" may give you more top end HP, but usually at the expense of things like a smooth consistent idle, and bottom end performance, even mid range. A smaller carb will be easier to dial on and given how often we normally ride in the highest rpm ranges, a better experience overall. The engine will not produce enough "suction", for the want of a better expression, at low rpm for the carbs to fuel properly. Of the two options I'd go with the 32's, but would be concerned even they would be too big, I see they are pre-jetted so the sellers may have experience to back it up, but normally 32's and 34's would be used on the higher end of that scale 650 and 750 twins. I definitely would not consider the 34's at all for your application, if you were having issues with your stock ones, you're not going to want to be over-carbed, and will not enjoy your new purchase..

Looking at Dime City's site they show a VM30 for the KZ400 and also for Honda's CB450 and 500 twins, that might be a better option than the VM32's if you are looking for good all round performance on a stock bike. IMO, that would still be a step up from you stock carbs.

Are you planning on doing a single carb or are you planning on keeping two?

I have had pretty good luck with the VM34 on my 440. I have thought that it might be a bit big for the bike at times though.

I wish kz400.com was still up, they had a lot of info on both swaps. That is really the only way I managed to get mine up and running.

I didn't consider he might be looking at one carb Still I think a 32 would suffice. My 750 Triumph Tiger runs a single 34mm Amal and period tests had them quicker than the similarly equipped twin carbed Bonneville. I don't think having one or two carbs makes much difference in isolation, performance has more to do with the overall ability to breath, normally a carb per cylinder gives a more direct route to the cylinder, improving flow, compared to a shared manifold. However, if the carbs are not working together properly, it's easy to have less power than with one carb on a twin. IIMO the reason for the performance of the late model Tigers being better, or virtually the same as the Bonnevilles was because the single carb gave better mid range torque, which was not subsequently made up for by any increase in power at high rpm by the Bonneville.

If going to one carb on a 440, how well it will breath through a shared manifold is a consideration, no point stuffing a huge carb on if breathing is somewhat compromised between it and the intake valves.

dom2570 wrote: I wish kz400.com was still up, they had a lot of info on both swaps. That is really the only way I managed to get mine up and running.

www.kx400.com works for me

Forum part is infected. Do not click on kz400 forum

redhawk4 wrote

It seems the forum part is infected with one of those malicious things to get you to call to have your computer fixed, based on my attempted visit just now, so take care.

It seems the forum part is infected with one of those malicious things to get you to call to have your computer fixed, based on my attempted visit just now, so take care.

You can't spit without hitting a scammer. :dry:
Steve

KZ440's have 36mm cv's, 750b's had 38mm cv's.
If you want a jet kit try Sigma 6 or use half a
Dyno Jet kit for a KZ750 E & H. They have a 34mm
cv carb on a smaller cylinder.


.

Thanks everyone! I didn't consider doing a single carb conversion. Are there any benefits to that besides simplicity?

Not really, maybe a little more low end. I have noticed mine struggles a bit more to get up over 70. It can do 85 to 90, but just takes its time to get there.

My old Triumph Tiger works well with the single carb because originally the head was designed that way, the two ports are really close together and work with the little intake manifold for good flow. It's also not a very high performance engine in terms of HP from the 744cc. If a head was meant for twin carbs usually the ports are going to be further apart and parallel or splayed out even, so to have smooth routing I'm guessing the intake would need to be longer than the existing carb rubbers, which in itself gives more low end torque, but will make it struggle at higher rpm. The other option is a sharp right angle for the mixture to take as it exits the carbs which is also less than ideal for good breathing. A single carb is great in terms of tuning, adjusting the idle and all round starting and idling, but I think if I had a 440 I'd stay with twin carbs, after all the type you are getting will be much more straight forward than the original CV units to adjust and being new, if maintained properly i.e., not left for fuel to go stale in etc. they should require little or no adjustment once dialed in other than the odd synchronization of the slides/cables. The 440 is a reasonably high performance motor too, in terms of power from the cylinder capacity, meaning it needs to breath efficiently, so I doubt you can go to one carb without losing some top end power, where as the replacement twin carbs promise extra performance over stock. Being a relatively small capacity engine, I would have thought you'd want all the power, within reason, you can get and are going to need to use the closer to the top end for good progress, where as perhaps on a 750 or 1,000 cc bike you may never push it that hard that a little loss in top end would ever matter. I think if you don't get two carbs, you'll always be wondering what if?

You hit the nail on the head -- a single manifold VM32 gives you a much better mid range but you absolutely sacrifice some top end power. I've run it on three different KZ440s with slightly different jetting but at a baseline for the '81 440 LTD I've got 32 pilot, 160 main (165 seems like better performance but it runs too rich), stock needle all the way up, 2.0 air and a 2.5 slide cut I can't improve on the stock CV top speed with the 2-1 manifold, K&N high flow pods and VM32. I highly recommend going dual to improve over the stock CV's; the single with 1-2 manifold is fun and head turning but the performance if you're riding highway leaves a lot to be desired compared to stock CV's with the airbox.