HELP! 1975 KZ400S Carburetor Rebuild Kits - can’t find the plug!

Hi All -

New to the KZ world with a project bike that we’re trying to turn into a road-ready machine. Going to  make a series of posts with photos and vids for folks to follow along if they want, or I’ll just be yelling into the void. Either way it will at least help us organize our thoughts. However, we’d love to pull from the massive knowledge this group has and would appreciate any insight if y’all are interested.

Context -

My buddy and I bought a non-running1975 KZ400s (I think the later models according to kz400.com) to wrench on/have an excuse to get together. Did not come with stock airbox and previous owner had put pod filters in it. Various missing stock parts, obviously someone had planned to cafe or tracker it out and did the “cosmetics only” move - bought some random seat online, put knobby tires on it, etc.. Now we’re left with the cliche aftermath of that. Payed probably too much for it but in the grand scheme of things it was a cheap roller with boxes of parts. Why the heck not.

Essentials thus far -

Electrical: when we bought, pretty much was a disaster. Pulled it completely and have rewired a basic drag setup just to test stuff (new keyed ignition, replaced the coil and wires, new batt) and - great spark! Will replace reg/rect and other essentials as we go. Oddly, there’s an electric start on it? Thought it didn’t come on this model but at some point somebody must have added one. It’s not wired up and we’re kicking it to run tests.  All in all electric/spark is ok thus far. 

Compression: it is good enough. 115 of the left/135 on the right. Probably need to replace piston rings and gaskets but it should be sufficient to get it running well enough. (Someone feel free to call me out if wrong). All the external components of the PCV system are missing but from what I’ve read it shouldn’t prevent starting and running.

Fuel - just not good. Tank interior has been re-coated, no visible rust. Had to put a new petcock in, non-vacuum, as the old one was junk. Added new fuel lines with inline filters to both carbs. Pulled carbs and cleaned all passageways with carb cleaner and blew them out. I believe this bike came with, and still has, the CVB 36 carbs (or Keihin 757c? Is there a difference?) Some of the jets are MANGLED - screw heads on the little mains were so bad that the passages were completely blocked, so we drove a small punch through just to open them up. Worst thing? The little rubber plugs for the mains aren’t there. Thinking that’s contributing heavily to the bogging. Bowls, floats, and slides are, somehow, SUPER clean and maybe the best I’ve seen anywhere for this year. We got gas to flow to both carbs, likely not evenly, but what I believed was enough to get her to start.

Outcome: we got it to start. I may have had to spray starter fluid in the carb bodies the first few times (don’t judge us) to get her going. But after a few cycles of that, she’d start with just a kick. But SHEESH is it rough (as expected) and is bogging badly with throttle.

Now, we’re looking for carb rebuild kits, but can’t find one that includes those little plugs. We’re going to standard jet it and then adjust as needed for whatever filter system we choose (any suggestions in lieu of stock box?).

Current questions:

1) anybody know of a carb kit that includes those little plugs? (And are those CVB36 or 757C)
2) anybody have suggestions on air filter systems in lieu of stock box?
3) I’ve read about the Venturi attachment in the stock box that helps smooth airflow through the carbs - do we need a solution for this?

We’ll keep posting as we go but wanted to get this out there and see if we could tap into the collective brain power of the internet.

Thanks in advance for any help offered,

The fellas at Rusty Pipez 

 

Welcome!

It is exceedingly unlikely the stock carbs will ever work well with pods. They just... don't. Your best bets are to find a stock airbox or to replace the Keihins with VM30s.

The frame will be flexible unless you weld in a hoop behind the seat. Thankfully the PO kept the fork brace portion of the front fender.

The clubman bars are on upside-down. Sounds like you guys didn't do it, but it's one of those things that make me cringe. They are supposed to emulate clip-ons, so they should drop outside the triple clamp, not rise.

Most KZ400s did come with an electric starter from the factory. They were commuter bikes, so they were meant to be simple/convenient/drama-free. Even if the '75 400S did not have a starter (to keep the cost down when new) it would have been easy to add one since most of the other bits were already there. For example, the Partzilla pages for the '75 KZ400S show a "starter cover" and a "grommet, starter terminal" even if they don't show the starter itself.

Good luck!

Thanks for all the feedback - love that folks are active on here.

Agreed on the bars. It’s an “ick” as the kids say. We don’t, however, hate what PO was going for - tracker bars maybe? What do you all think? If we go clip ons or anything low slung, we think riding position will be cramped without rearsets, now we have weird ergos… idk seems like more work than it’s worth. Would take any suggestions there.

“The plan” is to weld in a seat pan and a crossmember of some kind to add rigidity - I think K1 enterprises has a pan? We actually like the cropped fender thing the previous owner was designing in the photo where the tank is painted cream - might work up something along those lines. On the scale of welding expertise from novice to expert I’m at “only looked at them in boxes at harbor freight” so that’ll be an adventure - we’ll record for everyone’s comedic enjoyment.

Ok, good to know on the carbs. I think we’ll probably rebuild the stock ones while we’re doing the work, end up with the VM30s, and see if someone else here is in need of the ones we’ve got now.

Do the VM30s with pods run ok on these bikes? 

Thanks,
FMG

First welcome to the fun! There are others here who know much more about the 400/440 than I do. I will say the carbs are CVB36 carbs. The other ID you mention 757 are (as I understand) are CV32 carbs that were stock on later versions/years.

Here is a pic that shows where the carb ID might be stamped.

Maybe the PO was trying to do something like a old-school motocross or enduro bar like this one:
 

The "cropped front fender" is actually the fork brace portion of the OEM fender that I mentioned earlier. Seems suboptimal not to have a fender on a bike that is [sort of] intended to go off-road.

Welding in a seat pan could make maintenance a little awkward. Maybe you could modify an OEM pan that would allow you to keep the seat hinges.

VM30s are the sweet spot for 400cc-450cc bikes. For the older 400s I would almost recommend VM28s, but most of those kits are for bikes in the 350-360cc area (e.g. Honda CB/CL - also the KZ305).

thanks @gd4now and @Nerdy. Agree on the enduro bars, looking at something like this:  Z1 Enterprises: Handlebars - 7/8 inch - 1 3/4 Rise - Tracker - Non-Dimpled - Black . 

Good news (maybe) on the carb repair kit - amotopart.com has what they claim is a carb repair kit that includes the pilot jet plug. Also ordered a set from Z1 enterprises, so we'll see if they work! We had another look at the carb bodies based on @gd4now's pic and was able to find 7401 NCX on the right and 748E NO on the left. Looks like Keihin US was bought by Astemo and their website is... not super helpful.  Going to send the serial numbers to DCC just to be sure.

For the VM30s - we see they're installed individually with a 2-1 throttle cable assembly, no carb rail or bracket.  Any worries about stability there? Do they vibrate around a bit without that extra rigidity and weight? Reviews are overwhelmingly fantastic so not too concerned, but figured y'all might know.

Also, is this Murray's carb (photo attached) an option? Anybody used one of these before? Looks like it's a VM34 that's been tuned/jetted to work with a twin - Kinda cool.

OH - we have a front fender that came with the bike. When we get a chance we'll attach it and share a pic. Will also share some pics of the frame under the seat - hinges and attachment points have been ground off  .

Thx!

As I said there are others here that know much more about th 400.440 than I do. I think I remember posts from the past where some have made use of replacement carbs maybe a search will find more info on the subject.
As to the carb ID you found and posted  748E I do not have a listing for that ID, but it sounds like many I do have.
Here are some tables and a doc on the IDs used.

I'm not familiar with Murray's Carbs, but there was another company called 11th Hour Industries that made [nicer looking] single-carb conversions for KZ400/440, also with the VM34. I've read that the performance will be reduced slightly compared to a dual-carb setup but it is easier to maintain.

Unfortunately their web site is just a Facebook link ( www.facebook.com/11thHourIndustries/ ) and it looks like they haven't posted anything since 2022. I have one of their manifold+carb combos sitting in a box waiting to be installed.

Also IIRC the 11th Hour manifolds do not require frame mods, which Murray's do for some reason - not sure what's going on there.

Why is it that the twin guys seem to have the most carb problems?  There are only two carbs, not four, but yet they are the ones that talk about single carb conversions, or buying "VM" carbs (the china versions).  And they always want pods.

I suppose it can be a challenge at times finding parts for some 50-year-old carbs, but other than the diaphragms, the only thing needed most of the time are O-rings, and those can be found and/or reverse engineered.

Because Ed, that is the fashionable/cutting edge thing to do. Rattle bomb all the rust, hacksaw-off all the "tabs". buy a pile of Chinese knock-offs and throw a set of the unknown carbs on it then peddle it on Facebook. Happens Waay often in the inner city, at least around here. 

Nessism wrote:

Why is it that the twin guys seem to have the most carb problems?  There are only two carbs, not four, but yet they are the ones that talk about single carb conversions, or buying "VM" carbs (the china versions).  And they always want pods.

I suppose it can be a challenge at times finding parts for some 50-year-old carbs, but other than the diaphragms, the only thing needed most of the time are O-rings, and those can be found and/or reverse engineered.
 

Wow, you're awfully grumbly about something that doesn't affect you. No one is restoring the 400/440 twins with the same vigor as the bigger bikes; they are a simple and inexpensive platform for modification, so that's what people do.

Single-carb conversions are not common. In fact, until this thread the only vendor I had seen selling them was 11th Hour. Some people make their own, but again this is rare. If the stock carbs are in bad shape - as was described above - it's often easier to replace the old Keihins with new Mikunis, starting with a clean slate rather than futzing around with diaphragms and other potential unknowns. Some people do buy the knock-off VM carbs and they kind of deserve what they get; all of mine are genuine. And the only reason I have an 11th hour setup is my nephew was saving money for car stuff and asked if I wanted to buy it.

If it makes you feel any better, I have a set of Keihin CR26s ready to go on the GPz. And guess what? I'll be using individual K&N filters. Stick that in your pod and smoke it.