Z1A / Z1B-VM28 carburetors on a Z1000 with original airbox?

So, after this carbs running so well and functional,
I’ve now cleaned the carburetors and got them pretty spotless.
So, I can move on to fabricating the necessary adapters. I initially wanted to make them out of aluminum, but then I’d have to get them black-anodized as well...
Instead, I’ve decided to make them out of polyoxymethylene (POM). This plastic is fuel-resistant and easy to machine on a lathe. I need four pieces, each about 25 mm long; I’ve ordered a 150 mm length and plan to turn each one to the final dimensions before parting them off as finished components—
at least, that’s the plan ,..We’ll see how it goes—....I’ll keep you posted!


 

The plastic material I ordered arrived yesterday, so I stuck to my plan and machined the four "extension adapters" on the small Chinese lathe. As is well known, this material machines quite well with sharp inserts—though the depth of cut has to be shallow given the lathe's limited power, making it a bit of a tedious job.
Still, the parts fit well, so I’m optimistic that I’ll be able to successfully install the carburetors using them.

Nicely done Joe, I do like turning plastic, it's kind on small machines like ours

Well, I wouldn't be me if I’d waited until winter to do my carburetor swap, so I went ahead and got it done today. And it wouldn't be normal if a few hitches or problems hadn't cropped up along the way—but let's take things one step at a time.


When installing the old 28mm carbs, it was a pleasant surprise that I could reuse my throttle cables! However, when I moved on to mounting the rear intake rubbers, I discovered that a) without my extension adapters, the rubbers were—as expected—too short. I gave it a try, but it didn't work; the boots kept slipping off the carburetors due to too much tension. And b): *With* the adapters, the setup is a bit too long, causing the boots to compress slightly, but that’s tolerable... though getting those adapters and boots in place is a real hassle.



There should ideally be a maximum of 10 mm of "in-and-out" play, but that’s obviously not feasible from a manufacturing standpoint; I’d already built it to the minimum possible dimension. I installed new fuel lines and a Y-splitter, put the tank on, and then came the moment of truth: opened the fuel petcock to flood the float bowls, engaged the choke, and hit the button. It fired right up, had a brief warm-up, and then I took it for a spin around the block (2 km). Checked it over: everything was sealed tight—no fuel leaks! Great! ...But wait, what’s this? I hadn't noticed it when opening the petcock, nor when closing it "blind" during the final stretch of the ride (which is how I always do it)—the petcock is now pressing right up against carburetor No. 1! The petcock lever won't even clear the carburetor top cover! I didn't register it when opening the valve—everything worked fine... strange! And since it’s a Z1000 A2, I can't simply rotate the petcock to find a better position; the A2 petcock is fixed to the tank. Looks like I’ll have to cobble together a different lever from whatever spare parts I , or my freind, have lying around...


In any case, everything is running, and the spark plugs look pretty good for a first run; cylinders 1 and 3 could stand to be a bit leaner, while 2 and 4 are a perfect tan color. So, it’s just a matter of fine-tuning—the bike responds well to the throttle with no noticeable flat spots. I revved it up through the gears to fourth, and everything from idle to 5,500 rpm felt good to me; that’s really encouraging. And the best part is: if I’m not happy with this 28mm setup after all, I can always switch back to my perfectly functioning 26mm system.



I definitely wanted to find out if it’s possible, so I’m going to test it out during the remaining three weeks my seasonal registration allows—up until the end of September (weather permitting, as the rainy season is starting here...).

Holy crap Batman, ya made it work!! Great endeavor and completion! Actually before your venture realized there was a venturi length difference?? Thanks for that additional info!

Running "OLD SKOOL"

Hi Clay,
Thanks... yeah, I never used to notice that the entire 28mm carburetor body is about 10–12 mm shorter overall (because I always ran the specific carburetor setups designed for my Z1, Z1A, and Z1B models, and on my Z1000s I only use 26mm carbs or the "late-model" 28mm ones from the Z1R and MK2).

If you were to lay the parts—or the bodies—from your existing carburetor sets side by side, you’d see the difference!

 And as we both know all too well: you never stop learning—there are always surprises, even with old things...
 

I’ve just returned from a fantastic 120 km test ride. 19°C, nearly no wind, bst testing-conditions!

I am absolutely thrilled: full throttle response across the entire rev range, with responsiveness just like my old Z1 and Z1B—the bike really snaps to attention when you twist the throttle. During a stop after about 30 km, I checked the spark plug color again: cylinders 2, 3, and 4 were a perfect tan, but cylinder 1 was a bit too black. So, I backed the air mixture screw out about a quarter-turn on that one, lowered the idle speed slightly to just under 1,000 rpm, and carried on. At this point, while a stop by my friend, we´ve checked the petcock-clearance bye monting a Z1-Tank on: Perfekt fit, nothing disturbes, full succes!

I was lucky, too—the roads and highways were nearly empty, allowing me to test the bike at higher RPMs, higher speeds, and at full throttle. I know a lot of this is subjective, but the bike runs significantly better and more dynamically with these now installed early VM28, totally other than the late carbs from `77/`78!!! — which is logically due to the slightly larger 28mm bore. Great result; I’m absolutely satisfied. These carburetors have now the right to staying on the engine! And I’m going to get a Z1 fuel tank; the petcock is positioned further back on that one, so it won't interfere with the carburetors anymore. It’s decided!

I really hope to find a good one, and than , so the plan, i let it paint in the stripeline-design of a `75 Z1B,..maybe other colour, i must think about colour and conbinations....

Thanks for your interrests following this story!

Just wanted to add:

Here is the spark plug from cylinder no. 1—now a healthy tan color, whereas it was a bit black before the readjustment. The other three plugs look exactly the same; I’m happy with how the engine is running and the combustion. (The "trace of black" around the insulator indicates that the mixture is a tad too rich at idle—after the engine had been idling for a short while following a hard run...)
 


 
 

A great write up Joe, and perfect results.
And another day riding too , yay. 
Rich.

Yes, thanks Rich,
that sums it up perfectly: it was a fantastic ride and a huge amount of fun. I stopped to see a good friend along the way, and then rode another 20 km or so with another buddy on his fully restored '76 Z900. And the 40 km ride home—just me on the bike—was spectacular... If I had dropped dead and turned to stone right then and there, it would have taken them three days to chisel the grin off my face...

And for the picture-lovers: here´s the "fabulous 100% restored looks-like-new-Bike" from my Buddy :




 

So, I found and bought a Z1 fuel tank ...and of course, this tank will need a paintjob to match the rest....


...and I’m already thinking about how to have it painted;
I’d like to incorporate the Z1B stripe layout... keeping the base color black, with dark gold stripes...

I tried to simulate it using my most basic image editing software—very, very simple, given my rudimentary approach. (because I simply can't do it any better).




 

Yesterday, I derusted the interior of the newly purchased Z1 tank using citric acid — it worked really well again! After that, I neutralized it with diluted sodium hydroxide solution, and the inside of the tank remains rust-free!
 I’ve done this often enough over the years, whenever I bought a used tank.



Now you can clearly see the position of the fuel petcock on the Z1 tank, which is more convenient for me... And a direct comparison really highlights just how much "slimmer" a Z1 tank is compared to a Z1000 tank!

 



 

(The only downside, perhaps, is having 3 liters less fuel capacity available—though that’s something I’m used to from the Z1 I drove for the longest time....)