Boring out carbs

[richardwilson45]

Hi,
Any body got experience boring out 26mm Mikunis to 28mm?

[zed1015]

Yes!
Done a few for myself.
They work very well.
They can go out to nearly 30mm.
26mm carb bodies are the best to use as the casting at the boot side is slightly thicker than on the 28's.

[richardwilson45]

Interesting,
I'm building a Z750 motor for my son, and the 34mm CVs are pretty tired, so wondered about some bored out 26s.
I have spare 28s but didn't want to use them as they are the originals from my 1000A2.

When you bore them to 30mm will they match the flow of say the 29mm smoothbores?

[zed1015]

I bore them to 29.5mm
Any bigger and you risk breaking through the bottom of the boot groove, Especially with the 28mm castings.
I have not done any true flow bench cfm tests but seat of the pants performance they are just as good and have better response low down than the smoothies.
For a 750 you should try the 26mm slide carbs as they are first.
They work well enough on the 900 and 1000 motors.

[DeadZedDave]

I've no experience of boring out carbs but I'd agree with Zed1015: give the VM 26's a try first. To offer some perspective, I plan to fit 26's on my own 1000A this year - its been running 28's for ever, also, I'm going to put the 28's on a 1000J project (J's run 34mm CV's as standard like the z750). Kawasaki started with 28's on the Z1's but dropped down to 26's on the Z900 for 76 and kept the 26's for the z1000A1 / A2 for 77-78. At the time, Kawasakis logic was better mid range / driveability (also lower emissions to meet US legislation) In the meantime theres been much written in magazine and forum articles on the subject but generally its agreed that 28's give more top end but 26's are better low down / mid range - and thats based on the 900/1000 engines. Back in the mid 70's the z650 started out with VM24's and the z650's were pretty good ! When a lot of z1000 riders started to change to 28's after the Z1R was launched with 28's in 78 and the A3 and ST got them in 79, VM26's were fairly common 2nd hand and quite a few z650 riders put them on modified bikes - especially with big bores! Also, when Suzuki launched there first big 4 stroke in 77, the GS750 used VM26's and later the GS1000 again started out with 26's. So the 26mm carb sounds like a good place to start for the z750 you're building. It'll save a bit of work getting them bored out and also it'll save another set of carbs from irreversible modification !! If the 26's don't deliver enough top end you could try the 28's. I can understand you want to keep them for your A2 but just to highlight, so far as I know, VM26 was standard on the A2 and it was the Z1R which got the VM28's in 78 followed by the A3 in 79 so if its just for originality, you could just as easy put 28's on the 750 and put the 26's on the A2. With the 750's more revy nature / power higher up the rev range the 28's might just be better. The 26's should be fine ! Just remember that CV carbs (34mm in this case) NEED to be bigger than the old "slide carbs" because the butterfly - throttle valve in the CV carb takes up space / disrupts flow effectively reducing the flow capability so a smaller slide carb can equal the flow of a bigger CV carb ! - all other things being equal !!

[richardwilson45]

Thanks for the explanation Dave.
One issue I have is the inlet rubbers, although the carb sets are on the same pitch there are two body sizes:
The 650 bodies have approximately 24mm bores and are 31mm OD at the rubbers,
The Z1/Z1000 bodies with either 26mm or 28mm bores are 34mm OD at the rubbers.
I could fit the 26mm Z1000A2 carbs to the Z750 head, using the 34mm OD rubbers that are meant for the CV carbs.

I have a set VM28 from the Mk2, a set of 26mm from the Z1000 A2, a couple of sets of the Z650 24mm Slide carbs, a set of 34mm Mikuni CVs from a Z750 R1 GPZ, a set of 34mm Kehin CVs from a Z750H1 LTD, and various others.

The dilemma is that from what Zed1015 says, if I want to bore out a set for my Z1000 then the 26mm bodies are better than the 28mm bodies. The reason for the boring is to cope with the airflow needed by my Z1000.

My Z1000A2 is fairly non-standard, it has a Jim Wells tuned head from a MK2, Moriwaki cams, Harris pipe, 72mm Cosworth piston kit and the DFI from a ZX11. Last Dyno run in late 80's gave 148 BHP !
However, I think at some point I'll probably go back to carbs and I think I'll need all the flow I can get, so a set of 26s bored to 29.5 might work. It used to run 29mm smooth bores, but the DFI actually gave more bottom end tractability. Now being an old git I may need the torque not the horsepower.

[DeadZedDave]

Hello Richard, first, regarding the Z750, the Z1000 inlet rubbers are the way to go to put VM26’s/28’s on. Memory is a bit hazy but I think the inlet bore centres are slightly different. A NEW set of inlets are probably flexible enough to push fit the carbs but a set of old original inlet rubbers would probably need the mounting bolt holes elongated to adjust the positions.
I picked up from your earlier posting that you plan to put the 750 motor in a z650 frame. Back in the mid 80’s I briefly owned a z650 with a GPz750 R1 engine fitted (using the R1 Mikuni CV’s) and can recommend this combination makes a great street bike. You might find you need some of that extra BHP for your A2 though because as I remember, the bike was as quick as the big Z’s up to 120 or so (maybe just a touch slower off the line) ALSO, it’ll go around corners better as well !
Regarding the 1000A motor, first a question : what do you plan to do with 148 BHP ? Jim Wells is a name I’ve not heard in a long time and was awell respected tuner back in the day building race winning engines. 148BHP is pretty good going for a 1075 motor - even a Jim Wells motor and I don’t imagine it was a case of just throwing together those parts you mentioned and strapping it on the dyno. It takes some skill and know how for sure.
When you asked about bored out carbs matching the flow of 29mm smoothbores I’d have agreed with zed1015 for most instances, HOWEVER, with the set up you have I have to say the 29’s will flow more than bored out standards, simply as a result of the smoothbore design. At maximum throttle / flow, the race designed smoothbore has less turbulence as the result of the smoothbore design so in the case of your modified engine, the 29mm smoothbore will (in my opinion - others may disagree) produce more peak BHP. I doubt you’ll manage 148BHP though as the 29’s are a little small I think - maybe 140 ? HOWEVER, as you also mentioned, the DFI works a bit better lower down the range; smoothbores are a race carb (though plenty used them on the road) and although wide full open throttle performance is better, small throttle / mid range / high gear cruse conditions can be less than perfect. In these situations the 28’s or bored out 26’s might be better !
Alternatively, though I’ve no personal experience, I’ve read much about Mikuni RS Flatslides (34 or 36mm) being a good carb for modified old Z’s. The people who write these reports in magazines and forums say the RS’s overcome the low speed / part throttle issues and have the capability to make the big BHP. These are expensive carbs though ! (some RS36’s for sale on this forum)
I get the idea from your posts that you’re hoping to put a couple of bikes together from the parts you’ve got without necessarily buying more ?
It’s tempting to suggest changing to some milder cams, maybe the whole head back to standard, but then you’d have your high compression pistons to consider (maybe a base spacer)
It gets complicated.
I have a 150 BHP 1260 motor sitting on the floor for similar reasons and I can’t bring myself to take it apart or sell it and I’m currently putting together a standard 1015 engine !
Coming back to the question, what you’ll do with 148BHP, theses no amount of chassis / frame mods that’ll make an old Z handle like a modern sports bike and unless you’re drag racing, a modern 600 will be faster in the twisties.
If it were me in your position, especially as it’s a Jim Wells motor, potentially worth something, I’d leave that motor as is and use any cash you might have spent boring out carbs / buying alternatives etc. to put together a standard engine.
You could even put some VM 26’s on it and get some of that low down / mid range torque you mentioned !
Don’t let me discourage you though, I’m the wrong side of 60 now but if you build the 650/750 right, you’ll probably have a tough time keeping up - just because it’ll handle better !
I hope some of this is of some use .....