Boring Out Carbs

Can anyone chime in on their experience or advice on boring out Mikuni 26's. I am interested in what the procedure is, how much material is removed by the machinist and what other changes need to be made such as manifolds or slider mods if any?

I understand that this might be another option to consder for good low end responce instead of simply going up to larger carbs.
Thanks Kzr gang
Cheers-Colin Firth-Ontario-Canada

Colin-

Don't recall what bike you have.
Generally, and speaking more from auto experience, smaller carb with longer intake gives better low end on a normally aspirated engines. Too large a carb will kill your low end. You just don't get as much charge into the combustion chamber when the air is moving real slow.

First thing I check when low end suffers is ignition. Plugs, timing, and dwell/gap (if points ignition). Assuming mix is right to begin with and it was pulling better before.

-Duck

Hey Duck....Currently I am running a '77Kz 650 with 700 Wiseco kit and jetted Mikuni 24's. I am going to bigger jugs. 750/4 head/valvetrain that I will port and clean up. The bike also has a pipe on it. I am kinda torn between opening up the 26 Mikuni's I recently bought or a set of Mikuni 34's that I also have.

The bike has a smaller sprocket on the front so this should tell you that I prefer a little acceleration and good low end response with my ride. This is one of the reasons I have been exploring the smaller, modified carbs over the bigger 34's.

Cheers-Colin Firth-Ontario Canada

how about just putting a set of 29smoothbores on it?theyre a lot lighter than 26s and all the hard work is allready done for you they would be more responsive than the 34s!hey duck:theres a formula for figuring cfm requirements on cars but i cant think of the exact numbers right now, anyway i wonder if there is a formula for motorcycles other than than this is what i use try this?goodluck ,happy wrenching!

Reborn650, I think boring out a 26mm carb is a bit risky. I've had a look at this. there is no doubt it can be done, however there are a few considerations.
The nominal bore size is measured from the engine side, meaning there is a difference in the air-box side. What is the ratio of this difference? Do we try to keep it the same. Further the needle seat will protrude, probably causing unwanted (or wanted) turbulence. There are other obstacles, that escape me right now. I would just go for Mikuni 28's.

RegardZ

heres that carb formula i was talking about for figuring cfm !rpm x displacement x .85(efficiency 85%) divided by 3,456 =cfm requirement ,now can one of you mathematician types with the sliderules built into your glasses figure out how to convert this from inches and cfm to centimeters and mms? goodluck,happy wrenching!

Post edited by: wireman, at: 2006/01/16 00:21

Post edited by: wireman, at: 2006/01/16 00:22

Just a personal observation... the 26s can be bored to about 29mm. The 28s can't be bored. The 29s can't be bored. The reason the 26s can be bored is that they have a tad more material (thicker walls) on the venturi. The cost of boring is about $200 and I only know of one place that still does this out in California. The benefit to boring is about $5 imho. You gain a tad on the top but the carbs won't flow significantly more gas as their main gas limitation is the size of the fuel pipes... the pipes on a 26 are SMALL. You will also change the orientation of the needle jet and it would probably need a little fiddling to work right. I have worked with one set that was bored and really feel you would be better using later 28s w/accelerator pump and removing the pumps and redoing the innards (needle jets, opening up air jets, different slide cutaways, etc) than just opening up a stock 26 and using stock 26 innards. The effect is somewhat underwhelming from a performance standpoint.

wireman-

I have always heard that the venturi size should be no larger than the intake valve. I have no clue why but now that you've prompted from memory, I'm curious. Is this rule of thumb based in math, good practice based on experience, perpetuated myth, or ... ?

-Duck

Colin-

These are variable venturi carbs.

I have not played with slide response but suspect you could tune the 34's to do what you want. At least if the venturi is small enough to give good response at low rpm when you first open the throttle fully and vacuum is weakest.

What I'm thinking is all we need is a slower response from the slide, or more damping. A smaller hole in the bottom of the slide would do this. The slide will still open fully, but it will lag behind a bit and keep the velocity up.

WG, you might know. Do the same model carbs, say the ubiquitous BS34's, have different size ports in the slide depending on the application? How about a lesson on slide tuning!!!

-Duck

The slides are the same diameter in all of the BS34 applications. I am not a theoretical person but know that these carburetors are used on bikes from 750cc to 1100cc with different jetting combinations. I tune and test BS34 carbs on my stock KZ900s all the time and they work well although BS32 have better low/mid throttle response and I like their feel much better. The problem with the smaller carbs is that MOST of them have no choke lever and most of them have no sync ports on the carburetors. They are too hard to modify to add a choke lever (they use a remote choke w/cable mechanism) and there isn't room to add sync ports. You have to modify regular intake manifolds to hold sync tubes instead of using ports on the carbs for synchronization.

WG-

Are the ports in the slides are the same size for all applications of say, the bs34? What about the springs? Same 'k' and length for all apps?

Curious because the air volume, hence velocity, depends on engine load, RPM, and engine displacement. Or is all this variation taken care of on the fuel metering side?

Guess I could compare pn for different bikes and see if spring and slide have the same numbers. Will have to scare up some on-line parts diagrams/lists for that.

Before motorcycles the only variable venturi carbs I messed with were SU ad Stromberg.

Thanks,

-Duck

Thanks guys for all of the advice.

I was speaking with Steve Darlington (a fellow Kzr member with a lot of experience on the 650/750 upgrades) who is giving me some pointers on my 810 mods. Steve has experience boring the 26's and says there is enough metal to enlarge them for added flow.

The performance of the bored 26's will give me some good, responsive low to mid range grunt which is what I want. The 34's on the other hand would probably get me slightly more on the top end range but they might not pick up the revs as quickly. Also I am familiar with this style of carbs having lots of experience working on my 24's which I spent countless hours fidgeting with the jetting last summer until Wiredgeorge gave me some advice on the pilot jet-thanks WG

I am looking to build a really potent little 650 sleeper. That will include an extra set of bodywork painted factory '77 Silver with red pinstriping the way the bike came when my brother bought it new. I want a little runner that will give the bigger Kz's of my friends a licking.

I want to have the motor look bone stock to the trained eye except for the filters and pipe, so the 26's are probably the way to go for the factory look. Now if I could just figure out a way to get the 652cc displacement tag put into the 750 cylinder head casting...that'd be the ultimate in sleeper detailling! I plan on running the bike in a couple of our 1/8 mile drag weekends in northern Ontario.

Looks like I am going with the 650 head (smaller combustion chamber and intake runners) with the 750 valvetrain innards.
I put both sets of carbs the 34's and the 26's up against the motor and clearly the 26's have the factory look. I will be keeping the 34's to maybe try swapping them in later as a comparison if I am not satisfied with the bored 26's.

Thanks again everyone-Cheers-Colin Firth-Ontario Canada