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smoothbore carbs 19 Oct 2005 13:40 #2809

  • marko
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Can anyone tell me how to tell if a set of carbs are smoothbore or not? And also what makes the smoothbore carbs better than other carbs?Would like to see some pics if anyone has any. Thanks MARKB)

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smoothbore carbs 19 Oct 2005 14:23 #2812

  • Jeff.Saunders
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Look part way down this page for pix of the 29mm carbs.

www.z1enterprises.com/reference/z1-carb-guide.aspx

The carb is a better overall design than the stock carbs in a number of ways. There is an insert (called the jet block) that fits into the carb to keep make the venturi as smooth as possible. The regular carbs have irregular shapes in the slide area that cause turbulence in the airflow through the carb. This allows the 29mm carbs to give excellent throttle response down low yet pull hard at wide open throttle.

The carb design was very good in other ways. They have individual filters that go over the float needle to reduce the chance of particles getting into the float needle valve assembly. The carbs also have O rings on all the feul rails & tees, and the choke plungers. Something that no other carb had in the middle 70's when they first came out.
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smoothbore carbs 20 Oct 2005 06:34 #2969

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Jeff, some of the VM24 assemblies came with rubber o-rings over the fuel pipes from the same era. The reason for the fuel screen that must be there on a VM29 (or BS34 Mikuni CV carb) is they have no overflow system. The main cause for overflowing is crud getting between the needle and seat and if a VM29 overflows (or a BS34 CV carb), there is no where for the gas to go except up through the needle jet and into the main venturi...

While Jeff touts the VM29 for having the jet block to make the venturi smooth, I want to point out, in my opinion, that the MAIN reason that the VM29 is able to deliver MORE GAS and thus a larger amount of air/fuel mixture at HIGHER engine speeds is the larger fuel pipes ID. The fuel pipes on a smoothbore are far larger diameter than any of the OEM installed VM series. If you have a highly modified motor, the issue becomes one of fuel delivery more than venturi smoothness, I suspect. What this means is that if you have a stock bike, the OEM carbs are likely fine but if you have made modifications, you don't want your fuel pipes to become your MAIN JETS! Nor your petcock or gas hoses or cheap fuel filter....
wiredgeorge Motorcycle Carburetors
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smoothbore carbs 20 Oct 2005 07:28 #2975

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George's point is a good one - the 29mm carbs do respond well to jetting changes whereas some of the stock carbs you can only go so big on main jet and the carbs will no longer flow enough fuel to make a difference. I have run into this with stock Z1 carbs - at wide open throttle on a hot-street bike the plugs always showed lean - inceasing the main jets made no difference 130, then 132.5 then 135 - all showed the same lean condition - changing petcocks didn't help. Switching to 29mm smoothbores and the problem went away.
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smoothbore carbs 20 Oct 2005 07:43 #2979

  • Duck
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George-

I looked at your carb ID section and still have a question.
I'm at the salvage yard the other day and see a bike with these Mikuni carbs on it that look like my 29mm smoothbores except....
They have lots of ribs on the outside of the body.

They do have the big bowl drain plug and otherwise look like mine.

Are there any MIC carbs that have the big plug and the ribs that are not 29mm smoothbore? What else should I look for?

I ask because they are sitting out in the rain in the heap-o-parts-bikes and I thought I might try to grab them for swap fodder if they are the desireable part.

Bike they are on appears to be kz750 but hard to tell based on condition, lack of ID plate, and not knowing what else to look for.

-Duck

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smoothbore carbs 21 Oct 2005 04:05 #3176

  • dgfischer
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The Mikuni carbs with ribs are usually KZ 650 / KZ 750 24 mm.
Daniel

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smoothbore carbs 21 Oct 2005 05:54 #3191

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The bodies on the smoothbore carburetors are very similar to those on a VM26... the bodies don't have ribs. There are several variations of smoothbores. The earliest had a different bowl. Jeff at www.z1enterprises.com has pictures of this variation I think. I have never seen a set of these. There is also an optional accelerator pump kit. The #2 bowl will be twice as large as normal and there will be fuel lines running between the bowls.

The most common smoothbore (VM29) has some extra casted holes in the front edge. Actually, only two of them are used (as is the case with all regular VM series carbs) but the extra casted holes (all plugged) are there. There will be a 17mm drain plug on the bottom of the bowl most of the time. Those are the most common features that externally distinguish a smoothbore assembly. The internal differences are:

Larger fuel pipes

No overflow

Screened float seat

Slide block

Slide about 1/3 the length of a regular slide

Shares the upper slide pivot mechanism with a 76 VM26
(only one that does... has screw/locknut for adjust and another screw to hold the position of the adjuster).

Different air screws (VM29 has holes/hollow)

VM29 uses same type pilot jets as VM24 (little)

The aluminum bodies are a different and lighter alloy

The main jet holder tube and jet needle are
integrated.

The slide pivoting rod is drilled/tapped for bolts to hold the slide actuators the same as 76 VM26 (different than other VM models) (I am pretty sure on this one but not positive from memory)
wiredgeorge Motorcycle Carburetors
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www.wgcarbs.com
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Thanks George 21 Oct 2005 10:04 #3235

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We'll get lucky another day ;-)

-Duck

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