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Smoothbore advantages

  • Thread starter Thread starter dtkid
  • Start date Start date
D

dtkid

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CAn anyone explain the advantages of "smoothbore carbs" over CV-carbs.

I always thought (problem #1) that smoothbores were the type of carbs used on older metric machines (like the old Honda CB750s). The jet-needles on smoothbores are directly connected to the trottle cable...ain't that right??

In addition, I thought CV-carbs were engineered to be more responsive and provide smoother accelaration through the throttle range. Are smoothbores simply a newer/better generation of the old-type carbs.

Does the term "smoothbore" denote that the throttle body is smooth and without "cut-outs"??

I have a set of Keihins' that I have never used...I picked them up from a buddy about 10 years ago. Sometimes I see these go for big bucks....are mine worth big bucks? Will somebody send me a whole lot of money or stuff for these old Keihins?? I could use a whole lot of money or a whole lot of useful stuff.
 
Smoothbore refers to the fact that there is no throttle plate in the airstream. The throttle does act directly on the slide so you don't have to wait for vacuum to raise the slides. You can't just whack the throttle open down low with the newer RS and CR flatslides. In most applications they are a little oversized to deliver great topend. If you whack the throttle down low though you will lose velocity and the engine will bog.
 
Ahh......so they are not really the best for general street riding??

Is it that they are engineered primarily for racing (when the machines rarely use bottom-end torque)?
 
dtkid said:
Ahh......so they are not really the best for general street riding??

Is it that they are engineered primarily for racing (when the machines rarely use bottom-end torque)?
Click to expand...
For the average rider the stock CVs are more than adequate. Very few mods you can do will wake a bike up like a set of smoothbores.
 
The RS's are made for the street but are a mechanical carb that will let all the flow in as opposed to a vacuum operated carb that only opens as the engine can pull it open. The mechanical actuated carbs (ones that open as fast as you want) take some planning or thought when giving them WOT. Use a lower gear before opening them up all the way or just don't open them up as much when in a low rpm situation. The response is faster and the top end performance is strong. The vacuum actuated carbs when given full throttle actually are not open all the way and take a bit of time to open all the way where as the mechanical carbs are there right now. The thing about vacuum carbs is that they forgive when given full throttle at low rpm and don't bog. Mechanicals aren't so nice. For full on performance get the mechanicals and learn how to operate them, but for ease of operation get a good set of later model vacuum operated carbs.:wink:
 
I recently purchased a set of RS36 Mikuni Flatslides for my 1981 GS1100EX. I love the throttle response and power they have added to my bike. It doesn't even feel like the same motorcycle. I will point out that it took several weeks of changing jets and adjusting the needle to get the right combination. The other issue is I didn't do a baselin dyno before changing the carbs so I don't really know how much difference they make. The other comments are true about the full throttle in low gears. If you just roll it on instead of a quick twist it doesn't bog nearly as much. It just takes a little getting used to. I think they are worth the trouble.
 
i have a set of 29s on my gs 1000. they will bog at WOT below 5500 rpm and have been tempted to go back to stock 26s.however i can't kick the adreneline rush from 5500 to redline.{sigh}
 
roadron said:
i have a set of 29s on my gs 1000. they will bog at WOT below 5500 rpm and have been tempted to go back to stock 26s.however i can't kick the adreneline rush from 5500 to redline.{sigh}
Click to expand...

Get tuning then, I can wack the throttle open at 4k on my 1000S with 29's.
 
Got 33's on my 83 gs1100/1260e. At 3 grand open it up and it is GONE I mean snap your wrists gone.I love it !!!!!
 
Hi,
Last weekend I raced my GSX1100 equipped with 36mm CV's on the track.
The carbs are really well set up and the motor revs cleanly. I was up against other GSX1100's with Keihins smoothbores.
On top speed there is no difference, where you lose out is opening the throttle off the over run, the time it takes for the vacuum to work and move the slides the smoothbores have made 4-5 bike lengths on you, over a 5 lap race its too much to get back.
On the road the CV's would be perfect, fitting smoothboreswould give you about 10% more power, but do you really need it?

Noel
 
Mikuni made the Smothbore carbs for some years, but they stopped production some time ago. You can get them on ebay still. My understanding is that Suzuki and Kawasaki use the same spacing. If you see Honda in the listing forget the carbs. Honda uses 80-100-80 spacing on the DOHC bikes and 78-78-78 on the SOHC bikes. Another thing to consider is what direction the draft of the carbs are. Down draft carbs are no good. These are for bikes that the carb bodies are off at a pretty good angle. What the GS needs is a good set of sidedraft carbs, where the float bowls are level.

If you want new carbs Keihin still makes CR and FCR carbs. The CR is a normal mechanical carb. I know of a guy that can get them for the Honda's for a little over $700 USD. FCR are another story, they have an accelerator pump. The bottom line of this is that the FCR carbs will like WOT at lower RPM's much better then the CR (go ahead and whack the throttle).

I've talked to several Honda guys that have CR's and are very happy with them. One guy builds the old DOHC bikes and has the CR's with Dyna 2000 and many other upgrades on the bikes, and likes them alot.
 
SUDCO still offers RS flatslides. I believe the FCR line are downdrafts.
 
roadron said:
i have a set of 29s on my gs 1000. they will bog at WOT below 5500 rpm and have been tempted to go back to stock 26s.however i can't kick the adreneline rush from 5500 to redline.{sigh}
Click to expand...
Sounds like the mains are too big. Though a rich jet needle setting can create a momentary bog when rolling it on, even if the mains are right.
Ever marked your throttle and grip and taken some 1/3 throttle plug reads? If the plugs are burning right at 1/3, then you know the jet needles are set right and you need to try smaller mains.
If the 1/3 tests shows rich plugs, then you need to lower the jet needles a little.
In my experience, most "bogs" as in your description, are because the mains are too big.
 
Smoothies

Smoothies

Smooth-bores add around 8-10 hp and are smooth inside with no curves for the slides.(see pic) Besides there rare and trick, so that 23 reasons why to put them on.
 
I believe that Keihin is making a FCR sidedraft. Some of the Honda guys have them.

There are another type of mechanical carb, Flatsides. The deal with them is that instead of a cylinder regulating air flow you have a flat slide that is doing that. My understanding is that they tend to create some turbulance in the bodies and give up some power, but still a gain over any CV.

My thoughts are that if you do any racing get the flat/round side and ditch the CV's. I've seen someone go through the work of getting a turbo mounted on a GS750 with CV carbs.
 
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