Regular carbs vs Smoothbore carbs questions

Smoothbore carbs are obviously the choice for high performance and well tuned engines. But, what exactly is the difference between a regular carb and a smoothbore carb? Why are they so much better and how much added performance would swapping them onto an engine actually add? Where do smoothbore make the most difference? If a bike is capable of running an 11 second 1/4 mile with regular carbs, how much quicker would it run with smoothbores?

Thanks!

KraZyRIDER wrote: Smoothbore carbs are obviously the choice for high performance and well tuned engines. But, what exactly is the difference between a regular carb and a smoothbore carb? Why are they so much better and how much added performance would swapping them onto an engine actually add? Where do smoothbore make the most difference? If a bike is capable of running an 11 second 1/4 mile with regular carbs, how much quicker would it run with smoothbores?

Thanks!

Comparing carbs of close size,you'll notice mostly stronger midrange acceleration with smoothbores. As for how much power you gain, that's really relative the the overall health and tuning of the engine.

They use a different slide arrangement with a jet block in the body to streamline the air flow past the needle jet. Effectively, that produces higher velocity which in turn reduces static pressure in that portion of the carb body. The reduced static pressure translates into a stronger signal at the needle jet. Atmospheric pressure then has and easier job of forcing fuel up the emulsion tube to feed the engine.

The more fuel you can draw and efficiently burn [not waste], translates into more torque.

The carb bodies are cast with just a circle in the top for the slide instead of a whole, hence "Smoothbore".
Mark

LarryC wrote:

KraZyRIDER wrote: Smoothbore carbs are obviously the choice for high performance and well tuned engines. But, what exactly is the difference between a regular carb and a smoothbore carb? Why are they so much better and how much added performance would swapping them onto an engine actually add? Where do smoothbore make the most difference? If a bike is capable of running an 11 second 1/4 mile with regular carbs, how much quicker would it run with smoothbores?

Thanks!

Comparing carbs of close size,you'll notice mostly stronger midrange acceleration with smoothbores. As for how much power you gain, that's really relative the the overall health and tuning of the engine.

They use a different slide arrangement with a jet block in the body to streamline the air flow past the needle jet. Effectively, that produces higher velocity which in turn reduces static pressure in that portion of the carb body. The reduced static pressure translates into a stronger signal at the needle jet. Atmospheric pressure then has and easier job of forcing fuel up the emulsion tube to feed the engine.

The more fuel you can draw and efficiently burn [not waste], translates into more torque.

...OK, so mid-ranger acceleration is increased by better fuel flow through the carb. Could the same be achieved by using a larger conventional carb?

As for how much power you gain, that's really relative the the overall health and tuning of the engine.

...again, with a well tuned engine capable of running an 11 second 1/4 mile with regular carbs, how much quicker would it run with smoothbores? I'm not holding you to a 1/10, or anything. I'm just curious as to how much of a difference there really is. Is it a huge difference, how noticeable is it?

Mark Wing wrote: The carb bodies are cast with just a circle in the top for the slide instead of a whole, hence "Smoothbore".
Mark

I'm not sure what difference this would make to fuel pick up, or fuel atomization. Can you explain what would be gained by the difference?

due to the hollow bottom on the smoothbore slide, the air passes by it smoother, when it is all the way up into the carb body. so with less turbulence it can pass more air full throttle. but this same hollow slide creates a dead spot at low lift, so it has a poorer vacuum signal at very low slide lift, and a better signal at full lift.
this is exactly the opposite of a stock solid bottomed slide.

as for 1/4 mile improvements. not very much on a stock motor. We would only be talking a few HP I would think. it takes a combination of parts and labor(porting) working together to get a big dif. the engine has to breath better(must be able to accept more air and fuel) to create more power. but u can feel the add acceleration. I don’t think The full advantage of the carbs would not be realized, with out a bit of cam and porting.

mark1122 wrote: due to the hollow bottom on the smoothbore slide, the air passes by it smoother, when it is all the way up into the carb body. so with less turbulence it can pass more air full throttle. but this same hollow slide creates a dead spot at low lift, so it has a poorer vacuum signal at very low slide lift, and a better signal at full lift.
this is exactly the opposite of a stock solid bottomed slide.

as for 1/4 mile improvements. not very much on a stock motor. We would only be talking a few HP I would think. it takes a combination of parts and labor(porting) working together to get a big dif. the engine has to breath better(must be able to accept more air and fuel) to create more power. but u can feel the add acceleration. I don’t think The full advantage of the carbs would not be realized, with out a bit of cam and porting.

...OK. So, if we were talking about a '79 650 - 4 that had degreed cams, a pipe and velocity stacks, would smoothbores be worth the cost? Or, would 750 pump carbs be just as good for the fairly stock application?

degreeing stock cams wont bring u any power, unless they were grossly out of time, for some strange reason.. stacks r not recommended for the street. u will just suck in dirt and ware it your pistons.
what size carbs r stock for the 650 and 750?
keep in mind that as the carbs bore gets larger, u will gain top end and loose bottom end. so its hard to say which carb is best. its up to , how or where u want the power.

mark1122 wrote: degreeing stock cams wont bring u any power, unless they were grossly out of time, for some strange reason.. stacks r not recommended for the street. u will just suck in dirt and ware it your pistons.
what size carbs r stock for the 650 and 750?
keep in mind that as the carbs bore gets larger, u will gain top end and loose bottom end. so its hard to say which carb is best. its up to , how or where u want the power.

...well, I was recently introduced to an old time drag racer who build motors for people today. Specifically, Kawasaki motors. I said I wanted stock reliability with a little extra zip. He mentioned several ways to do just that. Cams and carbs were obviously one way. He also mentioned getting slotted cam gears so he could degree the stock cams. Or, he could slot them himself. He said he could degree the cams and change their timing in such a way that it would run like it had one size up cam in it. Doubt the man, not me. He's a long time legend in the area. He has several different bikes that his son runs that run in the 8 and 9 second brackets. I'm just trying to figure out what I want done and how much money I need to spend. Not looking to break any records, just have some fun and surprise a few people. Not sure on the carb sizes, I'll have to check.

KraZyRIDER wrote:

mark1122 wrote: due to the hollow bottom on the smoothbore slide, the air passes by it smoother, when it is all the way up into the carb body. so with less turbulence it can pass more air full throttle. but this same hollow slide creates a dead spot at low lift, so it has a poorer vacuum signal at very low slide lift, and a better signal at full lift.
this is exactly the opposite of a stock solid bottomed slide.

as for 1/4 mile improvements. not very much on a stock motor. We would only be talking a few HP I would think. it takes a combination of parts and labor(porting) working together to get a big dif. the engine has to breath better(must be able to accept more air and fuel) to create more power. but u can feel the add acceleration. I don’t think The full advantage of the carbs would not be realized, with out a bit of cam and porting.

...OK. So, if we were talking about a '79 650 - 4 that had degreed cams, a pipe and velocity stacks, would smoothbores be worth the cost? Or, would 750 pump carbs be just as good for the fairly stock application?

All the old 750's had CV carburetors on them. All but the GPz had Keihins and they were not the performance units.

All the old 750's had CV carburetors on them. All but the GPz had Keihins and they were not the performance units.

...so, you're saying the 750 carbs would be of no advantage?

The carbs that came stock on the (K)Z650s were either Mikuni vm22ss (on the B2A and the D1A), vm24ss (on all other models up to 82) or BS32 (on the 82 and 83 models).

Here to the best of my knowledge are the carbs that came stock on the 750 (4 cylinder) bikes.

1980 E1 and H1
Keihin cv34

1981 E2, H2 L1
Keihin CV34

1982 E3, L2, H3
Keihin CV34
1982 R1
Mikuni BS34

1983 L3
Keihin CV34
1983 H4
Mikuni BS34
1983 ZX 750 A1
Mikuni BS34

1984 Kz700 A1, kz750 L4, ZX750 A2
mikuni BS34

1985-1988 ZX750 A3 - A5
Mikuni BS34