Whats a flat side carb?

[The Pastor]

What is a flat side carb? How do they differ from normal (slide?) carbs?

Are they also called open carbs?

[James Deuce]

Flat Slide.
Round Slide Carb


Literally the bit that moves up and downwhen you twist the throttle is either flat, round, or flat on on side and round on the other (semi-flat slide carb).

An open carb is one without a filter on it or connected to an airbox with a filter in it.

[skelstar]

So the slide-y part is like the butterfly in an automotive carb?

[xwhatsit]

I think, skelstar, you are getting confused with CV (constant depression/SU) carbs. Yes, the slide is the bit that you control with the throttle. In a CV carb, the needle and slidey bits are controlled indirectly by strength of airflow (wait, here come the pedants), the airflow being controlled by the butterfly. A slide-type carb (or proper real man carb as they are correctly known), the slide/needle etc is controlled directly.

Simpler mechanism, but easier to tune, more compact, and supposedly better throughput for a given bore size (when you open the throttle, you're guaranteed that the whole carb is open). Drawbacks are the direct control; fuel jets have an inherent lag. So when you crack open the throttle, you have a lot of air but the fuel hasn't caught up yet, so it will gasp a bit before it catches up. You have to be smooth. CV carbs are better in this respect, as the slidey bit will gradually open up as fuelling comes on. However many modern slide carbs have a pumper, which squirts extra fuel in like a water gun when the throttle is suddenly cracked open. Instant response -- way better than CV carbs. S'why they use them on dirt bikes apparently.

A broad generalisation you tend to hear is that CV carbs are more resilient things than slide carbs. Slide carbs are so simple, that whilst they give the best performance when tuned properly, they go out of tune when elevation, air pressure, air temperature etc changes. CV carbs, because they are (in a way) self-regulating, are less likely to be phased by such things. They break down more gracefully in the face of such issues. That's what one hears on the intarnets, dunno if it's true or not.

Flatside, roundslide, same thing really. Apparently flatsides are the hot shit for modern racers. Dunno what the benefits are compared to the roundslide variety -- less disturbance of airflow, perhaps? Anybody?

[The Pastor]

yeah on my cbr250 forum this guy put on some flatsides saying "60hp" I just kinda want to know how they work better than normal carbs.

Also, strictly speaking performance gain (not performance to cost) what is the benifit to buying a set of performance carbs as to rejetting stock carbs?

[James Deuce]

You won't get 60HP by changing a set of carbs. You'll get a CBR that is an arse to ride in town and needs to be in the top third of its rev range to work correctly. It will still put out 40-odd HP.

There's a reason manufacturers put CV carbs on street bikes and flat slides on a race bike. CV carbs meter out a fuel air mix that works from whoa to go.

Whacking a set of flat slides open from low revs will have you bogging down or stalling.

[Fatjim]

Scolloped Potatoes.

[quallman1234]

Go the RG150! < It has flatside carbs.

[Ixion]

,,
Flatside, roundslide, same thing really. Apparently flatsides are the hot shit for modern racers. Dunno what the benefits are compared to the roundslide variety -- less disturbance of airflow, perhaps? Anybody?

Pretty much. A flat slide carb has a (duh) , flat slide. As in very thin, like a guillotine moving up and down in the carb mouth. This (in theory) gives less impediment to the air flow than a round slide. Downside is that a flat slide can't have much of a cut away. Look at the bottom of a round carb slide. It's not just a cut off cylinder. At the bottom, there's a slopey bit cut out off the cylinder (always faces the air box BTW- I've seen them put in back to front, which causes strange behaviour). The cutaway is variable , model by model, and controls the air-mixture ratio at low throttle opening. A bugger for tuning cos you can't fiddle with it as easily as the needle or main jet.

So flat slides are good for bikes that tend to be WOT or idling. Like racers and two smokers. Not so hot for bikes that need fine throttle control. As everything, that statement is horribly simplistic and someone will take issue with it.

[Motu]

Are they also called open carbs?

Open carb could possibly be referring to perhaps the Amal GP and similar.On these the needle isn't in the center of the slide,but in a chamber to the side - so when the slide is wide open there is nothing at all in the carb mouth apart from a small main discharge tube to the side.Deorto also made this type of carb.

One small point with flat slides - they seldom have a cutaway,this is one reason why they are fairly abrupt in their response.Ooops,belay this reply,I see Ixion has beaten me too it.....sneaky bugger,did he see me typing or something?

[The Pastor]

FYI open carbs mean no air filter attached.

[Ixion]

Oh, I call those engine grinder carbs

[Motu]

FYI open carbs mean no air filter attached.

Why not say no air filter....kinda says it all with no mystery attached.

In days gone by most bikes didn't have an air filter - and this has probably fueled some of the folk law about British bikes needing constant engine rebuilds....kinda obvious when you know they didn't have air filters,and us being so much more intelligent these days.I've had trouble getting barrels off bikes because the lip at the bottom of the bore was so much,in like 10,000 or so kms.

[xwhatsit]

Oh, I call those engine grinder carbs

Somewhere I asked advice about using velocity stacks (bell-mouths) on a carb, no air-filter. You said `perhaps a little more engine wear' or something similar. Obviously nothing's as good as having a proper filter, but modern tarmac roads and less exposure to metal roads has to be worth something?

[merv]

Go the RG150! < It has flatside carbs.

Well at least one of them I presume?