Flat side carbs?

[swanman]

Yeh so what's the deal with flat side carbs? What are they? What's the advantage?

[k14]

Thats flat slide, cause the slide is flat as opposed to conventional carbs having a round slide.

The advantages are (i think, im not really an expert on this subject) normally better top end with a sacrifice for the bottom end. Thats why its used for race bikes, don't need any bottom end. Not the best for road bikes but there are cases of road bikes coming out of the factory with them. Not very useful these days with FI, quite a few 2 strokes have them.

[Motu]

I used to know,back when it was important to me to know such things - basicly the air spends less time under the slide....if it's a big fat slide that's a long time.A guillotine slide is very narrow and the transition between one side of the slide and the other small,good for throttle response.A lot of guillotine slides don't use main jets,just use the needle for fuel control - Lake,Gardiner,Magnum are some brands of guillotene slide....I have a small selection of Blue Magnums,I like them ..easy to tune,start etc,but poor idle and need to be retuned when the sun goes behind a cloud.

[Coyote]

Do you know where I could get a flat slide kit for my 250?

[TwoSeven]

'Flat Slide".

If you look inside the end of the carb where the air goes in you will see a brass butterfly valve and when opened bit of plastic with a little groove cut out of the bottom and a needle hanging from it. The bit of plastic is the slide [correctly known as a throttle valve] and is by ways and means indirectly linked to your throttle.

Old carbs have the throttle cable attached to the slide directly (I mean really old carbs) and no butterfly, newer carbs have a butterfly valve in front to which the cable is attached. Its newer carbs you are interested in.

When you twist the throttle, it turns (opens) the butterfly allowing more air to be sucked thru the carb. As this air passes thru it hits the slide and tries to squeeze thru the gap given by the cutout at its bottom. As more air gets sucked thru and under the slide the additional pressure causes the slide to lift. As the slide lifts, the needle attached to the end lifts as well, opening up a bigger gap in the needle jet it was seated in to flow more fuel which then spills up into the carb and mixes with air.

The cutout has a specific size and angle - the size is what dictates your idle when closed and the minimum throttle position at which the throttle actually works and the angle of the cutout dictates the speed that it [the slide] lifts up. It also affects fuel metering at 1/8 and 1/4 throttle.

Get the cutaway angle to steep and the slide wont lift unless the throttle is well open, get it too shallow and the slide will flutter under the slightest throttle change.

As I said, it affects idle. The normal height of the cutaway (on a honda) is equal to about 1200 rpm idle, when you adjust the idle rpm adjuster on the side of the bike [as opposed to the one on the carbs], it opens the butterfly which causes the slide to lift more. Its the same as slightly moving the throttle (cept you dont have hold it).

There are three types of slide. A round slide, which looks like a small oil drum with the cutout on the front side. A TMX flat slide and a TM flatslide. The TM has two flat slides either side of the needle and the TMX only has one on the front side.

I personally like the TMX flatslide because it responds immediately to throttle pickup (response) although you have to follow the power curve up with it. If you wack the throttle open it will lift too quick and you'll get a lean/rich/lean scenario causing the engine to shudder (my theory only). On the round slides, they have a bit of delay so you can wack the throttle open and it will chase the throttle up - but you kind of lose feel as to what the engines doing. Its personal preference really.

[SimJen]

not much point sticking them on a CBR250 though....you'll notice possibly an extra hp at the most but more likely no gains and you'll lose the bottom end.
My old NSR250SP had flat slides, they seemed to work well and as small two strokes don't have bottom end you don't notice it missing plus twostroke 250's got a lot more potential for power than fourstroke 250's.....
want power in your CBR then turbo it, i heard of an Ozzie one with 110hp!!!

[TwoSeven]

The CBR250RR has a round slide 1/4 turn throttle. Its made of plastic instead of copper/brass so has pretty good lift qualities (I liked it).

I forgot to mention - there is one other kind of slide - the half round variety. It has a flat face, but the back side is round. I think this is to give good lift and a nice cuttaway angle. I think the cibby 6 may have used it (but cant remember).

[Kickaha]

Old carbs have the throttle cable attached to the slide directly (I mean really old carbs) and no butterfly, newer carbs have a butterfly valve in front to which the cable is attached. Its newer carbs you are interested in.
.

Your so called "newer carbs" have been fitted to bikes for well over twenty years

[Ixion]

Your so called "newer carbs" have been fitted to bikes for well over twenty years

Yeah, that's right. new, as in just introduced the other day. As in I haven't gooten used to them yet.

What was wrong with the Amal GP I ask.

[Motu]

Your so called "newer carbs" have been fitted to bikes for well over twenty years

Even longer than that,my old 1971 XS1 had CV carbs,so we can go back to the mid 60s,even to the 50s as the Triumph Thunderbird had an SU.CV carbs slow down throttle response and make the bike easier to ride,it's seldom you'd find straight pull carbs on a thou,just too hard to control.Same power,just softer delivery.Likewise you won't find CV carbs on a dirt bike,they need instant throttle response.Flat sides don't have a cutaway setting,they don't need it.

Um...of course you can get the worst of both worlds and have twin carb singles like some trail bikes had in the 80s and 90s.,one cable slide and one CV.

[pete376403]

Yeah, that's right. new, as in just introduced the other day. As in I haven't gooten used to them yet.

What was wrong with the Amal GP I ask.

GPs would flow more air than any other carb of equivalent size. Even the needle was out of the main venturi (it lived off to one side) to avoid disturbing the flow. WOT was really good, anything else wasn't so good. For racing that ok. They also had a remote mounted float chamber so the carb could be mounted at any angle. Often vibration would cause the float chamber to slide down what ever it was mounted on, playing merry hell with the mixture.
Early 2 valve speedway Jawas had a Dell'Orto that was a (poor) copy of the GP, with twin float bowls mounted on the frame downtube just under the seat. Often by the end of a race the chambers were near the bottom of the down tube. I binned the one on my bike and fitted a 40mm Mikuni - solved all my carb problems forever. The damn thing would even idle (sort of)

[TwoSeven]

Your so called "newer carbs" have been fitted to bikes for well over twenty years

The modern c.v. or butteryfly valve carb has been around for about 25 years to be more precise, which makes them newer if taken over the period that carbs have been around for (about 112 years) and not newist, because they have since been replaced with FI systems. Before the modern c.v. carb the skinners union c.v. carb was the most common type as it had a variable choke. This style I mean as being the older type.

[soundbeltfarm]

i think my bike has flat slides.
jap import gsxr 750 1986.
anyone know?

[Ixion]

GPs would flow more air than any other carb of equivalent size. Even the needle was out of the main venturi (it lived off to one side) to avoid disturbing the flow. WOT was really good, anything else wasn't so good. For racing that ok. They also had a remote mounted float chamber so the carb could be mounted at any angle. Often vibration would cause the float chamber to slide down what ever it was mounted on, playing merry hell with the mixture.
Early 2 valve speedway Jawas had a Dell'Orto that was a (poor) copy of the GP, with twin float bowls mounted on the frame downtube just under the seat. Often by the end of a race the chambers were near the bottom of the down tube. I binned the one on my bike and fitted a 40mm Mikuni - solved all my carb problems forever. The damn thing would even idle (sort of)

Yeah, it was a rhetorical question. Horrible things the GP for road use. They were racing carbs , as you say full throttle or full closed on teh overrun. Nothing subtle about them.

[F5 Dave]

Depends what you mean. People refer to flatslides & either mean a roundslide vs flatslide (or other shape). But the other distinction touched on above is CV vs Slide carbs (or straight pull) where the cables directly operate the slides.

Assuming you are talking for your YZF R1 they have CV carbs (pre injected models of course). My YZF750 is the SP with the straight pull flatslides Keihen FCRs. The R std road model has CV flat(ish) slide carbs.

The Flatslides are better for a racebike but dubious on a roadbike as they are prone to bogging if you carelessly whap the throttle open at low revs. Strangely the YZF setup doesn’t seem prone to that & is pretty sweet as a roadbike with great throttle response, loads better than a late model GSXR.

If you buy them be prepared to spend some time setting them up on the dyno & road.