Flat VS Round Slides

highdesert said:

Thanks Hobot.

I bought a pair of 32mm flat slides today from JSmotorsports.

Jim Schmidt there has been very helpful with my questions.

I am hopeful they will be as easy starting and idling as my single Mikuni, but with significantly more performance at higher rpm.

If so, I will have a well sorted out Mikuni 34 for sale.

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> Four strokes produce a relatively strong vacuum signal, and there is a very definite pulsing to it at low speeds. Flat slide carbs lend themselves more readily to the addition of rollers on the slides to decrease the drag, and reduce the possibility of the slide sticking.

> Space is always a problem when you want a lot of engine in a small place. Flat slide carbs are shorter in length, particularly in larger sizes. A round slide in a 39mm carb would need to be a minimum of 42mm, probably larger. That's more than 1 5/8".
>It seems to me that even without the rollers, a flat slide would wear the carb body less, and may well seal better, giving more accurate mixture control from half throttle downward. With the rollers, there is very little wear.
> There may be advantages to having a shorter distance from the face of the slide, over the discharge nozzle, and from under hte back side that relate to air flow dynamics, but I'm not sure.

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I've read on Sudco's catalog that the flat slide has a higher air velocity thru the venturi, so, wouldn't that mean that it is restrictive at higher cfm demands? Or no?
I've also read that flat slides (due to higher flow velocities) are more precise at lower throttle openings. Is this true?
I've also read that flat slides are more finiky to tune out the lean spots... Which makes them a poor choice for a cruiser steetbike who desires a constant throttle opening.
I've also noticed 2-strokes seem to have a round slide configuration (or at least a triangle where one side is round or oval). Are these observations true?

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flat slide carbs are for racing where you are at wot most of the time, they do not give good response in the middle therefor not making them good for street applications. this is the case for motorcycles but with mopeds you are almost always at wot the reason lots of bikes come with sha dellorto carbs they use a flat slide and make for easier wot tunning. round slide carbs will give better midrange performance and a more uniform power band from idle to wot. with less flat spots in the powerband

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"Some guys swear by flatslide carbs, others at them. They do flow more air but throttle response and midrange tuning has more of an on/off temperament than a round slide type. It's a race piece, great for blasting out of a tight corner, not really intended for seamlessly smooth road manners. Otherwise, the OEMs would spec them for their road bikes. You could get lucky and get a flatslide carb that has just the right fuel curve straight out of the box. If not, it's going to be a bastard to get dialed-in."

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One man's opinion. Both my '06 YZ450's run exceptionally cleanly under cruise conditions
In the sudco catalog I was looking at, the HSR is the Harley carb to have.

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Round are more efficient at higher throttle openings. Flats more efficient at lower throttle setting. I qualify that statement because on the same motor, with the same size carb, a flat will need a smaller pilot jet and a bigger main jet than a round. For example, 40mm on 700 rotax's I've tuned. Round 45 pilot 280 main. Flat 35 pilot, 480 main. Efficiency in this example would be defined as the carbs ability to pull fuel through the nozzle.

Flats have easier throttle pull and are easier to package being shorter. I find them slightly harder to dial in than good ole rounds. They supposed give a little crisper throttle response but if both are jetted right I can''t tell. Flats are lighter weight. Flats are available with tps. Flats tend to be sensitive at idle mixture and tend to hang the idle on twostrokes unless the idle circut is slightly rich.

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I qualify that statement because on the same motor, with the same size carb, a flat will need a smaller pilot jet and a bigger main jet than a round. For example, 40mm on 700 rotax's I've tuned. Round 45 pilot 280 main. Flat 35 pilot, 480 main. Efficiency in this example would be defined as the carbs ability to pull fuel through the nozzle.

On the same quad, I had a flat 28mm and it required a #10 pilot jet. The smallest made! I recently replaced the 28 with a round 32. Now the pilot is a #25. The needles are the same (5L1) and the needle-jet in the round is a bit larger. Not much, but a bit. Both are bleed types of needle-jets. I can't comment much about the mains.

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On two strokes flats usually work better with a hooded needle jet, fourstroke without the hood. The hood creates some turbulance above the nozzle which can help it pull fuel in weak vacuum conditions like partial throttle midrange. Some flats have a pressed in needle jet so it's hard to make this change, but can be done.

That's what mikuni says too. They call the hooded jet a "primary needle-jet" and the non-hooded a "bleed type".
The primary type has a hole that pipes in air from the air jet and injects it, without mixing it with gas, straight into the venturi. The bleed type, the air from the air jet has to find its way thru all the gas surrounding the needle-jet and then thru little tiny holes on the side of the jet... Even then, its still mixed with gas as it comes up between the needle and the needle-jet hole.

Some people confuse the air jet with the pilot jet, air screw, etc. The air jet is its own jet ranging in size from 0.0 to 2.0. All it does is let air in and seems to affect throttle openings right after the pilot circuit, but before the slide cutout comes into play. On a bleed type jet, it doesn't do much of anything.

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Did use flat slides around 93 to 96. Is a 2 stroke carb with an adjustable air bleed system to control low speed fuel mixtures. Signal to the jets is a bit sharper so some MX guys liked them. Off roaders did not as they were sensitive to alatitude changes. I traded mine to Barry at Premier for a 38 roundslide. Bike was a 96FC501. Installed a 36 Mikuni pumper on my 96 FE400 (Was quite a chore) and scrapped the Flat slide on that bike as well.
I had a 38 Flatslide on an '96 FE600. They seem to give more HP but it also made them harder to start and hesitate when giving the bike full throttle

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the flat slide I liked, but the slide wears out and does the needle and needle jet. when th e slide wears it becomes hard to start the bike. check the slide and needle. the flat slide means it is like a shutter that fires fuel into the bike. The round slide that came later was more friendly and I think was used to help electric starting. Great carbi and less maintenance that the flat slide.

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Mikuni’s original flat valve (flat slide) carburetor, the TM Series Carburetors provide significant performance improvements over older design round slide carburetors. Air flows faster and smoother through the TM Series venturi due to the flat slide configuration and the jet blocks which help create a smoothbore effect. The high velocity of the air flow means a stronger vacuum at the needle jet, providing more precise metering and better throttle response.

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