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Flat slide carbs on an old 2-stroke

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Not sure where to put a question like this. It's about carb technology. 

I have an old bike, a 1986 Yamaha TZR250, which is a 2T parallel twin. It has 2 Mikuni TM28SS flat slide carbs on it. What I'm curious to know is why they aren't the VM type round slide carbs. I thought round slide carbs were better for 2-stroke performance? Does the design of these flat slide carbs fit a particular purpose or are they an antiquated idea?

I know it's a road bike that wasn't even available in North America but I thought there might be quite a few people on TT that know a bit about stuff like Banshees and snowmobiles to know a bit about new and old 2T twins.

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Well, think about the fact that no modern performance oriented 2t comes with round slide carbs. And a popular upgrade to old ones is to go to a flat or D shaped slide design. Your bike is ahead of the game! Yamaha knew what they were doing when they built that bike.

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8 hours ago, William1 said:

A TZ idles well abut 3,000 rpm and rarely is run under 6,000. A dirt bike engine idles closer to 1,500 rpm and spends a lot of time at mid rpm.

So does that mean there would be a higher intake velocity through each carb, and that would work better with a flat slide design?

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Think of the surface area under a slide. How the air would move. Think 'line' compared to a wide 'flat' area.

Some of the newer 2S carbs have come up with 'work arounds' to where the slide cut-away (which really is a funnel sort of thing to focus the venturi point at low rpm) is no longer needed. A round slide really messed up the mixture at high rpm, it caused air to wildly and unpredictably tumble at large throttle openings.

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That last statement doesn't seem right because at full throttle the slide is completely out of the way for the air to go freely thru the carb venturi. So I don't see how slide design affects air tumbling (as if turbulence wasn't a good thing due to its helping the gas mix with air).

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5 hours ago, jaguar57 said:

That last statement doesn't seem right because at full throttle the slide is completely out of the way for the air to go freely thru the carb venturi. So I don't see how slide design affects air tumbling (as if turbulence wasn't a good thing due to its helping the gas mix with air).

Ever seen the bottom of a round slide ?  That's why they sell those plastic things that go in the bottom of that type of slide , to smooth the flow like the bottom of a round slide in a CV carb. That's also why the smooth bore carb evolved into the flat slide , the Keihin CR evolved into the FCR.

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yes but when the slide is all the way up it presents just a little bit of flow resisting turbulence due to that "step".

I made my own little bump under the slide to lessen turbulence there. It only affected the first 1/4 throttle opening.

round slide or flat side, I see them both as equal causes of air turbulence unless the design smooths the transition so that the air doesn't experience an immediate "bump" in the flow path. Looking at a PWK slide I see they designed it to have a smooth transition but I don't know about flat slides.

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The problem with a round slide  at full throttle isn't the bottom of the slide  it is the side of the carb body the slide slides in

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