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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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I assume you are saying that recessed area is a source of turbulence. And flat slides don't have a side notch they run in?

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37 minutes ago, jaguar57 said:

I assume you are saying that recessed area is a source of turbulence. And flat slides don't have a side notch they run in?

I do believe the side recesses for the slide cause some turbulence , but not nearly the turbulence that the bottom of the slide does , at wide open throttle at least.  Although if you remember  smooth bore carbs like the Keihin CR , the predecessor of the FCR , they had  round slides.  The bore was straight through with a cut out for the round slide on the sides  , shaped like a slice of the side of a circle on each side that actually was inside the slide when at anything less than full throttle.

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13 hours ago, jaguar57 said:

I assume you are saying that recessed area is a source of turbulence. And flat slides don't have a side notch they run in?

yes I believe you understand what I meant   if you look at a new style carb that has a D or an H slide the area of the runners  is less detrimental  than the relatively large 1/2 moon shaped runner of an older round slide.   Understand I did not do any flow bench testing  or anything like that just trying paraphrase A Graham Bell's book Two Stroke Performance Tuning

 

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off topic, but Mikuni had a version of the TM that ran totally on handful of needle's and air screw, eliminated the pilot and main.  TMD with a pro kit.   Sound familiar Lectron ?   

Difference between a 40mm VM and TM on the same motor (series 3 rotax).  Both use the same style jets,  the Round 280mj 40 pj,   Flat 480mj 25pj.  Both made the same power.  The round had more suction at the nozzle wfo, the flat had more throttle closed.  Only thing you really noticed between the two was the weight of lifting the round slide.  The TM's were kind of a PITA and Mikuni and BRP said the nozzle height were pressed in wrong. By the time they sent out a special tool we had figured out how to put a keihin hooded nozzle in them that worked better.  ymmv

Edited by highmarker

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The modern Mikuni and Keihin carbs will run better and make more power than the same size round slide carb from decades ago.  I can't think of any application where I would choose a round slide carb if I had the option.

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well I'll take highmarker's dyno experience any day over others theoretical ideas

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Where do you see the word ''dyno'' ? You're the one that usually posts the inane theoretical BS any way.

Edited by JoeRC51
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Here are a couple links to some test of complete intake track airflow on an older 4T race head. One is with a 40mm Keihin FCR and the other is a 40mm Dellorto PHM. There are no other changes to the test set up. Test pressure is ~28" H2O and valve lift is .6".

Flatslides do just what you have always heard, they flow more, about 6 cfm in the comparison. My experience is that modern ones, typified by the FCR also have better atomization which means better mixture quality and all the good things that arise from there like better throttle response, higher combustion efficiencies and ultimately a more predictable and easier to ride machine.

https://www.youtube.com/watch?v=9c-8zp72IPg

https://www.youtube.com/watch?v=vLkQ0maN_6k

 

Edited by pcnsd
poor wording choice
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in those videos an increase in cfm from 250 to 300 is only a 1.7% increase which is not going to make a noticeable difference in power.

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7 hours ago, jaguar57 said:

in those videos an increase in cfm from 250 to 300 is only a 1.7% increase which is not going to make a noticeable difference in power.

Your assumption is that a 1.7% increase in airflow will result in a 1.7% increase in PHP. It is not the case, but even if it was I would still chase it down. Over at NASCAR they spend millions annually looking for just such incremental increases. The 6 cfm difference between the FCR and Dellorto on this motor is a bit less than 2 HP potential. Not bad for a part swap. The 50 cfm difference in your assumption is pushing 13 potential HP.

See link. The equations refer to crankshaft HP potential and are based on data tables built primarily from the automotive side. Practical reality is often less for workaday 2 wheelers.

https://www.superflow.com/aspx/news.aspx?id=15

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man, you had me worried for a moment there. you're good. 

the formula from the site is horsepower per cylinder = Coefficient of power x Test Flow in  cfm

So if the CFM increases 1.7% then the hp increases 1.7%

So for a 25hp engine a 1.7% difference is .425 hp

Do you think you would even feel that small a change? (hint - the answer is NO)

Edited by jaguar57

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