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Discussion Starter #1
hi all,

very curious !
i want to mount flatslide carbs for obvious reasons, but dont wanna spend well over 1000 bucks.....

are there any members who have, or know one who has, experience in using Mikuni flatslides TM 40-6 ?

TDMRs are well known but also very expensive.

with using short(er) intake manifolds and possibly other alterations, it should be possible to mount those with good result right?

they are very affordable and i can do quite some things myself when it comes to metalwork.

lets hear it !
 

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Because of familiarity the Keihin is the Go To choice but Mikunis are good carbs and I don’t see why they wouldn’t work well. The issue is that you’ll be paving new ground installing and tuning them. If you don’t mind that challenge it sounds like a great project. You say they are cheap, would you elaborate on that ? Were they OEM on another bike ?
 

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I have bee working on Ducati's for over 20 years professionally, even at a top north american race/service shop. I have seen 1 Ducati in all that time run Mikuni flatslide carbs. I have a customer who is in the process of building a TT1 replica and he is going to run Mikuni flatslide carbs for the same reason you mention price. I should get the bike some time late spring/early summer for dyno work and some experience but cannot give you much more than that.

I have seen Keihin FCR's used 95% of the time as they simply are the best carburetors, but pricey. Next would be Delorto's and they work well for not much money with possibly the same design flaw that the Mikuni's have. Throttle springs. One of the benefits to the FCR's is that the slides run on rollers so you do not have issues with throttle hang-up. To get around this on the Delorto and Mikuni carbs they use very strong throttle springs which means a strong throttle hand is required to open them. If you use light springs the throttle slide gets sucked against the carb body and does not return with a light spring. It becomes a balance of stiff throttle vs slide drag.

I have run Delorto's on the track and just installed a set of Mikuni RS flatslides on a vintage GSXR750 the power gains will be similar to Keihins and both can be jetted to work. As long as you are fine doing the legwork that others before you have not and do not mind a "mans throttle" then I say go for it. Hey with increased costs someone has to start the ball rolling to help build a data base of alternate carburetors and educate us on the good and the bad. I look forward to getting the TT1 on the dyno just to start answering this question myself, but getting the bike on the track to judge throttle return will also be important as throttle hangup does not always happen on the dyno.
 

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There you go, he man slide springs. That is something workable, IMO. You may be able to get around this with a longer lower rate spring. I have a Mikuni flat slide on one bike and no issues but it’s a single carb set up so I’m only squashing one spring. I don’t see this as a deal breaker, just something to work out.
 

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Discussion Starter #5
With cheaper i mean a set of 2 TM40-6 carbs can be bought now for 416us dollars. Ex shipping.
Im not an experienced tuner but i dont mind to do some dirty work and try things.
 

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I saw many that specifically said they have pumps and the larger displacement snowmobiles have been 4 stroke for a while.
Mikuni TM Flat Slide Carburetor 40mm with Accelerator Pump
CARBURETOR TYPE FLAT SLIDE
CHOKE TYPE PLUNGER
COUNTRY OF ORIGIN JAPAN
HEIGHT 165MM
INTAKE BELL O.D.55MM
MAIN JET165 (N100.604)
MANIFOLD TYPE SPIGOT
MANIFOLD TO MANIFOLD 100MM
NEEDLE 9DJY4-96
NEEDLE JETY-6 (568)
PILOT JET17.5 (VM28/486)
SIZE 40MM
SLIDE TM40/81
SPIGOT MOUNTING O.D.44MM

Here is a picture of a used one in a pair off of an Arctic Cat but there are new ones available too.

 

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Tried a pair way back. Almost took both hands to wind them open, and you would need a very large diameter throttle tube to keep the required turn ratio down to something reasonable. Also as there is no data for jetting, that would take a while on the dyno to get sorted out. Their design just isn't as efficient as the FCR's. I'm sure with diligence you could make them work. The Keihins are almost plug and play. Money and time getting the Mikunis right or spend a bit more on FCR's and be done.
 

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Discussion Starter #10
Ok after reading a bit...ive decided to up my budget and go for something i know that will work.
Fcr 41s it will be. Then there is still a choice to be made......
Split or banked. Ive read that split is more difficult to tune? And its more expensive because of intake manifolds and airfilters.

What would be your recomendations?
 

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Bank of fcr's appear to be more midrange
split singles appear to be more top end.

Banks are easier to maintain as sync almost never changes, splits will go out of sync rapidly if you use the idle whips.
Splits look cooler (IMHO)

my track bike has splits and my street bike has a bank. Application is important to think through first as they are not cheap so do your home work and be honest in where you will be riding (rpms). A common mistake is to build a bike for how you think will make the most power even if you seldom ride there. I will also throw another wrench out in that I would chose 39's 90% of the time over 41's based on acceleration (part throttle accelerations) over again a peak number. I have a couple posts on just this question not long ago complete with dyno graphs for comparison.

What will be the use of the bike?
what rpms will you use the bike at?
what are you looking for out of the carbs?

A built 985 will want a different carb than a stock 900 so do not buy more than you need unless you have future plans to spend more money.
 

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in my race days most people were faster on cv's then those very hard to control mikuni flatslids. i wouldnt wast my time on them.
 

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Ok after reading a bit...ive decided to up my budget and go for something i know that will work.
Fcr 41s it will be. Then there is still a choice to be made......
Split or banked. Ive read that split is more difficult to tune? And its more expensive because of intake manifolds and airfilters.

What would be your recomendations?
whatever are the cheapest.lots of second handies floating around. never had a problem tuning either. linked carb are a little harder to get to the mixture screws.
 

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Discussion Starter #16
I will be using the bike 99% for road use. Not really sure where i use the engine most rev wise. I think its from halfway to 90% revs. Accelerating out of bends often WOT. Top speed isnt my goal. I like to ride curved roads in a sporty way but dont care about insane topspeed.
 

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Discussion Starter #17
What would be your recomendations?[/quote]
whatever are the cheapest.lots of second handies floating around. never had a problem tuning either. linked carb are a little harder to get to the mixture screws.[/QUOTE]

In all honesty...i dont find used sets atm. But maybe i must use some patience.

I love the looks of a split set, but if it would be better for my use to use banked then i would do that.

Right now the only engine mod is high comp JE pistons but who knows what the future brings.
 

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Most 900 supersports live in the 5000-7000rpm band and spend less time 7000-9000rpm. If you plan on spending time below 7500 then the bank would probably be better. If you want to try and make it a additional 1500rpm then you likely need splits, and cams .. and porting. The 2-valve heads really do not make much above 8000 unless you build it that way (it isnt in its nature) as the head design is not designed for high rpms. Of course 8000rpms is high rpms compared to some motors.


Run one is a long manifold 900 monster vs a short manifold 900ss. the SS with short manifold makes and carries its power higher and the long manifold makes its power earlier and stops making power sooner. Run 2 is a experiment with 2 800 monsters. 1 is a s2r800 with short manifolds and injection, the other carbureted with stock cv carbs and long manifolds see if you can guess which is which?

Depending on what rpms you ride at the power should be where it will do you the most good. If you have short manifolds and build a highrpm bike you will likely be slower than a stock bike at lower rpms. This is why it is important to first decide on what rpm band you are going to play in and then you need to ride it that way.
 

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Discussion Starter #19
I actually like the power to be not all in the above rev range. Headwork cams etc is also expensive and i think with only fcrs and high comps the bike is good for my needs.
So that would suggest banked with longer inlets would be the better choice?!
 

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Personally I don’t think fcr’s on a near stock motor is not good bang for bucks. Lighten the flywheel and shorten the gearing would be my first mod for a street bike. And maybe a shorter throttle.
 
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