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  #1  
Old 1st March 2012
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Default Mikuni flat slide carb setup

Ok Mikuni guys please speak up.
There is not much info on tuning these carbs specifically for dual carb sportsters.

I have a set a Branch modified 39.5mm flatslides.
The bore has obviously been machined and blended.
The throttle slide chamfer (4.0) has been machined ( I assumed to match the larger bore)
The mains are 300, pilots 30, needle is 6DP4, holder is 389 q-2

Does any one know exactly what branch did to these carbs?

My problem is : This set of carbs are very well tuned for both of my XR dual carb Sputhe sportsters with minimal tweaks when swapping.
One is a 77ci, 3.1875"x4.8125" the other is a 92ci, 3.625"x4.4375"

So I bought a set of stock 38mm flatslides.
Mains are 300, same needles, they work well from 1/4 throttle up.
The idle circuit on the new 38's is not working the same as the 39.5mm's.
I have 20's (pilots) in the 38's and the idle is still extremely rich.
The air screws won't even effect the afr much.
I'm going to try 15's (pilots) but I am thinking the 38mm throttle slides not being machined like the 39.5's are the reason for the overly rich idle?

Also on the bored carbs the small passages in the inlet have been blended because of the machining and one of the passages have been epoxied.
Anyone know what these passages are exactly?

Well thanks to anyone w/ any details.
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  #2  
Old 1st March 2012
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Default

Quote:
Originally Posted by chevelle View Post
Ok Mikuni guys please speak up.
There is not much info on tuning these carbs specifically for dual carb sportsters.

I have a set a Branch modified 39.5mm flatslides.
The bore has obviously been machined and blended.
The throttle slide chamfer (4.0) has been machined ( I assumed to match the larger bore)
The mains are 300, pilots 30, needle is 6DP4, holder is 389 q-2

Does any one know exactly what branch did to these carbs?

My problem is : This set of carbs are very well tuned for both of my XR dual carb Sputhe sportsters with minimal tweaks when swapping.
One is a 77ci, 3.1875"x4.8125" the other is a 92ci, 3.625"x4.4375"

So I bought a set of stock 38mm flatslides.
Mains are 300, same needles, they work well from 1/4 throttle up.
The idle circuit on the new 38's is not working the same as the 39.5mm's.
I have 20's (pilots) in the 38's and the idle is still extremely rich.
The air screws won't even effect the afr much.
I'm going to try 15's (pilots) but I am thinking the 38mm throttle slides not being machined like the 39.5's are the reason for the overly rich idle?

Also on the bored carbs the small passages in the inlet have been blended because of the machining and one of the passages have been epoxied.
Anyone know what these passages are exactly?

Well thanks to anyone w/ any details.
Just a bump for you. I wish I could help but I can't. Those mains seem awfully big though
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  #3  
Old 1st March 2012
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Quote:
Originally Posted by BWP 5p View Post
Just a bump for you. I wish I could help but I can't. Those mains seem awfully big though
Thanks,
All is well at WOT.
I will most likley go to 320 mains.
My issue is the idle circuit.
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  #4  
Old 2nd March 2012
KRTTXRTT KRTTXRTT is offline
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Default

It does sound like the pilot jets are too large. I have a set of the Branch modded (40.5mm with external sleeve) carbs too and the jetting is close (from what I remember) to what you are running - 315 main on a nasty XR motor with open megs. One size on the pilots goes a long way on Mikuni's.

I have a set of the stock 38mm Mikuni flatslides on a Krauser BMW that is about 950cc etc. Oddly it runs a 45 pilot and is just barely rich? Lower vacuum I suppose.

The altered circuit is likely the air bleed for the needle jet and needs to be there but the opening size is not real critical unless it's plugged!

If you are still having trouble, I will go pull a float bowl instead of guessing.
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  #5  
Old 2nd March 2012
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Thanks krttxrtt,
I installed some 17.5 pilots and idle is much better.
I believe I will be able to fine tune now.
I would still like to know the specifics on the Branch Mikunis.
I have some photos I will post soon.
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  #6  
Old 14th March 2012
KRTTXRTT KRTTXRTT is offline
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Once I figure out how to post pictures, I will shoot a pic of mine. I have two sets that are Branch modded and they differ. One set has a larger 40.5 mm venturi and has an external sleeve. The other set has a tapered venturi and is closer to yours at 39.5 mm.
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  #7  
Old 15th March 2012
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Quote:
Originally Posted by KRTTXRTT View Post
Once I figure out how to post pictures, I will shoot a pic of mine. I have two sets that are Branch modded and they differ. One set has a larger 40.5 mm venturi and has an external sleeve. The other set has a tapered venturi and is closer to yours at 39.5 mm.
Here is a Branch Carb bores and blended (also has a 55mm long velocity stack), the center hole at 7:00 is epoxied
And the factory 38mm carb, the hole plugged w/ a piece of aluminum in the stock 38mm.

The design of the carbs are different now.
The new Mikuni design has a idle adjuster in the side of the slide housing.
The earlier style has the idle adjustment in the slide cover w/ the funky rod and jam nut.
The slides are different as are the slide guides(blocks) and the slide covers obviously.
I will get more photos showing this.


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  #8  
Old 13th April 2012
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Default

Im still having trouble w/ setting up new dual 38mm flatslides for 0-2000 rpm.
I have a set if Branch 39.5mm (38's bored).
The 39.5's have 30 pilots and 300 mains w/ 6dp4 needles.
The 39.5's work flawlessly through rpm range.
I cannot seem to get idle/ pilot circuits to work well in the 38's.
If I tune idle circuit w/ 15 pilots the transition to mid range suffers.
If I run larger pilots to compensate the plugs foul.
Now I notice the small bypass hole (just behind needle jet) coming from up from the pilot circuit into the bore on the 39.5's is about twice the size (.050")of the stock 38's (.025")
If I enlarge this hole, I'm thinking it will add more air, then I can run a larger pilot.
Any details appreciated.

Here is a photo of the bypass holes in question,
its the small holes behind the brass needle jets.
The carb on the left is new 38mm, right is the modified 39.5mm Branch.


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  #9  
Old 17th April 2012
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Here is a link I started from another site for reference.
If there is anyone with a Branch carb that could verify the bypass hole size I would really appreciate the help.

http://speedtalk.com/forum/viewtopic.php?f=15&t=30761
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  #10  
Old 17th April 2012
KRTTXRTT KRTTXRTT is offline
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Chevelle,

I will pull one when I get home and have a look. I do know some people run the accelerator pump on these as you can always 'beat ' them if yank the throttle really quick at low revs. I see the size of the motors you are running these on and it seems you are not close to running too large on the throat size? Mine are both on 61" motors and work well.
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