Carburetor swapping question

S

SuzookKING

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Aug 31, 2002

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• Nov 28, 2002

• #1

I am swapping a Keihin 28mm round slide carb for a Mikuni 28mm flatslide, is there some sort of corelation between the jet sizes on the 2 carbs.

In other words, if I am using a 130main/52.5pilot in the Keihin, what would be a good starting point for the new Mikuni carb?

 

J

Jman271

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Oct 18, 2001

317

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• Nov 28, 2002

• #2

You should be close, I did the same thing for my sons CR80, and used the same jets, it worked great. I had a 127/125 main in the flatslide. Good luck-

 

S

SuzookKING

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Aug 31, 2002

82

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• Nov 28, 2002

• Thread starter
• #3

This is what came in the new carburetor:

Mikuni VM28-418
28mm
180 Main Jet (4/042)
15 pilot Jet (VM22/210)
175 P-8 Needle Jet
5DP39 Jet Needle
832-33001 2.5 Throttle Valve

We have put an identicle carburetor on a Derbi GPR 50cc and it took a 170 main jet for extended WOT usage.

 

D

DEANSFASTWAY

LIFETIME SPONSOR

May 16, 2002

1,192

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• Nov 29, 2002

• #4

Usually the Mikuni # is like2xs larger dont just swap straight across or it will be too lean and youll possibly burn down. I think there is a coversion chart somewhere .Maybe do a search as I think I saw something here before .

 

Rich Rohrich

Moderator / BioHazard

LIFETIME SPONSOR

Jul 27, 1999

22,839

16,904

Chicago

• Nov 29, 2002

• #5

Mikuni hex head jets are numbered by bulk flow, while Mikuni round head jets are numbered by inner diameter in mm.
The flow difference between a Mikuni 175 hex jet and a 180 is about 6cc ~3%) .
Keihn jets are numbered by jet id, and the difference between a 178 and a 180 is about .0008" .

I don't remember if the VM28s use round or hex head jets.

Basic math looks like this:
weight of fuel = jet area * ( SQR Root (head pressure * fuel density) )

 

Last edited: Nov 29, 2002

S

SuzookKING

~SPONSOR~

Aug 31, 2002

82

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• Nov 29, 2002

• Thread starter
• #6

small hex jets are in it