My Carb seems to have a different main jet than the dealer wants to give me for Mikuni BS40 he says it should use this jet

But the carb has one of these in it now @ 147.5 with a fairly thick brass washer - I want to give 152.5 a go - what's the deal on these sizes?

Mine had the thick one originally, but I used the smaller ones when I was playing with the jetting.  It still has a smaller one in it.  It fits and holds the washer in just fine.  Appearance is different, but it works.

FYI - My nearest bike shop only had Keihin jets - they work and fit, too, and look like the smaller picture.  The numbers are not the same, though.  a 155 Keihin was about the same as my 145 Mikuni.  Mikunit is in it now, a 155.

(I measured the jets with drill bits - NOT turning at the time.  I know it ain't exactly right, but it worked for me)

Don't argue with Your dealer. Just tell him You want  BIG ROUND MIKUNI JET 152.5 and that's it . You don't have to explain what, when, how. If the dealershop doesn't have 152.5 , then You will have to go with something else. The small ones will do the same job, but they do  not have exactly the same thread pitch /nothing scary/, so You will feel a little bit of tightness when You screw it in.

bobo383 wrote on 12/31/69 at 16:00:03:


As a point of interest and if you are drilling out your jets with micro drill bits rather than purchasing four of the same size each time!
  The Mikuni jet sizes are denoted in tenths of a millimeter and increase 2.5 tenths of a millimeter per size. i.e. a #155 main jet is actually 1.55 mm. diameter, the next size up(adding 2.5 tenths of a mm.) is a #157.5 main jet, which is actually 1.575 mm. diameter, and so on.

The 2 types of jet styles will work fine.  I don't know why the dealers seem to have only the smaller ones, but the overall length and threads are the same, so it doesn't matter....just as long as it sits down in the fuel and sucks gas up it's little hole