Do I need Mikuni "small round" or "large round" main jets?

[TheRadBaron]

When I got my FJ it had DJ main jets that were physically the same size as Mikuni "small round" jets.  I just got a set of new carbs from Randy, and they have "large round" main jets installed.  The threads are the same for both styles, so they can at least physically interchange.
I did some searching around on the internet, and I've seen conflicting information as to whether they're functionally interchangeable, though.  What style is the FJ supposed to have?  Is there any reason that they shouldn't be swapped?
Everything that I found said that the metering system between the small and large round jets was identical, so a 125 small will flow the same as a 125 large, but I seem to remember reading in the past that they were different.  I'd like to actually have the carbs right this time around, so what do you guys think?

[racerman_27410]

large round..... the outside diameter of the jet is what holds the emulsion tube in place.


KOokaloo!

[TheRadBaron]

That's true in the sense that if no jet is installed, the emulsion tube isn't secured.  But there's a brass washer between the main jet and the emulsion tube that also serves to hold it in place.  Small or large round jets will both secure the emulsion tube in place just fine. 
The bike had Dynojet mains in it when I got it that were the same dimensional size as the small round Mikuni and it ran fine for the PO for a long time. 
I already have a set of 125 small rounds that I'd like to use if I can, but if there's some reason that 125 large rounds would be better I'd just assume get them instead.  Looking at the jets, the orifice design looks to be identical and assuming that the metering standard is the same I don't see a reason that they'd flow differently, but maybe someone knows something that I don't.

[racerman_27410]

lol.... then use what ya got !



KOokaloo!

[Ralph]

125's seem large. Butt what do I know

[candieandy]

i use 135 small jets, but i will find out this spring what works exactly as i will be tuning with an NGK AFX box. ill post the video after we make it.

[racerrad8]

When I got my FJ it had DJ main jets that were physically the same size as Mikuni "small round" jets.  I just got a set of new carbs from Randy, and they have "large round" main jets installed.  The threads are the same for both styles, so they can at least physically interchange.
I did some searching around on the internet, and I've seen conflicting information as to whether they're functionally interchangeable, though.  What style is the FJ supposed to have?  Is there any reason that they shouldn't be swapped?
Everything that I found said that the metering system between the small and large round jets was identical, so a 125 small will flow the same as a 125 large, but I seem to remember reading in the past that they were different.  I'd like to actually have the carbs right this time around, so what do you guys think?

The small and large jets are the same as far fuel delivery rate if they are numbered the same. The smaller jets are for a different application and/or for the air bleed application in the throat of the Mikuni Carb.

Keep in mind as long as the jets are numbered; 112.5, 115, 117.5, 120, etc they are Mikuni jets. If the are with even numbers the are a company specific jet.

Randy - RPM

[Ralph]

Yes Dear

[fj11.5]

i have posted elsewhere on here about my fj running rough around 7k and up, had been toooo rich ever since ive had her po had drilled the 112# main jets, compared to a set of 125 mains the hole was huge, , tried the 125 but still rich, so fitted a set of the 115 small size mains, these are slightly shorter than standard mikuni mains, could that cause rough running  up high in revs , as im guessing they dont sit as far into the fuel bowls, will see what the plugs say tomorrow after todays run in the tripple digits  , ,

[Grey runner]

I don't have clue about what size jets you should have but I bet Randy does, As he supplied the carbs i would be pretty darn sure that they are the right jets for the job! If after riding around or a while you think they arn't right then start "fine tuning" but for now stick with what you have.

[andyb]

large round..... the outside diameter of the jet is what holds the emulsion tube in place.

I thought the washer between the emulsion tube and the jet did that job?  My DJ jets are quite small OD, but I use the larger mikuni jets.  No problems either way.

[fj11.5]

could of just been burning of the carbon from running rich  ,, ill rip the plugs out today anyway,, are your dj jets shorter andyb, , the ones i fitted a shorter by a mm or so

[andyb]

I can't tell the difference in height after two beers!

The DJ jets measured about 9.3mm tall, the big mikunis were 9.18 or so. Not that it'd matter really, so long as there's enough thread to screw into the needle jet, you're fine.

[fj11.5]

must be big beers mate  ,   these are about 7.5 8 mm all up , just thought they may not sit low enough in the fuel, but the actual metering hole is the same depth as standard jets, just less head height

[racerrad8]

must be big beers mate  ,   these are about 7.5 8 mm all up , just thought they may not sit low enough in the fuel, but the actual metering hole is the same depth as standard jets, just less head height

You have nothing to worry about with different external height or diameter main jets. The jets are submerged into about 25mm of fuel, so there is no chance the jet can ever be "out" of the fuel supply...

unless you run out...

Randy - RPM