Mikuni Jet Sizing

[racerrad8]

There was a recent post regarding the numbering system for Mikuni jet sizing being in reference to the number indicated as a reference to the hole size. I.E: 1.125mm holes = 112.5 jet size

Well, unfortunately I cannot see how that works. I have two different versions of the genuine Mikuni Carb Fuel Pilot jets in my inventory. Both are indicated as size "40", but one has a significantly larger hole to control the fuel flow.

Fuel Pilot Jet 4HR-14142-40 is the "40" jet we use in the FJ carbs. This jet orifice size is for lack of a better word microscopic.
Fuel Pilot Jet 193-14242-40 is the pilot jet I purchased from Yamaha thinking it would be the same orifice size, but it is much bigger.

Much to my disappointment, the orifice size on the "193" is at least two times if not three time larger, even though it is clearly stamped on the body as "40". The 4HR jet is also marked "40" and has the small orifice.

When I got the 193 jets in stock, I looked them over and noted the larger hole by visual comparing them in the light. We then tried them in a set of carbs and the bike will not even run as is floods out. I even took out the air bleed to see if it would run with a significant amount more of the air bleed feed.

My number drill bit only go down to #60 which is .040" and it is too large to fit in either jet.

It is very difficult to get good photos, but the "193 (40)" jet is on the left and the "4HR (40)" jet is on the right as I hold them in my fingers.

I was told a long time ago that Mikuni rates there jets by CC flow rates. That too doesn't work out since both of these jets are marked 40 and they have significantly different orifice sizes.

I also have this same issue with the 42.5 jets using the same prefix above...I have 1500 of the "193" jets that I cannot use in the FJ carbs because the orifice is too large even though marked the same as the jets we normally use.

David, I will gladly send you both versions of the 40 jets so you can possibly measure them if you have higher number drills than I do.

Randy - RPM

[FJ_Hooligan]

Randy,
My number drill set stops at #60 also.

[T Legg]

I have a set of orifice drill bits for gas orifices that ranges from size 40 to size 80 made by Anderson & Forrester.I'd be happy to measure them for you,or you can usually buy a set at a heating or refrigeration wholesale supply house like Johnstone Supply or refrigeration supply distributors.

[racerrad8]

For reference from the other post regarding Mikuni jet sizing.

Randy, do the 155 pilot air jets actually measure 1.55mm? (0.060")

No, the 155 fits a #51 (.067") pretty nicely, but a #50 (.070") will not fit.

I pulled a 160 jet and it too fits the #51 which is sloppy and the #50 will not fit.

The #53 (.059") drill bit which is as close to .060" I have is really sloppy in both the 155 & 160 jet.

Randy - RPM

[racerrad8]

I have a set of orifice drill bits for gas orifices that ranges from size 40 to size 80 made by Anderson & Forrester.I'd be happy to measure them for you,or you can usually buy a set at a heating or refrigeration wholesale supply house like Johnstone Supply or refrigeration supply distributors.

If you are willing, I might send some jets over for you to measure.

Let me know, Randy - RPM

[56 CHEVY]

If that doesn't work Randy, we have gage pins all the way down to .011 in.

[Pat Conlon]

For reference from the other post regarding Mikuni jet sizing.

Randy, do the 155 pilot air jets actually measure 1.55mm? (0.060")

No, the 155 fits a #51 (.067") pretty nicely, but a #50 (.070") will not fit.

I pulled a 160 jet and it too fits the #51 which is sloppy and the #50 will not fit.

The #53 (.059") drill bit which is as close to .060" I have is really sloppy in both the 155 & 160 jet.

Randy - RPM

Sounds like the 155 pilot air jet is really a 170 (1.70mm=0.067")

[T Legg]

I have a set of orifice drill bits for gas orifices that ranges from size 40 to size 80 made by Anderson & Forrester.I'd be happy to measure them for you,or you can usually buy a set at a heating or refrigeration wholesale supply house like Johnstone Supply or refrigeration supply distributors.

If you are willing, I might send some jets over for you to measure.

Let me know, Randy - RPM

i have an account with rpm under Travis Legg .Just send them to me at 1990 Temple Hill road Reno Nevada 89521 and i'll be happy to measure them.

[Pat Conlon]

I found this, here:
http://www.historicmotorcycle.org.au/Adler/carby/jets.txt

CARBY JET SIZES
  ---------------

Everybody uses different methodology to establish their number for their given type of jet.

Mikuni for example, use a few different type of main jets in the various carbs commonly found on motorcycle engines.
Each of their different jets are tested and calibrated in different ways.

So for example, the Mikuni Long Hex, Short Hex, and Round-SlottedHead Jets in #71 size are 3 different Calibrations.
Besides the Physical differences, the Flow Rates are different.


Anyway, by whatever method that's used,
Jet Numbers are representative of a VOLUME of fuel.

CC's or Grams, usually per minute

So #71 Mikuni jet would flow about 71cc's of Gasoline per minute.
_________________________________________________________________________________________________________________

Mikuni, Dynajet, and Kehin all have different drill sizes for the same jet number:



drill             Dell'Orto #        drill    Adler
inches --- Kehein # -- DynoJets #  -- Mikuni # --  mm  --- Bing #

0.035 ----- 92.5 ------- 92 ---------- 86.3 ----- 0.92
0.036 ----- 95 --------- 94 ---------- 88.1 ----- 0.94
0.037 ----- 97.5-------- 96 ---------- 90.0 ----- 0.96
0.038 ---- 100 --------- 98 ---------- 91.9 ----- 0.98
0.039 ---- 102.5------- 100 ---------- 93.8 ----- 1.00 ---- 110
0.040 ---- 105 -------- 102 ---------- 95.6 ----- 1.02
0.041 ---- 107.5------- 104 ---------- 97.5 ----- 1.04 ---- 115
0.042 ---- 110 -------- 106 ---------- 99.4 ----- 1.06
0.043 ---- 112.5------- 108 --------- 101.3 ----- 1.08
0.044 ---- 115 -------- 110 --------- 103.1 ----- 1.10
0.045 ---- 117.5------- 112 --------- 105.0 ----- 1.12 ---- 120
0.046 ---- 120 -------- 114 --------- 106.9 ----- 1.14
0.047 ---- 122.5------- 116 --------- 108.8 ----- 1.16
0.048 ---- 125 -------- 118 --------- 110.6 ----- 1.18
0.049 ---- 127.5------- 120 --------- 112.5 ----- 1.20
0.050 ---- 130 -------- 122 --------- 114.4 ----- 1.22
0.051 ---- 132.5------- 124 --------- 116.3 ----- 1.24
0.052 ---- 135 -------- 126 --------- 118.1 ----- 1.26
0.053 ---- 137.5------- 128 --------- 120.0 ----- 1.28
0.054 ---- 140 -------- 130 --------- 121.9 ----- 1.30
0.055 ---- 142.5------- 132 --------- 123.8 ----- 1.32
0.056 ---- 145 -------- 134 --------- 125.6 ----- 1.34
0.057 ---- 147.5------- 136 --------- 127.5 ----- 1.36
0.058 ---- 150 -------- 138 --------- 129.4 ----- 1.38
0.059 ---- 152.5------- 140 --------- 131.3 ----- 1.40
0.060 ---- 155 -------- 142 --------- 133.1 ----- 1.42
0.061 ---- 157.5------- 144 --------- 135.0 ----- 1.44
0.062 ---- 160 -------- 146 --------- 136.9 ----- 1.46
0.063 ---- 162.5------- 148 --------- 138.8 ----- 1.48
0.064 ---- 165 -------- 150 --------- 140.6 ----- 1.50
0.065 ---- 167.5------- 152 --------- 142.5 ----- 1.52
0.066 ---- 170 -------- 154 --------- 144.4 ----- 1.54
0.067 ---- 172.5------- 156 --------- 146.3 ----- 1.56
0.068 ---- 175 -------- 158 --------- 148.1 ----- 1.58
0.069 ---- 177.5------- 160 --------- 150.0 ----- 1.60
0.070 ---- 180 -------- 162 --------- 151.9 ----- 1.62
0.071 ---- 182.5------- 164 --------- 153.8 ----- 1.64
0.072 ---- 185 -------- 166 --------- 155.6 ----- 1.66
0.073 ---- 187.5------- 168 --------- 157.5 ----- 1.68
0.074 ---- 190 -------- 170 --------- 159.4 ----- 1.70
0.075 ---- 192.5------- 172 --------- 161.3 ----- 1.72
0.076 ---- 195 -------- 174 --------- 163.1 ----- 1.74
0.077 ---- 197.5------- 176 --------- 165.0 ----- 1.76
0.078 ---- 200 -------- 178 --------- 166.9 ----- 1.78
0.079 ---- 202.5------- 180 --------- 168.8 ----- 1.80
0.080 ---- 205 -------- 182 --------- 170.6 ----- 1.82
0.081 ---- 207.5------- 184 --------- 172.5 ----- 1.84
0.082 ---- 210 -------- 186 --------- 174.4 ----- 1.86
0.083 ---- 212.5------- 188 --------- 176.3 ----- 1.88
0.084 ---- 215 -------- 190 --------- 178.1 ----- 1.90
0.085 ---- 217.5------- 192 --------- 180.0 ----- 1.92
0.086 ---- 220 -------- 194 --------- 181.9 ----- 1.94
0.087 ---- 222.5------- 196 --------- 183.7 ----- 1.96
0.088 ---- 225 -------- 198 --------- 185.6 ----- 1.98
0.089 ---- 227.5------- 200 --------- 187.5 ----- 2.00

DynoJet Brand of Re-Jet Kits is very popular among motorcycles tuners.
They use Hole Size in Millimeters to label their jets
So a Dyno Jet Brand of jet for a Mikuni or Keihin Bike Carb ,,labelled as a #71 Jet,,,,
is actually 0.71 MM orifice diameter

I guess Bottom Line,,,among all the variety of jets,,,there's No Rhyme or Reason,,,or Standard.

They ALL are ultimately based on Fuel Flow Rate.
Because THAT is what is most convenient and accurate to MEASURE.

It is always most practical to number each graduation of Flow Rate sequentially.

But each Mfgr has their Own Scheme for translating Tested Flow Rate into a "Jet Number".

"A Rose by any other name....",,,
However they do it,,whatever they Call it,,,,it's all a Measurement & Ranking of Fuel Flow rate.

[racerrad8]

That chart doesn't seem to coincide with Mikuni jet sizes, they do not make 121.9 or 123.8 sizes...

Here is a jet chart that I found years ago and refer to when trying to decipher other than Mikuni jet sizes.

http://jetsrus.com/FAQs/FAQ_mikuni_vs_dynojet_vs_keihin_sizes.htm

But as mentioned in the article linked, CC flow rate seems to be the sizing factor not the drill size.

Randy - RPM

[FJ_Hooligan]

In the data that Pat posted, the second column looks like Mikuni numbering to me.

I think I have a mixture of Mikuni and DJ jets.  I'll try to do some measuring on my own.

[Pat Conlon]

.....I have two different versions of the genuine Mikuni Carb Fuel Pilot jets in my inventory. Both are indicated as size "40", but one has a significantly larger hole to control the fuel flow.

Fuel Pilot Jet 4HR-14142-40 is the "40" jet we use in the FJ carbs. This jet orifice size is for lack of a better word microscopic.
Fuel Pilot Jet 193-14242-40 is the pilot jet I purchased from Yamaha thinking it would be the same orifice size, but it is much bigger.

Much to my disappointment, the orifice size on the "193" is at least two times if not three time larger, even though it is clearly stamped on the body as "40". The 4HR jet is also marked "40" and has the small orifice.....

Randy, I found this image showing the profile on the different Mikuni pilot jets. Does this appear to be what's going on?

[FJ1100mjk]

(170mm=0.067")

[FJ_Hooligan]

Love the meme Marty!

Pat, you slipped a few decimal points.  1.7mm = 0.0669"

[Pat Conlon]

Yep, those pesky decimal points change everything.... Made the correction, thanks!