What does the number on the main jet mean?

Question

i know the number means the size of the hole in the main jet...but ..what is the connection between the size and the number?

say...#71..is that mean the diameter of the hole is 0.71mm??

hope someone can explain it for me ,,thanks a lot!~

Answer 1

yep

Answer 2

Every carb manufacture and aftermarket jetting company use their own numbering to size methodology.

Answer 3

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.

.........................................

"Car" carb jets and the old Fuel Injection "Pills" are usually calibrated in numerical sizes according to o*****e Diameter.

Even then,,,,the Jets are Labelled according to a Code,of sorts.

Holley Carbs for example,,,I cant recall Exactly how they determine the Number stamped on the jet,,,

But they Drill a buncha jets during a production run.

Imagine the Hole Size being .071".

Then they will Flow Test all the jets in that batch.

Some will be  LABELLED  #70,,,some #71,,,some #72.

Meaning,,,they Drill & Finish the jets as precisely as practical,,,,and they are a NOMINAL #71 Batch.

Then Beyond That,,,they are Tested and labelled according to actual Flow Rates.

....................................

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 o*****e 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.