This post basically because of a post in another thread. Thought it might have interest.
Different companies had different meanings for the numbers stamped on jets. The chart below is correct as far as it goes, but in probably not totally inclusive.
CARTER - used a prefix followed by a suffix (eg 120-101, 159-43, 230-11). In the preceding sentence, the 120, 159, and 230 are code numbers representing the type of jet and make up a portion of the part number. The numbers following the dash make up the rest of the part number. Thus 120-101 is a part number, but unless one consulted a part number chart (never available to the general public) OR the original service literature for the carburetor by identification number, the meaning of the part number was unknown.
In the early 1960's, ON THE 120 TYPE JETS ONLY, Carter changed the suffix to have meaning.
HOLLEY - Holley jets marked with numbers up to and equal to 66 would have been the size of the jet in thousanths of an inch. So a number 56 jet would be 0.056 inch. Above number 66 (unless your memory is better than mine), consult a chart. As an example a jet marked 70 is 0.073 inch.
ROCHESTER - MOST Rochester jets were marked by Rochester in thousanths of an inch, thus a number 56 jet would be 0.056 inch. A FEW ROCHESTER JETS WERE MARKED AS THE LAST TWO DIGITS OF THE SEVEN DIGIT PART NUMBER!!! A jet marked 07 is a common example.
STROMBERG - Stromberg marked jets with either the size in thousanths of an inch OR numbered drill size.
ZENITH - Zenith sometimes used metric sizes, and sometimes used other meanings. I am not going into the Zenith meanings here.
AFTERMARKET - Various aftermarket companies have marked jets in thousanths of an in inch, parts of a millimeter, drill bit size, part number, and other numbers which I have never been able to determine.
THE BOTTOM LINE - NEVER BELIEVE A NUMBER STAMPED ON A JET .....MEASURE IT!
Purchase a numbered drill set with numbers from 1-60 (this will cover main jets fairly well). If you are also interested in idle jets (smaller than main jets), you will also need the numbered drill set with numbers of 61-80.
You will note there are gaps by thousanths in the 1-60 set. These gaps may be partially filled in by the purchase of individual fractional metric drill bits (I have never seen a fractional metric set).
And while we still have thousands of original jets, we have run out of many sizes and have had these reproduced. The jets we reproduce are left unmarked!
And while we are considering jet sizes, NEVER BUT NEVER consider the old wives tale (actually, this is an old husbands tale, old wives are too smart for this one) about soldering a jet and redrilling the jet smaller. Solder is soft. Gasoline running through the solder would erode the solder like the Missouri River erodes levees! One pass down the drag strip, and the solder would be gone (guess where it went!!!). If the jet is too big, put it in a bag with the size marked on the bag, and replace it!
Jon.
