OldIHC

Stripping down my 1904 I found it fitted with a new #79 main jet (previous owner rebuilt the carb). The original spec called for a #68 I believe, not sure why the change.
What are typical main jet sizes? Were these on the lean side from the factory?

Try the Google for Mikes Carb. Lots of info on the 1904, including jeting.

The Holley 1904 came in a myriad of different versions.

Typically, the carburetor is first sized to the manifold (physical sizing); then the venturi is sized to the air flow requirements of the engine.

Once the above is done, both the pressed-in air jets, and the removable fuel jets are sized to the fuel curve of the engine at a given altitude.

It is quite possible for two carbs with the same venturii and fuel jets to have totally different fuel curves depending on the values of the air jets.

To determine the jetting for any specific carburetor, one just needs to know the identification number of the carburetor. Holleys will have a "List" number, where the word "LIST" is stamped on the casting (generally on the bowl side of the fuel inlet boss for the type 1904), followed by the actual list number. Generally, the O.E. number will also be stamped in the same general area.

In the case of the 1904, required tools consist of: a strong light, a strong magnifying glass, and a pair of "young eyes".

Will the engine run without all of this trouble? Probably even if the fuel jets are off a few thousands, but never as well as designed by the factory engineers.

Post the "List" number, and someone will have the jetting information.

And NEVER believe what is stamped on any jet, measure it. Enthusiasts have been drilling jets since just slightly later than Noah landed the Arc!

Also remember that with Holley, the stamped number on the jet would represent the actual jet size in thousands, up until a certain point when it doesn't! Since I do few Holleys, I never remember the exact point without looking, but is in the 65/66/67 area.

Jon.

Here is probably the most helpful page on the web, as well as a great source for kits and info, if you're dealing with the Holley carbs and their applications over the years. Should be the "go to" shop for the old IH's. http://www.thecarburetorshop.com/TruckkitsI.htm#TkitIHC