Carb: Main Jet Number?

[Marty Levenson]

Dear Don,

While I have my carb off, I am wondering about this bit of information from Robert Hess's site:

"Carburetors with fixed high speed main jets should have the main jet removed and checked to see what jet number it is. The jet number is stamped in very tiny numbers on the face of the jet. If it is not a #21 main jet, it should be replaced with one."

Thanks yet again,
Marty (Vancouver)

[Don Moyer]

Marty,

Zenith uses index numbers to identify all their jets and nozzles with respect to orifice sizes. An index number of 21 relates to an orifice size of .041".

In the 35 (or so) years that Zenith has been producing carburetors for the late model Atomic 4, I don't believe that they ever changed the specification of the fixed main jet. They have however, changed the specifications of their well vent jets and main discharge nozzles several times.

The current index number of late model well vent jets is 21 (coincidently the same as the fixed main jet), and the main discharge nozzle has an index number of 55. If you have a copy of our Newsletter Compendium, you can find a table of all the current jets and nozzles of both early and late model carburetors on page 136.

I'm not sure what Bob's concern was in mentioning the index number of late model fixed main jets. He may have been worried that someone could inadvertently install an early model main jet (with an index number of 23) in a late model carburetor. Such an oversight would make a late model engine run very rich, since early model main jets are designed to be used with a needle valve to adjust the mixture.

Best regards,

Don Moyer

[Marty Levenson]

Thanks, Don...that's good to know.