Zenith Main Jet: Design and Nomenclature

[Brian R Adams]

Question about Zenith Main Jets: My jets, marked "122", have an orifice at each end: The large hole at the screwdriver slot end measures nominally 122 (actually 0.119" to 0.120" in my set of 4) but hole in the other end measures only 0.047" across the set. I consider the large orifice to be the inlet from the fuel well, and the small orifice to be the outlet jet. So, why are these main jets specified as "122" rather than "47"? Wouldn't the smaller hole do the metering?

Brian

[Ron LaDow]

.47" is pretty close to 1.22mm (from memory) and they are spec'd in metric; sounds right to me.

[Brian R Adams]

.47" is pretty close to 1.22mm (from memory) and they are spec'd in metric; sounds right to me.

Oops. Been a while. Metric, huh?

Never mind.

Thanks!

[Brian R Adams]

So using standard (non-metric) pin gages, "minus" type (each gage is 0.0002" below nominal) in 0.001" increments as go/no-go gages, my "122" mains will readily accept the 0.047 but not the 0.048. The 0.047 is 1.190mm, and the 0.048 is 1.215mm. So the "122" is probably closer to a 121 or 120. Another way to look at it is to call it 0.0475" which is 1.2065mm or rounded to 1.21mm. Precision is elusive.
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[Harlan Halsey]

The flow through a jet is complex, depending on inlet, length, and outlet. The minimum diameter is not definitive. Back when the world was young, we just went by the number, bigger, more, smaller less. In the steps available. But now, after the racers have been drilling them out, or soldering them up, and with current QC, it may not be so easy.
I recall that Bruce Smith published an article on measuring jet flow in gasoline with a simple rig, not too long ago-probably in the Registry. If you really want to know what is what, I'd check out that article.

[Alan Hall]

Harlan is correct. The flow is dependent on size and inlet shape and surface finish etc. I recall that the carb companies checked each jet on a flow rig to determine sizing, which is why you can't just use size measurements directly. However, measuring the size does let you know if the jet has been drilled out by someone in the past.