Can anyone explain why Olds would use a smaller main jet on the 442s, including W30s than on the regular 455s in 71? The 442s on carburetor 7041257 have .069 jets and the 7041251 carb on the SX and 88's have .070 jets. All use 49B rods. The secondary metering rods are richer on the 442s though. The part numbers are different between the two carbs for the air horn, float bowl and throttle section. Could there be differences in those parts that counteract the leaner jet mixture in the 442 carb?
It just seems to go against my common motoring belief that a higher performance engine would require a richer mixture. Thoughts?

 

While I don't have a specific answer, keep in mind that 1971 saw the drop in CR, milder cam, and other changes to allow the use of lower-octane low-lead gasoline. Emissions standards were also being imposed, which required careful metering selection. TCS disabled spark advance in the lower gears.

Also, you need to consider how the ENTIRE carb meters fuel, not just the jet size. For example, if the 442 carb uses a stiffer power piston spring, the primary rods will be higher in the jet sooner, resulting in a richer mixture. Also, check the air valve spring wrap adjustment on the 442 carbs also. If the air valves open faster, the secondary side will be metering sooner, which will pretty much swamp anything happening on the primary side.

 

I was thinking, as you stated, there may be other differences in the carb to counter the smaller jets. Thanks for your explanations Joe, all makes sense. Yes, it was that time when emissions had to be factored into the equation and not in a good way.