I posted earlier about my new-to-me 570 Street Avenger on my '64Dog's 318 Poly motor. I am still in shock because even though I thought I had the old Holley 1850 600cfm carb leaned out, mileage with the 570 is hugely better. What has changed? Metering block design? Boosters?
It's weird that the 600 came stock with 66 jets and I got it all the way down to 60s without going lean, while the 570 came stock with 54s and is really happy with 52s at my altitude of 4300 feet.
I have other Holleys that I'm building. Should I be looking for different metering blocks?

Thanks,
R.

I'll jump in on this.

First, the fuel today is a lot different than when the 600 was made.
Now there has been new idea's on fuel mixing inside of carbs.

I would guess the idle/off idle circuits are much refined on the 570.
Below 2000 rpm I would say the carb is still running on the off idle circuits so the jets don't effect it very much.
Are you using a wide band sensor o2 to tune it?

Im guessing your carb is running better because of different idle feed restrictions and possibly a different setup in the emulsion and air bleed areas. There could also be small gains from different booster shape and your overall cfm could be better suited to your engine's needs and optimized better for atomization.

These are all areas I had to learn more about to try and get my double pumper to run well off idle when the transfer slot is doing a lot of the work. I am just guessing I don't have 1st hand knowledge of the specific metering blocks that come with that carb.

To tune these circuits you need aftermarket or hp metering blocks or you can use pieces of wire or tap in lead pellets and drill them or drill and tap for home made jets. The air bleeds also affect these areas but dont be too cavalier messing with those.