I have a Holley 4778-2 and would like to start out with the stock jetting of 66/76. I noticed that the 4778-3 & 4778-4 have 69/78 jets. Does anyone know what the difference is or what the #s -1, -2, -3 mean?

 

1st design, 2nd design, etc. Some carbs (like the 3310, which has been around since before they invented dirt) have even more design changes than that. I think that one is up to about -7 or -8 now.

You make jet changes based on what your particular motor wants. There's no "magic" correct jet combo.

Experiment with your carb on your motor until you find what works best. Don't bother with what came in different design iterations, or what your friend says worked in his (hers), or any of that.

 

I wasn't really looking for a "magic" number just a good place to start the tuning from. Thanks for the info!