I just rejetted my Holley on my 406. It's a 1850-S (600cfm, vacuum secondary). I know it's a little small, but the new 750 isn't in the budget until summer. Anyway, I changed the jets from 66s to 65s and put in a 6.5 power valve instead of the 4.5 that was in there, all this and a Holley rebuild kit (blue gaskets, of course). It seems to run a lot cleaner (and a LOT faster) than it was (it was DEAD rich). Anyway, I haven't had much experience with Holleys, I just don't want to be too lean, that's bad, does my combo seem OK? It's not pinging and it is running like 100% better.

 

The difference between 66s and 65s is very tiny. With Holley carbs, 3 sizes is a standard tuning step; 4 is a big one; 2 is a fine one; and one size - well, some 65s are almost as big as some 66s, so that doesn't do much.

You went from 0.003421 sq in of jet to .003318 sq in. That's about a 3% change. So, if your A/F ratio was (pick a number) 13 before, it's 13.3 now. Very minute difference.

The 6.5 power valve will make it run richer. That number is the amount of vacuum required to close the power enrichment circuit. That is, any vacuum below that, you have enrichment; vacuum above that value, jets only.

I'd be inclined to think that just cleaning a little crud out of it and sealing all the internal passages is what actually produced your improvement.

A larger motor will tend to want a leaner calibration in any given carb than a smaller motor will, all else being equal. That's because a larger motor draws in more air, and therefore creates a larger signal at the venturis, than a smaller motor will. The situation in the idle circuit is exactly reversed, however: the idle feeds fuel in direct proprtion to manifold vacuum, so a lrager motor with the same manifold vacuum as a smaller motor will always idle too lean with the same carb, because the carb has no idea how much air the engine is drawing in.

Go to the store and buy some 63 and 61 jets; keep leaning it down until it either surges at highway speeds or starts to stumble at light throttle tip-in, then go back to the next larger size. Then tune the secondaries for max MPH in the 1320, or over a known stretch of road that you can time.

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