In tuning my carburetor, I have found that for maximum RPM and vacuum, my mixture screws are only turned out about 1 full turn from all the way in to lean. Does that mean anything? Should I go with a smaller size jet? Would difference would I see economy wise or performance wise if I went with a smaller jet, and say, 2-1/2 full turns our from lean?

Also, for a 4-barrel carburetor, what would be the typical difference in jet sizes between the primary and secondaries? For example, if I am running a size 50 jet in the primaries, how much bigger would the secondary jet be from the primary jet?

 

sound s like u know how to tune ur idle mix . changing jets, well each engine is a littil diff and will run at a wide range of air fuel ratios . for best tuning of ur setup ? well this is what i did . got an air fuel gauge and tuned for what the gauge said and drivablity for the way i drive i have an edel 1406 600 stock jets where .098 prim .095 secon with a .075x.047 metering rod. now its .092 prim with a .068x.052 rod same .095 sec and it is still a littil rich but it runs good

 

When I set up a Holley I start by establishing the proper primary jetting first, then once primaries are set, I typically jet the secondaries 6-numbers higher than the primaries, which is a good baseline to start with.

I'm with wackodog, an A/F meter is a beautiful tool to help you tune but if you're like me and can't justify dropping a hundred bucks into a gauge, set the primaries where you want them and putz around town for an hour then pull a spark plug and see how it looks - tan is ideal - a perfectly clean plug is too lean, and a black sooty plug is too rich. Secondaries require you to jump on the highway, flat-foot it, slow down, and repeat multiple times, then read a plug.

Also what carb and size engine are you running?

 

Main metering jets have no function at idle if the carb is right.

Relationship of primary to secondary jet size varies with type, brand, size of carburetor and application in question. A 4 bbl Holley might have same size front and rear bores and same size jets frt and rear or may require larger rear jets .... depending on whether or not it's equipped with a secondary power valve or not or if it has vac secondaries or linked secondaries and whether or not it's a single pumper or double pumper.

Your truck and mine might have the same engine, same cam grind, same exhaust, same heads, same trans, same gearing, same tires, same carburetor and mine may still require several steps leaner or richer jetting at either the primary or seconmdary or both sides of the carb.

Unless you have a good handle on how to read a motors needs and lean versus rich and carburetor operation at idle, part throttle cruise, and wide open acceleration and top end .....
..... you should stay with the out of box jetting on a carb. You can gain a good start on understanding and applying jet changes by reading the instructions provided with the carb and theree are many books out there. Mistakes are often costly in burnt valves or fouled plugs and poor mileage maybe, but they also provide opportunities for learning.