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"You can never be too rich or too lean."
-- generally wise advice

Tuning Holley carbs is fun. Depending on how rich or lean you are (that is, the carburetor) you'll probably want to get a few jet sizes bigger and smaller (a pair of each) and test each one. Make changes one step at a time. Depending on how much you pay for the pairs of jets, it may make more sense to get the whole kit -- you get #64-99 jets (a pair of each) for like $30-40. You'll also need a new set of bowl and block gaskets (maybe get the neoprene ones). You might get away without a block gasket, but it's better to have one around and not need it than the other way around. Spray gasket remover will make the job a lot easier. Have a can of carb cleaner handy. You may also need float bowl screw gaskets, I like the Moroso plastic ones if you're going to open the carb up a lot.

To do this you'll need a 5/16, 3/8 and 5/8 wrench (preferably box), a large flat-blade screwdriver (around 1/4" tip), optionally a stubby flatblade the same size, a small flatblade screwdriver, a small can to catch some fuel (tuna cans work well), and about an hour if you've never done it before and have all the tools and parts.

Take off your air cleaner assembly.

Disconnect and plug your fuel line.

You'll see four screws on the front of the float bowl. Most newer Holleys have 5/16 heads on them, some older ones use flatblade heads. Unscrew one of the lower screws, when you loosen it some fuel will drain. Use your catch can to get most of it. If one side of your manifold is lower than the other, do that side first. When the fuel's drained, remove the other three screws. Be careful not to drop the screw gaskets, if you're going to re-use them. Using a screwdriver, carefully pry the float bowl from the metering block. Be very careful not to gouge the float bowl or metering block! If you work at it carefully, it'll come off.

Note that if you're working on a 4bbl Holley with a side-hung bowl, there's a silver tube with an O-ring that connects to the float bowl. Don't lose it. Now that you have the bowl off, you'll see the two jets. Using your large stubby flatblade, unscrew them. The screwdriver should span across both sides of the jet, and catch both sides of the slot.

Screw in your new jets.

If there's any gasket material on the metering block, carefully scrape it off. Plastic works well, metal tends to scratch up the block. I use a case separation tool for a Compaq LTE computer, but you may not have one of those. Spray the gasket remover around the edge of the float bowl to soften the adhesive. Let it sit a few minutes, then the gasket will come off easily. Spray it down with carb cleaner to remove the residue.

Place the new bowl gasket on the metering block. There are pins around the outside edge to hold it in place.

Hold the float bowl upside-down. The top edge of the float should be even with the top (now bottom) of the bowl. If it's not, loosen (don't remove) the large flatblade screw and turn the nut to adjust it until it is. Then tighten the screw.

Now, replace the float bowl on the carb. If you've got a 4bbl, put that O-ring back on the silver tube, put some lubricant on it, and line it up with the hole. I've heard of some people using oil or Vaseline on the O-ring, but I've been doing it long enough at the side of the road that I've done it with gasoline. Watch that the O-ring doesn't lump up over the edge of the stop. Hold the accelerator pump linkage in place, and slide the bowl on. Install all four screws, then tighten them evenly, NOT TOO TIGHT! If you overtighten them you'll warp your carburetor.

Check the accelerator pump arm. With the throttle closed (choke off) you should have no play in the arm. Then, hold the throttle fully open, and make sure there's at least .005" before the pump bottoms out. If you need to adjust it, use your 3/8" wrench and screwdriver to adjust the screw on the spring. Check both adjustments again when you're done.

Reattach your fuel line and tighten the clamp.

If you have an electric fuel pump, turn it on and check the carb carefully for leaks.

Make sure you didn't knock off any vacuum hoses.

Start the car. Check again for leaks.

Remove the brass sight plug from the side of the float bowl, being careful not to drop the gasket. The fuel should be right at the bottom of the hole, maybe dribbling out a little. If it's not, with your 5/8" wrench and a large flatblade, loosen the large screw on the top, turn both the screwdriver and wrench as a unit, then tighten the screw. Clockwise will lower the level, counterclockwise will raise the level. Note that if you lower it, you'll have to rev the engine to use the fuel that's already in there. When you get it right, put the sight plug and gasket back in.

Check for leaks.

With the car warmed up, adjust the idle mixture using the two screws on the side of the metering block.

Reinstall the air cleaner.

Take a test drive. Drive around and listen carefully for pinging at all throttle positions before you beat on it. Pinging means you're probably way too lean (assuming it didn't before and nothing else changed). Also check to see if you've got a haze of black smoke coming out of the exhaust, that means you're too rich. Once you're good at it, you'll probably be able to tell by the exhaust smell if you're way off.

Read your plugs, to see if you picked the right jets.