Carburetor Tuning

Carburetor Tuning

OK, I'm gonna assume you know how to rebuild a carb, there are a million books out there to tell you how to do it if you don't. This is here just to tell you what to do with your rebuilding knowledge. Also, I'm gonna assume you know that the idles screws should be about the same number of turns out, and how to adjust your floats. It really doesn't matter if it is a carburetor, FI, or whatever, my way to tune for street/strip is such:

  1. First thing, adjust the fuel pressure! On carburetors, proceed to set the float levels too. Everything is relative to the float level / fuel pressure. If you change that, you start over. If you think it should be OK, skip this step. Remember though, if it still doesn't run right in the end, you may need to do this step.
  2. Adjust mains by vacuum gauge, pick a RPM (I usually use 2800 or 3200 in top gear, depending on gearing and where I normally cruise), and adjust the mains so you get the maximum average vacuum at that rpm. On metering rod equipped carbs it is a little more complicated, you will find that increasing the rod diameter size is the same as decreasing the jet size (and vise versa). The metering rods also have a step for full power enrichment, so you effectively have a jet size for full throttle and a jet size for cruise. Take the numbered jet size, subtract the thick part of the rod (low enough to be the part that sits in the jet, but not the very end, that is the fuel enrichment step) and use that number for tuning (Example, 100 jet, and a rod that's .050 thick, use the number 50 for your jet size). After you've figured out what the magic number for cruise is, remember it, we may need to come back to it when tuning the full throttle acceleration.
  3. Adjust idle to get maximum vacuum in gear (autos)(Note1) or in neutral (man). You will have to play with idle speed and richness settings, because they are interdependent, and when you change one, you change the other. Divide that number by two, and use that number for power valve selection (or enrichment activation). If the idle screws have no effect, or a perfect tune causes more than a 300 RPM drop in RPM when shifted into gear (note 2,4), remove the carburetor and turn over. Where are the throttle plates? There should be an idle discharge hole, and a transfer slot. If there is more than .004 showing of the transfer slot (Note 3), you need to drill a hole in each throttle bore on the transfer slot side, near the center. Start really small (1/32) and work your way up in size until the idle screws work again and you can idle properly.
  4. Adjust accelerator pump so you have the shortest duration and volume that won't cause a bog under normal acceleration (no secondaries and normal throttle action). You should not be able to tell when the pump shot ends and the mains come on line. If the car bogs then goes, you need to increase the squirter size. If it goes, bogs, then goes again, you need to either decrease squirter size or increase pump capacity, possibly both. On FI, move the dial or map, on a carb, change the squirter or pump cam.
  5. Adjust the secondary jets to give maximum acceleration or highest MPH in the 1/4 mile. Sometimes the secondaries will be leaner than the mains, esp. if a secondary PV is used. On carbs or FI without secondaries, try a high flow PV if lean. On some FI units, you can control fuel enrichment. Tune the enrichment circuit as if it were the secondary jets. Failing that, you'll have to decide if you want performance or mileage, and tune for that. This will also be true for 4160 Holleys, and other carbs with non-adjustable secondary jets, as you will have to tune using a high flow / low flow / two stage power valve and main jets. Metering rod equipped carbs can also have the primary rods changed at this point to either lean (thicker rod) or richen out the primary bores. Because the TQ rod sizes are not part of the part number, I've listed the sizes below. When changing the rods, you only want to change the effective step size for the fuel enrichment. In other words, you want a metering rod to have the same thick diameter as was called out in step 2, but change just the thin step. Sometimes, to get the correct size, you need to change the main jets along with the rods so the cruise ratio stays the same (here's where that number you needed to remember comes into play). Say you needed a 50 before, but you don't have the rods that will do it. But you do have a power step rod that will be perfect, if only the cruise rod was right. Measure the step, compensate by jet (Example : rod .060, you need a 50, use a 40 jet), and start over.
  6. Adjust the secondary accelerator pump / diaphragm spring / air door to give the quickest 60 foot time (also without bog or hesitation). Sometimes, it requires a lot of fuel to cover up the lean pump shot on the primaries. On Holley vacuum secondary carbs, the spring will also affect mileage, lighter the spring, the more performance and lower mileage you will get.
  7. Note vacuum on top end at full throttle. If carb is way too small, the vacuum will climb above the number you got in step number three. If this happens, take the highest reading you have, and add one. Replace (or add) the secondary power valve with this number, and go to step 5.
  8. Adjust choke so under startup, no black smoke pours out, but no bog occurs either.

All out drag tuning requires all jets to be equal on square bore carbs, jetted for best MPH, and the accelerator pumps tuned for fastest 60-foot times. Chokes and air horns are normally removed. Sometimes the PV is (mistakenly) removed. If the PV is removed, the car will load up in the pits, because jet sizes need to come up about 10, causing an over rich condition at part throttle.

General thoughts on the matter:

Increasing cam duration requires more jet.

A dual plane manifold can use a much bigger carb.

If you decrease the metering signal, you need to increase the jet (as you do when converting from a dual plane to a single plane manifold).

A blown power valve will have the greatest effect at idle and low RPM. You can check the carb by unbolting it from the manifold, putting the nuts back on the studs, and plopping the carb onto the studs. If you have an electric pump, turn it on. For a mechanical pump, completely remove the coil wire, and crank for about 15-30 seconds. The manifold and carb will be separated by a 1/2 inch or so, enough to see if any gas drips into the plenum when the engine is off (Don't try to start it!). If gas is present, check where it is coming from. There is a hole near the throttle plate that leads to the PV chamber. A blown PV will cause gas to drip from this hole(s). If it is dripping from the bores, the float level may be too high, or your carb has a crack.

The biggest city mileage killer is the accelerator pump.

A bigger jet requires less accelerator pump, a smaller more.

A dogleg style booster requires the most pump shot; the annular style the least.

The higher the velocity through the barrel, the better the atomization is. Better atomization means more efficiency (more MPG and Power). That's why dual planes typically give better MPG and performance up to the point where they start to choke off the upper CFM (and HP).

If you have a 600 CFM 4BBl carb on a true dual plane, the most any one cylinder can draw through the carb is 300CFM, where a single plane allows it to draw 600 (At its rated in. Hg, that is).



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    (R) = Key On Engine Running test
    (M) = Memory code

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