Holley Carb Baselining/Tuning

It's been a while since I've been asked a carburetor-related question, but this one came up this week so I figured I'd do a little writeup. This is how I baseline a Holley 4150 Double Pumper carburetor. I'm not going to go into great detail, there are many sites and books do that, but by following these steps, you should get the car running fairly well with a double pumper Holley and then you can take it from there.

Before you get started, I recommend that you buy a new set of spark plugs (or two) a Holley Jet Assortment kit, a set of reusable bowl gaskets (so you don't have to replace gaskets every time you change jets) and once it's baselined, an accelerator pump cam assortment and a few power valves in various sizes will come in handy! That stuff will set you back about $100, but once your carb is dialed in, replace the two sets of jets you used, along with the pump cam and power valve, and you can resell the rest as complete kits!


Step 1: Set/check the fuel pressure. Make sure you have 5-6 psi of pressure or you will force the needle/seats open and it will dribble fuel into the air horn from the bowl vents. This will foul plugs and the car won't idle.

Step 2: Set the floats. Turn your fuel pump on, or crank the engine over enough to fill the bowls with fuel. (I don't like to let the engine start in case the floats are so far off the carb won't work correctly.) Remove the sight plug (the round plug on the side of each bowl up towards the top) and see if fuel pours out. If so, the fuel level is set too high. If no fuel comes out, bump the car with your hip slightly (just a little bump) and see if any sloshes out of the hole. If not, it is too low and needs to be raised. Ideally you want the fuel level to be right at the base of that hole, so a tiny bit will seep out (not pour) when you remove the plug. The floats are adjusted by loosening the large screw on the top of the front and rear bowls. (Just back the screw off enough to allow you to turn the nut.) You raise or lower the float level by turning the nut up or down. Lock the screw back down when you are done. If you have an electric pump, you can do this with the pump running and engine off, but on a mechanical pump, the car has to be running to do it that way, so with a big lumpy cam, it can be hard to tell when it's right on due to vibration. In that case, I think it's easier to crank it a few times to pump the fuel and then check it, and repeat until it's right.

Step 3: Set the timing. If you're just replacing the carb and it was previously running well, leave it alone. If not, start the engine baseline it to the recommended base timing for your engine.

Step 4: Install new spark plugs. This makes it much easier to "read" them and tune your carb. Do this after you get the fuel pressure, floats and timing right to keep from fouling them.

Step 5: Set the idle mixture. There are two screws (one on each side) towards the bottom of the front metering block. Turn each one in until the screw is seated, then turn each one out 1.5-2 complete turns. Hook a vacuum gauge to the intake manifold and start the car. One at a time, slowly turn the screws in until the vacuum starts to drop off, and then back the screw out a quarter turn and it should be good.

Step 6: Jet the primaries. Other how-tos have much more scientific ways to do this, but this method will get you really close. Hold the throttle part-way open for about 15 seconds (2500 or so RPMs.) You want it running fast enough that it is no longer running on just the idle circuit, but not hard enough to open the secondaries/power valve. Immediately shut the engine off and remove a spark plug. If the insulation (white part) around the electrode on the plug is white (like it came when new) the mixture is too lean. If it's black and sooty (or wet with fuel in extreme cases) it's too rich. If it is lean, go up two jet sizes and try again. You're looking for a nice gray color. If it's rich, go down two at a time until you get a nice color. (You go up or down two sizes at a time to make the changes more noticeable and to save time, if you overshoot it, just go up or down one size then and you're done.) Here's a picture of what the plugs should look like. The 2nd picture down on the left is a rich plug, and lean isn't pictured there, but it'll be white like a new plug.

Step 7: Jet the secondaries. This one is a little trickier because you can't do it in your driveway but can be approached the same way as the primaries. A good starting point is 5-6 jet sizes bigger than the primaries, but each combo is very different, so actually test it, that's just a baseline! (I've seen some the same size as the primaries, and some 12-14 sizes bigger!) Ideally, you should put the car on a chassis dyno and jet the secondaries based on wide-open-throttle (WOT) horsepower and wideband air/fuel ratios. If that's not an option, the second best way is to take the car to the dragstrip and make a bunch of test passes and look at max MPH. If that's not an option, as I mentioned before, you can approach it the same way as the primaries. Take the car to a deserted stretch of road and make a WOT pull in 3rd gear. As fast as is safely possible, pull over, shut the engine off and read the plugs. (Shut it off as fast as possible to get a good reading, so the plugs don't start showing the idle and cruise mixtures again.) I shouldn't need to say this, but DO NOT shut it off if you have power steering and brakes until you have pulled over and stopped since you will lose them if you turn it off, and DO NOT turn the key to the "off" position, because that will lock the steering wheel, just turn it to "acc" (one click back from the "run" position) to kill the engine. Read the plugs and adjust the secondary jets in the same manner as the primaries. This is much less precise, since at WOT the power valve and accelerator pump(s) will be squirting fuel too, but again, the point of this writeup is just to get you close, and this method will do that.

Step 8: Set your final idle speed. Once you have the rest of your carb baselined, you can set the final idle speed. I set the idle as low as it will smoothly idle when the engine is warm and not stumble and stall when you let off of the throttle. (If you give it some gas and it wants to die when you let off, raise the speed slightly.) It will take some experimenting to get perfect and will vary with your combo. You may have to set the speed roughly immediately after you get the car started for the first time if it's way too high or low, but you should revisit it after you have the rest of the carb ironed out. The idle is simply adjusted by turning the large screw that makes contact with the throttle linkage at the base of the carb. You can see how it works by looking at it. To speed it up, turn it in the direction that opens the throttle. To slow it down, turn it the other way.

That should do it. Your idle, floats, primaries and secondaries will be pretty close. (Close enough that if it's a new combo, you can go chase the other gremlins down and then come back to the carb for fine tuning later.) The car should be drivable as is. I used this method to baseline the 650DP on my old 83 mustang. A few weeks later, I had the opportunity to put it on the rollers with a wideband 02 sensor (IM240 smog checker, not a dyno, but hey, it worked!) I gained a little by fine tuning idle mixture and one jet size on the secondaries, but I was pretty close! (With the timing optimized and the carb tuned on the wideband, it passed the IM240 smog test with flying colors! Not bad for a dart-headed 306 with a Comp Cams 270h cam, offroad exhaust and Holley double pumper!)

Here's a great write-up that goes into a lot more detail than I have. It's a lot to digest, but that article will help you get it totally dialed in. I just wanted to give you the quick and dirty, shadetree version of baselining a double pumper... I hope it helps!