Carburetors may be simple, old-school, and dirty, but they're also cool as hell. BangShift forum member John Cerone walks us through the process of overhauling one of Detroit's most iconic fuel-squirters — the Holley double-pumper. Long live cheap speed! —Ed.
The BangShift forums are ripe with good tech info. This play-by-play is a nice introduction to carburetor overhaul, and it demystifies what most people see as a intimidating piece of equipment. All things considered, it's amazing that a machine so elementary is capable of so much.
Freshening a carburetor isn't that scary — anyone with a little mechanical aptitude, a set of basic hand tools, and an off-the-shelf gasket/jet kit can rebuild a carb on the cheap. There are certainly countless tricks and tweaks to improve performance, but this is a solid take on a basic overhaul.
(If you'd like to ditch our gallery viewer, the complete process can be viewed on one monster of a page here.)
Here you can see what makes a double-pumper a double-pumper — two accelerator pumps, one primary, one secondary. The pumps are underneath the circular covers on each corner of the carb. These are both standard-volume pumps — larger pumps are available for those who need them.
Looking down the venturis: You'll notice that this carb currently has no choke installed, and that it has the standard booster venturi's and air bleeds. Some higher-end carbs have downleg boosters, removable air bleeds, and such. This one is pretty standard-issue.
Here's the first disassembly task: Remove the four float-bowl retaining bolts, and with a firm whack from the handle of a screwdriver against the needle/seat fasteners and the bowl, the gasket will let loose. This reveals the primary jets and power valve in the metering block, and the center-hung brass float and needle/seat assembly.
Here you can see some of the nasties left over from leaving who knows what kind of fuel in this carb for who knows how long. This stuff has undoubtedly settled elsewhere in the carburetor, and I can only hope that wherever I can't see will be taken care of by a soak in a bucket of carb cleaner.
More shots of the nastiness — this shot also shows the bowl vent extensions (the white plastic things) that came on this carb.
After removing both bowls, I removed the throttle-plate assembly from the carb's main body. This allows access to the bottom of the metering blocks to remove them. Old gaskets really like to stick these things hard to the main body...
This shot shows the "power-valve saver" check ball between the two primary throttle blades. This was standard on this carb — on most older Holleys, (not sure what model year they made the change) you have to drill and install this check valve yourself to keep your power valve from being blown out if you get a back fire.
Here is an example of how I try to keep the parts organized while I'm taking the carb apart. Even if you're doing this on your picnic table like I was, keeping the parts organized and labeled, along with taking pictures during the tear-down, can really save you some time and frustration during reassembly.
Notice the small silver "needles" in the shot at left — these must go in the main body under the accelerator-pump squirters (shown above them on the cardboard) which are held in with phillips-head screws. There is a gasket between the squirter and main, and screw and squirter.
This shot shows both bowls, needle and seat assemblies, and standard inlet fittings for flare nuts. 5/8" nuts adjust the float height; slotted screws lock down the adjustment, like the squirters. There is a gasket under both screw and nut.
This is an accelerator-pump assembly removed from the bottom of the bowl. The little orange rubber thing is an umbrella check-valve seal. During assembly, these seals need to be pulled through the bowl bottom carefully with pliers. Don't forget the spring, and don't mix springs between pump sizes if they are different. (No worries here — pri and sec pumps are the same.)
This shot is my somewhat unconventional method of removing stubborn metering blocks. I have carefully clamped the main body upside down in a vice. Using the extra portion of casting that protrudes from the bottom of the metering block, I give it a couple taps with hammer. The metering block gasket then gives up and I can separate the blocks from the main body.
(Note: If you're going to do it this way, be very careful, as you can damage or chip the carb's body or metering block. It's also not the best way to take care of a screwdriver. We usually use a wooden drift and/or a rubber mallet for this sort of thing. —Ed.)
Below you see both metering blocks removed from the main body. The secondary block on this carb does not have a power valve. This carb has the most common factory installed power valve, marked with the stamp "6 5". This means it opens for enrichment when the manifold vacuum drops to 6.5 inches of mercury. For most carbs, this power valve works well, but if you have a really nasty cam with very little idle vacuum, you may need to change to a lower numbered valve to keep it from opening at idle.
Close-up view of the power valve. It's hard to see the faint stamping, but a 6 and a 5 are in there along with a couple other characters.
Here is the power valve removed from the metering block. This requires a 1" wrench.
This is a close-up of the pin that holds in the plastic vent tubes. Push those up with a screwdriver, then pull the pin out the top with pliers, then wiggle and remove the tubes. All plastic parts must come out of a carb before putting it in the chemical dip cleaner.
Here's the vent and pin removed.
Here's the gallon dip tank I have — it was originally purchased to clean the three one-barrels on my 90-hp Merc outboard. With a little creative packing, I've got both metering blocks, both accelerator pump housings, and both bowls in the can together. The main body and throttle plate won't fit in there, so cleaning them with carb cleaner spray and blowing everything out with compressed air will have to suffice.
Don't sweat it if you can't get all the gasket material off the castings before putting them in the carb dip. They usually come off easily after soaking for a while. Cleaning the gasket surfaces is tedious; I generally use a razor blade and a pick set. You have to be careful not to damage the sealing area, as these carbs are aluminum and pretty soft.
I like to go through the build process one section at a time, cleaning and blowing out each part right before assembly. I started with the metering blocks; in this case the primary has a power valve while the secondary does not. Here's a shot of the new power valve (a 65, which is stock in most Holleys) and gasket.
Gasket in place. Make sure it's centered.
I put the power valve and gasket in with the metering block upside-down to make sure the gasket stays centered.
Next, tighten it with a 1" wrench or the crescent hammer shown here. Remember that you're going into aluminum, and that it doesn't have to be 1,000,000 ft-lbs.
Moving along with the primary metering block…
…I then installed the gaskets for the idle-mixture screws.
Then install the mixture screws. Use a little oil or silicone spray and a little firm pressure, as you're threading the screws into the cork first. Once they thread into the metering block, lightly bottom out the screws, then loosen them 1.5 turns for a base idle mixture setting. Use the same setting on both screws.
Then, with the right size screwdriver, install the primary jets. (Oversize or undersize drivers can damage the jets' soft brass.) As you can see, I also installed the bowl vent — this metering block is ready to install. The secondary metering block goes together the same way as the primary.
The throttle or baseplate assembly basically just needs a new gasket before putting the main body on, but there are a few things to check first. Holding the plate up to a light lets you see how far open the primary and secondary throttle blades are in their bores. Ideally, you want everything even. it's tough to see exactly in the picture, but the secondaries on this carb looked like they were open too far.
With a small straight-blade screwdriver, you can adjust the secondary throttle stop from the bottom of the base plate. This is not something you can adjust once the carb is installed, and it's best to get to it now before putting the main body on the base plate. Loosen the screw until the throttles seat in the bores, then tighten it 1/4 turn. It's a bit tricky to see/feel exactly when the blades contact the bores, so I backed off the screw until I could see a tiny air gap between the screw and the stop, then went clockwise again.
Here's the secondary throttle stop from the top side.
This gives you an idea of how far open the secondaries should be: just barely exposing the transfer slots. (The small holes in the throttle bores.)
This shot shows the proper replacement gasket on the throttle plate, and a bunch of other gaskets that are wrong. It seems that the kits always come with at least three of these gaskets that you don't need. Carefully select the proper gasket — it should have holes for the two dowels that locate the main body to the throttle plate, the throttle bore diameter holes should match closely to those in the plate, etc.
Here's a shot from the bottom after the main body is screwed to the throttle plate. Start all the screws, then snug them down working from the center out in a criss-cross pattern. Again, don't over-tighten the screws — you want them snug, not leaned-on. You'll feel the gasket compress and then the screws will snug up. Go around turning each screw a few times until they are all evenly torqued down.
Now that the main body is on the throttle plate, we can install the metering blocks and bowls. Here's the secondary metering-block gasket in place and sprayed with a little silicone spray to keep it from sticking. The gasket will only go on one way — there are offset locating pins.
The metering block set in place on the main body, and the nonstick bowl gasket pressed in place over the locating pins on the bowl side of the metering block.
Now it's time to build up the bowls. Install the accelerator-pump seal: A little oil on the tab helps it slide into place while pulling it into the bowl with needle-nose pliers. Make sure you cut off the excess gasket material, otherwise it will interfere with the float movement.
Next, lube up the new needle and seat assembly o-ring. Screw it into the bowl, flip the bowl over, and set the float dry height according to the instructions. It sure helps to get this setting right the first time, so you don't have to rely on the site-plug method and get fuel all over your intake manifold. The float-height chart in this rebuild kit indicated 15/32" secondary, 11/32" primary. I've read other float settings within a few of these, and have always set them this way or using a drill bit as a gauge.
Once the float level is set, install the adjuster nut and set screw, remembering the two gaskets, one on each side of the nut.
Next, install the accelerator-pump spring and new diaphram, and screw the pump cover in place (again, criss-cross and not too tight). Be careful: I've seen too many of these screws stripped, at which point you're looking at buying another bowl.
The two bowls are built up the same way: After the floats are set and accelerator pump springs and diaphrams installed, you install the bowls onto the main body. Remember the gaskets under the screw heads. I like to use teflon reusable gaskets for the bowl screws and a bit of oil on the gaskets and threads. Criss-cross and snug up the screws with several passes to draw in the bowls and metering plates evenly. You're compressing two gaskets, and you want it to all compress squarely to the main body.
Next, finish up the main body with the accelerator-pump squirters/nozzles, and don't forget those check valves. They go in first, point down, then find the two gaskets, one on each side of the squirter. Use a little oil on the gaskets so they don't bind up under the screw head or stick, and snug the screw down making sure the locating tab engages with the main body to hold the squirter in position.
Needle going in.
Screw and gaskets.
Screwing down the top.
Then, it's time to make sure the accelerator pump linkages are set right, and to make sure that wide-open throttle is really wide-open throttle.
You want a .015" free play in the pump arm at full stroke. Open the throttle to full and make sure that you can get a .015" feeler gauge between the pump arm and the adjustment bolt head. If you can't, the pump arm is bottoming out against the seal and jamming it into the bottom of the bowl. If there's excess play, you're not getting the full volume of fuel from the pump stroke.
Bend the arm that actuates the secondaries to make it "shorter" in this case — to get WOT to be really WOT!
The last thing I did here was to make sure the primary throttles and idle speed screw were set about right. This will be adjusted to get the desired idle speed once the carb is on and the engine running and ignition timing set properly. Still, I like to start with a good base line — about .020" of the transfer slot on the primary side showing beneath the throttle blades. Then I know that if I have to open the throttles a lot to get it to idle, something isn't right.
This carb is ready to set on an intake and try out. I'm sure I've left out a few details, but this should at least be a good foundation to show that there's nothing mystical about a Holley, or even about most carburetors. There is a vast amount of information available on these carbs, and reading up before tearing into one is a great idea. The tuning that goes into making one of these perform perfectly has been written about by several authors. Because these carbs are so tunable, it is sometimes easy to chase your tail changing parts and making adjustments in circles while not ending up with the desired result. Approach each tuning change carefully — some of the books have some great trouble shooting guides that can save you a ton of time and frustration.