How to rebuild automotive carburetors

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Contents

[edit] Introduction

Sooner or later, every hotrodder runs into the steps of making induction and carburetion choices while building a performance engine.

For some, it's a tough choice whether to buy a brand-new carburetor, or to find an old dirty one and rebuild it. While buying a new carburetor may be convenient, at $200 and up, the cost can be prohibitive for the budget-minded hotrodder.

On the other hand, a brand-name carburetor rebuild kit may cost as little as $15, and the rebuilt carb, (if rebuilt correctly), will last many years to come.

[edit] Where to start

Things to consider when choosing your first rebuild victim...

  • CFM (cubic feet per minute of air or air/fuel mixture flow), and how much do I need?
  • Handy CFM Calculator
  • Spreadbore vs. squarebore -- which one to use?
  • Parts and rebuild kit availability

[edit] Holley squarebore carb

Holley squarebore Model 4160

The squarebore 4bbl Holley is a very flexible carb. The design of the model 4150 and 4160 squarebore Holley 4 barrel carburetor has changed very little since its introduction in 1957. Repair, rebuilding, modifying and tuning techniques remain basically unchanged as well. They are easy to rebuild and tune, and they have great aftermarket support- in fact a "Holley" carb can be assembled without using any Holley parts.

It has been many years since a Holley carb (or ANY carb) has been used on a domestic production automobile, but regardless of that, the Holley carb will remain in use for many more years to come- of that there's little doubt.

Often the best place to start is to baseline the carb back to factory specs. This eliminates any chance that a previous owner's changes will mess up the way the carb works on your engine.


[edit] Fuel transfer tube

Holley redesigned the fuel bowl transfer tubes and seals used on the 4160, 4150/4175, and 4165 carbs. The old style used a simple rubber O-ring on a transfer tube that had a collar on both ends to secure/locate the O-ring. This design was leak prone and somewhat troublesome.

The new design has a straight tube without the collar and a new type of seal. Details are shown in the photos below.

New seal transfer.jpg
New tube w seals.jpg

[edit] Power valve

Holley power valve vacuum rating

The Holley carb uses a power valve to supply additional fuel when the engine vacuum drops (and presumably the engine load increases). The amount of fuel it supplies is equal to about 7-8 jet numbers. If the rear power valve (if present) is removed, the threaded hole it was in has to be plugged. The jets have to be enlarged 7-8 jet sizes to compensate for the loss of fuel flow it provided.

If you want to check to see if the PV rating is too high and allowing it to open during idle, turn the idle mixture screws all the way in (count the turns so you can return them after the test). Then start the engine. If the engine won't idle, the power valve is not open.

If you want to check if the PV is open during idle, turn the idle mixture screws all the way in (count the turns so you can return them after the test). Then start the engine. If the engine won't idle, the power valve is not open.

[edit] Holley idle transfer slot

The drawing below shows the transition slot as seen with the carb held upside down. The drawing on the Left shows an overexposed transition slot. Baseline the throttle blades to give a transfer slot that looks like the image below, Right (approximately as long as it is wide):

Holley t-fer slot.jpg

Warning Note: Other carbs have similar transition circuits, and they need to have the throttle blades in the 'sweet spot' same as the Holley. If the throttle blades are open too far, a poor idle and off-idle transition can be the result.

[edit] Secondary vacuum chamber

On carbs that have a vacuum operated secondary, the secondary vacuum chamber is important to the function of the carb. It should supply only as much air/fuel as the engine needs, but if the calibrations are not correct, the engine may get too much or not enough.

Secondary vacuum chamber spring selection

The opening rate and opening point is controlled in large part by the secondary vacuum spring. There are several springs available from Holley and others that allow the camber opening rate/point to be tailored to the vehicle's needs.

Quick change lid for the secondary vacuum chamber

Quick change vacuum chamber lids have been developed that allow quick, easy secondary spring changes. They are well worth the money if any serious tuning is going to be done.


Arrow indicates cork O-ring that seals the vacuum chamber to the main carb body

Use a fresh cork O-ring to seal the secondary vacuum chamber assembly to the carb body. Using a light smear of Hylomar on the cork gasket helps it to seal and stay pliable. A small rubber O-ring may be used instead of the cork gasket to seal a Holley secondary chamber. If you try this, be sure the O-ring is sealing, w/o being too thick.

Secondary vacuum chamber check ball

Be sure the vacuum chamber is assembled correctly. The diaphragm can be torn or perforated; if that happens there will be no or very slow to activate with incomplete opening of the secondaries. Also, the check ball can get left out or can stick, or the check ball seat can be rough, allowing the secondaries to activate too easily. So be sure the check ball is present and the seat is well formed.

The seat can be re-formed by using an old check ball and a small punch. Drop the check ball into the check ball passage, then with the vacuum chamber solidly supported, lightly tap the old check ball with the punch and a small hammer. This re-forms the seat.
NOTE: There are small grooves in the vacuum chamber check ball seat. This is normal, so do not try to use this method to remove the grooves. They need to be there to allow the secondaries to work correctly.

A tuning trick is to use a deeper or shallower groove to change the opening rate. A tool can be made from a modified phillips head screwdriver to cut deeper and/or more grooves in the seat. Also, a lighter (aluminum) check ball can be used.

To check the vacuum chamber, with the chamber off the carb push the diaphragm link arm back into the chamber, then use a finger to block the vacuum port that goes to the carb body. Release the link while keeping a finger over the port- it should remain retracted until you remove your finger from the vacuum port. Only when you remove your finger from the vacuum port should the link extend. This will let you know the diaphragm is sealing.

If you use your lips or a vacuum pump to pull a vacuum on the chamber (chamber off the carb), there should be some resistance felt; the diaphragm shouldn't be too easily retracted because the check ball should be restricting the vacuum somewhat. That tells you the check ball is there and is able to help modulate the secondary opening rate.

[edit] Motorcraft

[edit] Model 2100/2150 2bbl

[edit] Rochester Quadrajet carb

Rochester.jpg

Because of its ability to be tailored to work with a variety of engines and circumstances, the Rochester Quadrajet- if properly built and tuned- is one of the best carburetors available. With its small primaries, you can get reasonable fuel mileage, or you can kick in those monster secondaries, forget about fuel mileage, and fly.

It does have its limitations, though. For instance, if you are running a cam that causes the vacuum to be low, the Q-jet power piston spring needs to be selected carefully- just as you would do a Holley power valve. Several companies make different power piston springs for such applications, and jets and metering rods are also available.

In maximum performance applications, one problem with the Q-jet that's difficult to overcome is the small float bowl. In cases where the engine is large and the RPM high, there's insufficient volume in the Q-jet fuel bowl. This is often countered by using a much higher than normal fuel pressure. This in turn requires the float and inlet valve (needle and seat) assembly to be modified to handle the higher fuel pressure, otherwise the carb will flood due to the fuel pressure overpowering the float/needle valve assembly's ability to shut the flow off when the desired float level is reached.

Used under the right circumstances, the Quadrajet is a great all around carb for many street machines. Although Holley carbs were seen on a few Chevy engines during the '60s and '70s- most notably on high performance small and big block Chevy engines (including the three 2-barrel 'Vette big block engines)- the Q-jet carb was original equipment on most GM muscle car engines from that era.

Once tuned, it tends to hold that tune unless dirt gets into the air bleeds or clogs the jets or other orifices. This is usually caused by a bad or missing air filter- so always use a clean air filter! Don't remove it at the track, either. Often a correctly shaped filter housing base and a free flowing filter will out perform no filter base and filter, so adding performance by removing the filter is a fallacy.

It's true that the Q-jet is somewhat more difficult to rebuild than a generic Holley, at least the first time. But after the first one, the process becomes much easier until after a few rebuilds, it becomes second nature. Some types of changes to the tune of the Q-jet requires drilling orifices and/or replacing air bleed and fuel supply tubes; this too will be daunting at first. This is where a book like How to Rebuild and Modify Rochester Quadrajet Carburetors by Cliff Ruggles is a big help. In addition to his book, Cliff Ruggles also maintains a website (Cliff's High Performance), with a forum dedicated to discussing carburetors. Another highly regarded title on the Q-jet carb is Rochester Carburetors" by Doug Roe/Bill Fisher.

Most rebuild kits come complete with instructions and application charts for whatever engine your working with, and you can always hold on to the parts you didn't use and use them on your next rebuild or experiment. Several companies make everything you need to tune the Q-jet to fit your needs, or you can buy a new carb from Edelbrock. Jet Performance also sells Q-jets in several stages to meet your application. If your running a nice little street engine with a little thump, you can't find a better carb, but if your getting into monster pro street territory, you need to look to carburetors that can handle the needs of these engines and/or have a very good understanding of the tuning requirements needed to allow the Q-jet to work well under these conditions.

[edit] See also:

[edit] Drilling jets

The flow through a jet is determined by the surface finish (small consideration as long as it’s very smooth), the length of the orifice (moderately important, depending on how much the difference is), the entrance and exit angles and their finish, and the size of the orifice (major consideration).

When a jet is drilled oversize, all of these things are changed except the angles of exit/entrance. The number stamped on a jet (Rochester, Holley, etc.) is an indication of the actual flow, NOT the orifice diameter. The manufacturers flow the jets to see what number they get stamped on them. The same orifice diameter jet may be stamped w/different numbers. This tells you they do not flow the same, even though the orifice diameter is the same. Subtle differences in the above parameters account for the different flows.

The difference between the correct and too rich/lean jet sizes may only be 0.002”-0.004”. There are no readily available drill bits that are that close in size to one another. So at best, the changes made to jets by hand drilling are going to be in steps decided by the availability of drill bits. Because drill bits are often available in 1/64” increments (the small numbered bits- which have a finer increment between bits- are too small for drilling the average carb metering jet), this will be on the order of a 0.0156” change between drill bits- which is HUGE (~40%) change in metering area when in the 0.070” jet orifice range.

There is no practical way for a person to casually drill an orifice and know what the change in flow will be. Drilling orifices by hand is mostly an anachronism. Nowadays, better options are almost always readily available. Only rare and/or extreme circumstances dictates such practices.

Highly educated engineers work with mathematical formulas to design correct sized orifices. During the formative years of our contemporary automotive racing environments, carburetors were not always made with a full range of ready made fuel and air bleed orifice replacements so carburetor tuning 'magic' was done with a drill. Because of the difficulty in obtaining precision, for those reasons listed above, a general recommendation to purchase an assortment of pre-calculated metering orifices (fuel or air bleeds) is emphasized. Nevertheless, carburetors exist that do not have readily available interchangeable orifices, and these warnings are made to caution the ambitious tuner.

[edit] Quadrajet Tuning and Repair Parts

[edit] Carter carb

[edit] AFB

[edit] Thremoquad

Thermoquad.jpg

[edit] Stromberg carb

Stromberg.jpg

[edit] Actual rebuild procedure

The overall steps are the same no matter what type of carburetor you are rebuilding.

  • Identifying what you have
  • Purchasing the right kit
  • Purchasing some sort of cleaning media
  • Rebuilding the carb.

To keep from losing small parts, work over a clean, well lit work space. An old cake pan works well for holding larger parts as the carb is disassembled. Small parts like the check balls, needle and seat, metering rods and springs, etc. can be stored in pill bottles, for example. It is a good idea to take some pictures as you go through the disassembly process, they could come in handy for reference later. Most rebuild kits will have an exploded view of the carb to help with the reassembly as well.

Usually the top (air horn) comes off first, after any linkages are removed like the accelerator pump link on the Q-jet.

Next remove the float and needle so they don't fall out when the carb is inverted to remove the base plate. Once off you can turn the carburetor over and remove the base plate. It is a good idea to look at the instruction sheet that comes with the kit so you know where all the check balls and other small parts that may fall out go. On Holley style carburetors with removable metering plates and fuel bowls these also need to be removed. Remove all needle valves and rubber parts from the outside of the carburetor.

Try to preserve the gaskets if you can. A lot of the kits come with gaskets for several models of the same carburetor and using the correct gasket is critical. You can use the old ones to match the correct new ones.

Once you have it torn down, all the parts need to soak in your carburetor cleaning solution. Follow the directions for the cleaner that's being used. It's best to not over soak the carb parts that have a dichromate finish so the finish isn't removed. While the parts are soaking, it's a good time to go over the instructions that came with the kit. They will include the adjustments that will need to be performed. A good kit will also have a good exploded view to use as a guide.

Check all the parts to identify what you will be replacing. At a minimum all gaskets, needle and seat and the accelerator pump need to be replaced. Some kits will have additional parts as well. Once the carburetor has soaked, rinse it clean with water and blow out all passages with compressed air.

Re-assemble using the kit instructions and your pictures as a guide.

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