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December 29, 2012

Rebuilding Holley94 (and other) carburators

I purchased a used Thermoquad for a 1974 Dodge Challenger I did a while back because I thought it would be cool to have period-correct parts on it. Of course everyone told me not to bother and would tell me things like: "they never ran right","get an Edelbrock", "they go out of tune quickly", and more....But, being committed to the process, I asked my neighbor, who happens to work for Ackland's and was the dedicated carb rebuilder there for many years, to help me rebuild the TQ.

While he did so, I watched him carefully and found his "secrets" included soaking the carb body in the parts washer overnight, then using twist-tie wire (stripping the paper off with his teeth) to clean out all the tiny passages in the body, the needles and rods, spraying carb cleaner through a straw into the passages, then using compressed air to force any remaining cleaner out. Secondly, he made sure there were no vacuum leaks around the throttle plate shaft and other through-passage parts. Lastly, he spent a fair amount of time setting the float level properly - which entailed getting the carb in a position to actually make a measurement in the required manner.

That Thermoquad carb ran perfectly the minute we started the engine and I never even had to fiddle with it after that. Since that time I've done several more carbs and - with the exception of one carb that I didn't heed the above steps - they have all ran great after the rebuild. The lesson here being this: you can do all the steps, replace all the parts and have it LOOK really good, but, if one tiny passage is partially occluded, or the throttle shaft is loose producing a vacuum leak, you'll never get it to run right and you'll likely blame everything except those items you could have done something about!

So here's a detailed look at relatively simple Holley94 from one of my Ford Flathead engines that needs a thorough rebuild -

First, you'll need some space to do the rebuilding - and like anything, once it's taken apart, it takes up far more room! Plus, you obviously need to keep things organized and still have room to work on the individual pieces. Below is my engine-parts rebuild area consisting of a dedicated parts washer (no large ultra-greasy parts in this one!), various rebuilding references, tools, oils, cleaners - and of course - the ubiquitous shop visuals ....

Looking a little closer, you'll see I like to use baking sheets to separate the job at hand from others on the bench. These pans have raised edges all the way around which stop tiny parts from rolling off into the abyss, can contain any spills and are easy to clean. Of course I always buy new ones for Mrs E-tek's kitchen before taking the old ones! (See my upcoming posts entitled "Secrets of a Good Marriage" - LOL!)

One thing that can really help is to take photo's of the parts before - and during - tear down. It may be weeks before you actually put this thing back together and I've found the exploded views of parts don't help you all that much during re-assembly. Each carb is totally unique and photo's help immensely.

Especially true for the above mentioned point is to photograph linkages. Once disassembled, they can be nearly impossible to get back together without an original reference to go on.

Before you started your rebuild, you would have purchased a rebuild kit. But not included in the rebuild kit are some key items that you may need, including new floats - and in an effort to reduce variables and having to re-do things - I always buy and replace them.  A new throttle plate shaft may be required and sometimes, a different sized power valve, also called an economizer valve, may be required. 

I took this photo below from the net - to illustrate an important point. Some Power valves - such as the ones I'm using here, have square windows in them and therefore require a "3-window", not a round, gasket as the one on the left is fitted with. Make sure you look into little things like this and try to visualize how the air and gas will travel through the various parts and orifices in the carb you're rebuilding.

The operation of the power valve is the second response to a call for acceleration. The first response is from the action of the accelerator pump, which squirts a double shot of fuel down the carb throat. The power (economizer) valve becomes operational as the engine vacuum drops, thereby opening that valve, which delivers a continuing supply of fuel until the vacuum levels out, indicating the returning balance of air/fuel supply.


As my set-up will be dual (2) carbs on an Edelbrock Slingshot intake, I was advised to replace the Power Valves that come with the rebuild kits with slightly smaller ones of 4.5, as shown above. This number corresponds to the vacuum opening point, which needs to be less, to compensate for the lesser amount of vacuum when shared by two carbs. Although this information is specific to my build and not something you'll need to worry about in a single carb, stock application, I thought it useful to the discussion at hand.

You're only tools at this point, beside a couple flat-blade screwdrivers, are cleaning supplies, carb cleaners and compressed air. I often use another, deeper pan to clean the parts from the parts washer one at a time. The cleaning brushes pictured below are used to loosen crud on the outside of the carb and in some of the more accessible parts of the carbs interior. The "PRE" pictured here is actually a pre-paint cleaning solvent used to remove wax and grease from surfaces to be painted, but many different solvents can be useful in the cleaning process.

After washing, removing and drying the parts off with paper towels then compressed air, I use a straw (from a can of WD40 of course!) inserted into the carb cleaner nozzle to force the cleaner through the small holes in the carb. *This is where I should tell you to use extreme caution-  including goggles, or better yet, a face-shield -  so as not to get this stuff in your eyes and face!*


Once that is done and the passages are well soaked, I use the wire from a twist-tie to ream out and loosen any gunk in the passages. Continue with the cleaner and the wire until you're sure there is nothing left in every orifice. You may need to use a bright light and a magnifying glass to find some of the more obscure (hidden) passages on your carb.

 Some of the passages on this carn where so small that I needed a particularly small twist tie to get all the way through them.  The key is to use whatever you need to, to make sure they are clean and open.

Lastly, I used the compressed air to blow out each passage. I use a tapered blow gun to really get the air into each orifice and I make sure to feel for air coming through the other side.

Another key piece in any rebuild is to check the play in the throttle plate rod. When you check it before dis-assembly, remember is is likely slightly worse than you can feel then - due to the dirt that is still inside the throttle rod passage. Any slop here means it is a source of unwanted air entry, creating a false vacuum which plays out as uneven idle and running lean, requiring you to richen the mixture, creating even more problems...

Below - old and new throttle plate rods -

The rod from this carb after removal. It felt a little loose and you can see why - the end of the brass rod that rides inside the carbs throttle rod bore has been worn down over the years. This is actually not really bad - I've seen worse and I'm sure pro-rebuilders have seen some real loosy-goosy ones!

On this particular carb, the fix was to ream out the throttle rod passage to accept the next largest rod. Some carbs have bushings that can be reamed, or replaced, to restore a tight fit. There's no point leaving a sloppy throttle rod in a carb. Just changing gaskets won't do a thing to make it run better.

If doing it yourself, the key is to have the carb mounted perfectly perpendicular to the drill bit.


Here, the new rod is inserted into the newly drilled out passage way. It fits "almost" like a glove - not so tight as to bind, but tight enough not to allow air to whistle through between the rod and the opening.

Once the rod is in, the throttle plates (cleaned on the wire wheel) were re-attached. (A little thread-locker is not a bad idea here.) These rods are left long so you can custom cut them depending on your linkage set-up. They'll be left like this for a long while yet...

There are several adjustments you'll need to make during the rebuild, 
which are generally detailed on the rebuild kit instructions. 

Here the float level is being checked with the included ruler - 

Note the tine that can be bent up or down to get the float to sit at the correct level in the bowl. Make sure to read the directions carefully and measure the float both in it's hanging and "resting" positions.

Resting position measurement -

 A second measurement I do with the float resting on the needle is to check if it is square. If the float is twisted, one side will hold the other up and the levels will be off, but you won't know why. It's another variable you can correct now.

The third measurement you need to make is with the float hanging. This is where the top tine comes into play and can also be bent to get the float to sit at the right level.

Hanging position measurement -

With the float hanging down, I used the enclosed ruler again to measure how far the float hanged down - be careful here - the difference between the first measurement (with the carb horn flipped over) and with the float hanging down - was only 3/32nd of an inch in this case - the thickness of a dime.

With the float perfectly set it's time to put it back onto the base.....

Once you find the correct gasket! Most gasket kits cover a range of carb suh-types and so have a lot of "extra" gaskets that you won't use. so make sure you find the exact one for your application. Make sure you grab the one that has all the correct holes and that none of the holes in the carb get covered over by the wrong gasket.

Hee's a key part of the re-assembly that is easy to mis - or mess up: many carbs have return-springs that load the choke or throttle plate shafts and the linkages. This helps these parts return to their initial state once input from a cable, dashpot or vacuum source are removed . On this Holley94, the choke plate has a tiny spring that is both eas to overlook and difficult to set in the correct configuration. A couple of ties  - and having another carb to look at for reference - and I was able to hook it up correctly, using a small screw driver.

Once set properly, the choke plate snaps open when released -

As we move into re-assembliing the linkages, we find more spring-loaded mechanisms.

The final part I needed to tackle was removing the level that was press-fit to the throttle plate rod that we bored and replaced earlier in the rebuild and attach it to the new rod.

 At first I thought I'd just grind off the bit of brass that had been peened over to hold the metal lever...but it still wouldn't come off...

Then I thought, brass is very soft and has a much lower melting point than dfoes steel. So I grabbed the torch and softened the brass enough to knock the lever off.

The new rod is threaded for a screw so attachment was easy.

One Holley94 done and one to go so they can work together on the Edelbrock Slingshot intake.

Next up - The Holley 4 barrel carb from a Ford 390 destined for my Galaxie!

Don't forget to check out my website at www.E-tekRestorations.com !