A carburetor's condition directly relates to an engine's performance. If the carburetor is old, tired and gummed up, then the engine is going to breathe and run as about as well as someone who smokes two packs a day. The good news is that it doesn't take much to get it running like new again.
Rebuilding a carburetor might sound like an overly complicated task that takes a ton of time and money, but it’s actually pretty simple. More often than not, it boils down to a thorough cleaning and a fresh set of gaskets. And despite the minimal effort, it can totally transform the way a car performs, making it one of the most rewarding things you can do for a classic car. Here's how you do it.
Click here for some basic rebuild tips before you dig in.
Carburetor Rebuild Supplies
You will need some basic supplies to get started. First, pick up a carburetor rebuild kit specific to your carburetor. Next, you will need some carburetor cleaner, a chemical dip, soft bristle wire brushes and plenty of paper towels.
Tools are necessary to tear the carburetor down. The tools you need are specific to your carburetor. Screwdrivers, hand wrenches, pliers and picks are all going to come into play; but since you will likely be doing this near a toolbox, you should be able to pick what you need as you work.
Carburetor Disassembly
The rebuild starts with a simple inspection. Make sure everything moves freely. Work the linkages to determine if there are any issues, and get a feel for the working condition of the carburetor.
If anything is jammed up, now is the time to think about taking it to a professional or replacing the carburetor altogether. You can replace throttle linkages, blades and other major components, but that usually calls for specialty work specific to the carburetor.
Otherwise, any normal rebuild starts with the teardown. About the only pieces you want to leave in place are the linkages in most cases. Slowly start removing hardware and setting it aside in a way that makes it easy to remember where everything goes. (Consider taking pictures along the way for easy reference later.)
Old gaskets will often stick two pieces together. While light prying might not seem like a bad idea, it can easily chew up the tolerance-sensitive parts. Instead, use light taps to break things loose. Be sure to remove all the old gasket materials before moving on.
Note: Most Holley-type carburetors, like a Holley 4150, have small plugs to block off the fuel wells. You will want to remove these to do a thorough cleaning. This is done with a self-tapping screw and some pliers. Carter, Rochester and Edelbrock carbs are much simpler to tear down, but you still want to make sure to remove everything for the same reason.
Carburetor Cleaning
You can throw all the parts in the world at your carburetor, and it still won't run right if it's dirty. You need to make sure all the passages and surfaces of the carburetor are free of dirt. There are multiple levels of doing so.
Start by spraying the parts with some carb cleaner and removing any loose debris with a soft bristle brush and paper towel.
Soak the parts overnight in a chemical bath or, if you have access to one, you can use an ultrasonic cleaner for a fast and thorough cleaning.
After the parts have soaked, pat them dry and then give them another pass with some carb cleaner and tools. Make sure you spray out the many small passages to ensure they are free of dirt. If you used an ultrasonic cleaner, you should remove any lingering traces of cleaning solution by rinsing parts with fresh water.
Carburetor Inspection
It's a good idea to give everything a visual once-over before reassembling. If there are any signs of damage or warping to the components, you will want to replace them or move on to a new carburetor altogether.
Note: Holley carburetors are the most prone to warping, with the metering blocks being the primary concern. Your best bet is to lay a straight edge across the mating surfaces and use feeler gauges to determine if these components have warped. The good news is that they can easily be replaced if they have.
Carburetor Reassembly
If everything checks out, you're ready to reassemble! The rebuild kit for your carburetor will have every gasket and seal you need to get it running. It should also include new bushings and clips to replace all the old hardware.
There's no universal step-by-step process to this part, since your carburetor will have its own set of steps you need to follow. It might seem like common sense (and it is for the most part), but there are some special steps you need to be aware of to be successful. For example, Carter and Edelbrock AVS style carburetors require you to set float levels during assembly, as the only way to adjust them is to disassemble the carb again.
Don't worry if you're not sure where to start or what steps to take. Your carburetor rebuild kit will come with a set of instructions that breaks down the entire process for you – so be sure to keep them close by.
Set The Baseline
The last thing to do before reinstallation is universal to all carburetors: You need to get everything back to the base settings.
The idle speed screw should be set about one full turn in to get started. To verify it is in the correct position, flip the carb over and look for the idle transfer slot. It's a tall narrow opening in the bores of the main body that comes up against the throttle blades. The more the blades open, the more of the transfer slot is exposed. About .020 of the slot should be exposed, which would make it appear as a small square peering over the throttle blade.
The other adjustment you will want to make is to the idle mixture screws. They should be only about a turn and a half out to get started.
Once that's taken care of, you're ready to set it on the car. You're not going to drive off just yet, though. Even if everything has gone right here, you still need to fine-tune these adjustments to get the car idling just right.
The first step is to get the engine up to temperature and if the float levels are verified to be correct, then come back to these adjustments. You shouldn't have to mess with the idle speed screw much, just enough to get idle speed in the range the engine calls for. Once you're near that area, you start backing the mixture screws out in 1/4 rotations at equal rates until you've found the highest, smoothest idle.
You can use a vacuum gauge to aid the process, which involves the same steps, with the gauge hooked up to a manifold vacuum port. Finding the highest vacuum at idle signifies you've found the right mixture.
Once that's done, you make any necessary adjustments to the idle speed screw and get back to driving.