Cleaning and Rebuilding a Carburetor
The following article is designed as an overview of how to remove and rebuild a carburetor. Every model carb is slightly different, but the basic principles apply to all. We are not trying to give specific instructions on how to rebuild every carburetor. If after reading this you don't feel comfortable tearing down a carburetor, have a professional or a friend with carburetor experience do the work.
Removing and Rebuilding a Carburetor
With this issue we begin a series on making the fine adjustments so that a carburetor will operate at its optimum ability. But before any adjustments can be made, problems have to be solved, and that includes locating and sealing air and vacuum leaks anywhere in the engine, and cleaning the carburetor thoroughly, replacing damaged or questionable parts, and reinstalling the carb on the car.
If the car shows symptoms of poor idling, hesitant acceleration, stalling, lack of power, rough running or backfiring, the problem must be isolated and corrected. Begin with the electrical system. Check the wires, the points, the grounds, the condenser, the distributor cap and the spark plugs to be sure that all parts are in good condition and performing as expected. Often an electrical problem will manifest itself in a way similar to a fuel problem. When you are assured that all is well with the electrical, move on to the fuel.
First thing to check is that there is an uninterrupted supply of gasoline to the carburetor. Irregular or interrupted fuel delivery can be caused by a clogged pick-up filter within the gas tank, a plugged or dirty in-line or in-carburetor filter, a crimped fuel line, a defective or leaking fuel pump, a leak at the vacuum tank, or even an empty, or near empty fuel tank.
Check the entire engine for vacuum leaks. An open vacuum line – a cracked line, a line plug that has come loose, a disconnected line – can all allow air to enter the engine and cause a lean condition. You can often locate vacuum leaks by sound - a ‘whooshing’ sound - or by selectively spraying engine starting fluid or brake cleaner around the suspected leak [Editor’s note: be very careful with these flammable sprays, which also dissolve paint]. Don’t neglect the intake manifold gasket, the carburetor base, and joints between sections of the carb and the windshield wiper line.
Check and tighten the lines and screws on the vacuum tank. Follow each vacuum line to be sure that it is connected or plugged. If it feels loose, hard, or is otherwise suspect, replace the line and then again check with the aerosol spray. You may find that the entire rough idling problem is not the carburetor but just an open vacuum line. [Editor’s note: this is especially true in the 1970s-1980s engines.]
Okay, you’ve checked, and everything seems to be in order, but the engine still doesn’t run properly. Let’s check the carburetor. Before reaching for the wrenches, carefully examine the carb for cracks, a warped mounting base, or other evident problems. Try the air adjustment screw(s). There should be a definite change in the way the engine is idling as each air screw is turned in or out. If not, it’s a pretty good indication that there is an air leak. It may be internal and only repairable when the carb is disassembled, or it may be that the mounting gasket(s) are leaking, the carburetor is not securely mounted, bolts are loose or other easy fixes.
Also try tightening carb top screws or other carburetor body screws. Do not over-tighten [torque specs are usually in the factory repair manual]. But the carburetor body screws may have vibrated loose.
No luck? Let’s pull the carburetor. Remove the air cleaner, and disconnect fuel and vacuum lines from the carb. Be sure to catch the gas that will run out of the line in a small can or with rags. Make sure that the gas-soaked rags are hung out-of-doors to dry. Gasoline soaked rags are extremely volatile.
Disconnect throttle connections, making note of which holes the linkage fits into. Often there are two or three holes into which it can fit. Make a note on paper or take a photo. Otherwise you’ll never remember.
Remove the entire carburetor from the intake manifold. Do not try to partially disassemble the carb while it is on the car. As you remove the carb, make note of the direction of the carburetor on the manifold, of the way the gaskets are installed, of spacers and of throttle connections.
As soon as you have removed the carburetor, put a clean towel or rag over the intake manifold so that small parts, nuts, washers, etc. do not fall into the manifold.
Put the entire carburetor on a clean work-bench, and photograph it from all angles. This will help in reassembly. Keep a supply of zip-top plastic baggies, a pen and small pieces of paper nearby. As you remove screws or parts, make a note on the piece of paper as to what the part is, where it came from and the direction that it was installed, and put the part and note in a baggy. Take photos as you disassemble the carb; it will jog your memory in reassembly. Many parts look the same once they are spread out on the bench. Little ball-bearing seals may look identical, but the size may differ by a couple of thousandths of an inch. Don’t mix them up.
A word about carburetor repair kits: many kits are designed to supply parts for a variety of carburetors. It is generally less expensive or easier for the manufacturer to include the parts for several carbs in one package. Be sure that you carefully match up the parts in the kit with the parts removed from your carburetor. Often gaskets look identical, but holes or openings are not quite the same. Also flipping a gasket over will present a totally new perspective of where the holes align.
Carefully begin to disassemble the carburetor. Make a note of the sequence of disassembly. It will make reassembly much easier. The instruction sheet that comes with the rebuilding kit will be a tremendous help in locating internal jets and passages, one-way valves, etc.
Remove all gaskets, rubber or leather parts. The all-metal parts have to be cleaned in a special carburetor cleaner. I have used Berryman’s B-12 Carburetor Cleaner and Gunk’s Carb-Medic. A word of caution – from personal experience: these are strong, aggressive chemicals, and they dry the oils from skin. You must wear rubber gloves when working with carburetor cleaners. The one-gallon size of both products comes with a dipping basket. Take your time, and don’t try to throw all of the parts in at once. Keep them separate.
As gasoline gets old and evaporates, it can leave a varnish film or gummy residue on parts and in tiny orifices. It's also possible some small particles and rust could have gotten into the small passages and orifices of the carb. This means that parts inside a carburetor may not move like they are supposed to, and small passages meant to meter air or fuel can become plugged. The varnish or gum will not be completely dissolved or removed with the chemical. A brass wire brush is a big help in removing gum and varnish.
NEVER use a drill bit to clean jets or orifices. Although it doesn’t feel like the drill bit is removing metal, it could enlarge a jet size. Never use a steel paper clip or sand blast the parts either. If you have to get into a jet or tiny passage, use a piece of copper or brass wire. Repeated treatments with the wire and chemical are better than trying to do it all in one pass.
Check the float to be sure that it is not leaking. Immediately upon disassembling the carburetor, shake the float while listening for fluid sloshing around inside. After everything has been cleaned, submerge the float in hot water, and watch for bubbles that would indicate a leak. Rotate the float slowly underwater so that all surfaces are given an opportunity to be facing up. Air rises, and the bubbles will be obvious. Again shake the float and listen for fluid inside of it.
After all of the parts are completely cleaned, dried and inspected, it is time to begin reassembly. Use the new parts that came with the rebuilding kit instead of trying to reuse an old part that still looks good. A spring may have lost its tension, a valve may have developed a flat spot, a gasket may be too thin in spots. New parts are
NEVER paint the interior of a carburetor. If external parts are to be painted, use Plasticote’s Aerosol Lacquer spray paint. Lacquer is more resistant to gasoline and won’t peel off or dissolve readily. Make sure that you do not get any paint in screw holes, access ports or on linkage that has to move freely.
When you assemble and then reinstall the carburetor, tighten the screws in a pattern (generally opposite screws) that evenly distributes pressure. Tighten the screws a bit at a time, and take at least three steps to fully tighten each one. This will help put even pressure on the flanges and will help prevent breakage.
The adjustment specifications that come with the instructions or that you find in a Motor’s Manual or Chilton’s is a starting place. The details found in Orest Lazarowich’s column, Orest’s Carburetor School in Skinned Knuckles magazine will give you much more detailed and precise specifications.
This article originally appeared in the February 2011 issue of Skinned Knuckles. It is copyrighted by SK Publishing and may not be reproduced, in whole or in part, without written permission from SK Publishing. See Skinned Knuckles for more vintage and classic car tips. Also see vintage car repairs and these other articles from Skinned Knuckles: