In the early 1900s when carburetors were ubiquitous it wasn’t unusual to have to rebuild them every few thousand miles. The amount of dirt in the fuel, combined with poor fuel filtration, served to clog the minute passages within the carburetor. Being a mechanical device, it cannot compensate for restriction to fuel flow, as when a piece of debris clogs a jet. This would cause the engine to run poorly.
Carburetors were simple back then and most people who were mechanically inclined could rebuild one in an afternoon. Carburetors increased in complexity over the years until their pinnacle in the late 1980’s. They were a combination of a mechanical device and one laden with sensors and actuators controlled by rudimentary computers. By 1990 they were nearly non-existent on passenger cars.
Having started in this career when most cars were carbureted and working through the switch to fuel injection, I was lucky to experience both. Technical colleges and trade schools stopped teaching carburetors classes around the turn of the century as it was clear there would not be many on the road for long. Anyone younger that about 35 has likely worked on very few. Most could not explain how to set idle mixture on a carb or set up breaker points for that matter.
Carburetor rebuilding is not as easy as it seems to do right and there are many guys who create more problems than they solve. There is so much more to it than simply taking something apart and then putting it back together correctly. On the late-model computer controlled carburetors there is a lot of crucial setup required that not many people were good at even when these cars were new.
The following information is a guide to some of the steps that need to be followed to have a successful rebuild.
1. Get your information ready.
In advance of tearing that bad boy apart, you need to have a rebuild kit. In that rebuild kit should be an exploded view of the carburetor so you can figure out how the hundreds of parts go back together when you forget. If you think you can do it by memory you are wrong. I’ve found that there is lots of good information on the internet even for rare carbs. More information is always better than less. Print pages out so you have it close by on your bench and can take notes.
2. Have a good work area.
A bench that is about 4 feet wide should suffice. A fast food tray is nice to work out of because the raised edges will keep small parts from rolling away, which they inevitably will want to do. It will also capture spilled gasoline and prevent it from getting on your bench or your clothes.
3. Take pictures of how everything came apart.
I start by taking pictures of all six sides of the carb. You don’t need a fancy camera, just use your phone. Take more pictures as you progress through disassembling the carb. You can refer back to these during reassembly. It is always better to take too many pictures than too few.
I lay some butcher paper on my bench and carefully lay out all the individual parts as I take them apart. After everything is apart I take a picture of all the parts laid out. This way I have an inventory of parts should something go missing.
4. Cleaning is the key to a good rebuild.
As a carburetor ages, the small passages which meter fuel flow become restricted with dirt. Its similar to cholesterol as the passages become more and more restricted. A carburetor that needs to be rebuilt will almost always run lean because fuel is what flows through these metered passages. As they become restricted, less fuel flows in proportion to air.
Non-agitated cold soak carburetor cleaning solution does not work well anymore. The chemicals that used to be in it have been restricted. What you need for a truly clean carburetor is agitation. We used to have compressed air powered cleaning equipment that worked great, but then again the solvent was restricted and they went away.
The only good solution today is ultrasonic cleaning with a heated baking soda and water solution. This works effectively, but takes about twice as long as the old machines.
To do the job at home, you certainly can use the cold soak. If your carb is not too bad it will do the trick. I use it around the shop to freshen up carburetors that have basically been sitting on previously restored car with no miles driven. It brings back the patina of the carburetor if you let it soak for about 30 minutes. Rinse it in hot water once you take it out and then blow passages dry with compressed air.
5. Take your time during re-assembly
Refer back to your pictures. I like to use my tablet computer since I can place it on the bench right next to me. Be careful not to cross thread small parts such as jets.
Always write down the sizes of the different jets. Almost all carbs have replaceable main jets and they should have a number stamped on them. Other carbs have replaceable idle jets, emulsion tubes, air compensators, etc. You will need to refer back to these during tuning.
You can use silicone spray on the gaskets to keep them from sticking in case you have to take it back apart. If the accelerator pump is made of leather, soak the new one in gasoline while you are doing the rebuild so it swells and fits the accelerator pump bore.
After it is together, actuate the linkage and make sure everything operates smoothly. There should be no binding in the linkage.
The next step is to get it on the car and the engine running. After you’ve got the car drive-able, it’s time to tune. That is the subject of another blog post.