How to overhaul your own carburetor

In this instalment of Shop Class, we get back to the basics of carburetors

In the mid 1980s, carburetors quickly gave way to fuel injection systems in new cars and trucks. But it’s likely there are still a few carbureted engines on machines in nearly every Prairie farmyard. That handy ATV and small grain auger engine are probably two places they can be found. And, of course, there is likely to be an older grain truck or gasoline-powered tractor still parked in the back corner of the machinery shed.

While carbs can’t match fuel injection systems when it comes to precise delivery of vaporized fuel to engine cylinders, they’re still pretty good at what they do. The trouble is when machines sit for long periods without running, as often happens on the farm, fuel systems can develop problems. And that goes in spades for those with carbs. Evaporated gasoline leaves a varnish that can gum up the small orifices inside a carb and leave you with a dead engine.

So a carb overhaul is something most farmers will eventually need to tackle if they do their own equipment maintenance.

Basically, the function of carburetors is pretty straightforward and relies on a scientific principle called the Venturi effect, which is the reduction in pressure of a fluid as it moves through a restriction.

A carburetor uses the Venturi Effect, relying on vacuum caused by faster moving air as it passes through the barrel to draw gasoline up from the float bowl through the fuel nozzle, vaporize it and create an explosive air-fuel mixture.


A carburetor uses the Venturi Effect, relying on vacuum caused by faster moving air as it passes through the barrel to draw gasoline up from the float bowl through the fuel nozzle, vaporize it and create an explosive air-fuel mixture.
photo: Scott Garvey

Here’s how that applies to carbs. As air enters a carb, it passes through a narrow throat called a Venturi — or barrel — which causes it to speed up. That jump in speed creates a zone of low pressure, or vacuum, that “sucks” fuel out of the float bowl and vaporizes it into the air stream entering the intake manifold.

Sounds simple, right? Well, the reality is despite the basic concept behind what they do, many carbs are extremely complex; but others, especially those on small or older engines, usually aren’t that hard to work on. So servicing a gummed-up carb could be a good DIY job. Just go slow if you tackle it.

Even older, relatively simple carbs may use a lot of parts. Paying close attention to the order of removal and orientation of parts during disassembly is essential. Some carb kits include an exploded view, like this one, to aid with the rebuild (click image for larger view).

Even older, relatively simple carbs may use a lot of parts. Paying close attention to the order of removal and orientation of parts during disassembly is essential. Some carb kits include an exploded view, like this one, to aid with the rebuild (click image for larger view).
photo: Scott Garvey

Getting started

Start by spending a few dollars on a carb kit, which includes new gaskets, some other pieces and often an exploded view of the carb components. Along with that, buy some specialized carb cleaner. Those things together with a couple of hours work can get a carb back in good working order.

Before you begin unscrewing things and creating a pile of parts, be sure to have that carb kit in hand. There are very thin gaskets between carb body components that will shred when you open it up. They’ll need to be replaced during the rebuild. You don’t want to leave all those tiny, complex little pieces sitting on the bench for a week before replacements arrive, which will mean if you don’t lose something you’ll have completely forgotten how everything went together.

Make sure you end up with the right kit. There should be a metal tag attached to the carb, usually at the float bowl, which will have an identifying number. You’ll need to give that to your local auto parts store so they can order the right kit.

Keep your cell phone handy to take pictures of how components come apart when you first open up the carb. If you don’t, it can be pretty easy to end up with a pile of tiny parts that don’t seem to fit back together. Also photograph the control linkage arrangements before you take the carb off the manifold. That’s another thing that can leave you scratching your head during reassembly.

Using your cell phone to take a picture of complex linkage arrangements can make reinstalling an overhauled carb much easier.

Using your cell phone to take a picture of complex linkage arrangements can make reinstalling an overhauled carb much easier.
photo: Scott Garvey

Set up a clean surface on the workbench and lay out parts in order on it as you take the carb apart. Laying a light coloured cloth on the workbench surface allows you to easily see things. Once it’s disassembled, soak the carb in a good quality specialty cleaner. Just spraying cleaner into passages before reassembly may not be enough to clean out a badly gummed up body.

Before you decide to rebuild a carb, make sure it’s still usable. Carbs wear out just like any other component. Check the body for cracks around inlets. Also check the bushings around the throttle valve shaft, the shaft shouldn’t wobble. Space between the shaft and worn bushings can allow air to get in and lean out the fuel-air mixture too much. Not all carb kits include new bushings for this. If your carb has that problem and isn’t corrected, it will never work properly.

A tag like this one attached to the float bowl has a specific number identifying the model of carburetor. Use this number when purchasing a rebuild kit.

A tag like this one attached to the float bowl has a specific number identifying the model of carburetor. Use this number when purchasing a rebuild kit.
photo: Scott Garvey

Each carb has a float that maintains the correct fuel level inside the bowl. It works just like the one in a toilet tank, allowing more fuel in as the level drops and closes off the flow when the bowl is full. Floats can develop leaks around soldered seams, causing them to sink and allow too much fuel in. If there has been fuel in the bowl recently, you’ll be able to tell if the float leaks by giving it a shake. You’ll hear the fuel sloshing inside. Floats aren’t usually included in a rebuild kit, so you’ll have to order one if the original is bad and can’t be repaired.

When removing set screws, count the number of turns required to back them out. Then when you reinstall them, you can get the settings back in the ballpark so the engine will start. Fine tune the settings as the engine runs.

About the author

Machinery Editor

Scott Garvey is the machinery editor for Grainews.

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