For many farms, however, there is almost always an air compressor in the field. During seeding, spraying and harvest, the farm’s heavy truck is almost always around, and it probably has an air brake system. By making up a special air hose, it’s possible to use the truck’s compressor for other tasks, like inflating a low tire, using an air tool, or blowing out a clogged radiator. This low-cost line makes all that possible, and it stows away easily — much easier than an entire air compressor.

All it takes is a standard air hose fitted with a glad hand connector at one end and a standard quick connect coupler at the other. With a glad hand connector at one end, the portable air line can be attached to the emergency (also called the supply) line of any truck or truck tractor equipped with glad hand connectors for pulling a trailer. Open the red trailer supply valve in the cab, which allows air to flow through the line, and you now have a portable air supply.

The hose is cheap, simple to make, and very handy.

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Repairing that problem isn’t too difficult, even if OEM replacement parts are no longer available. There are plenty of aftermarket components that can do the job. However, even though all fuel gauge systems use the same two basic components, a gauge on the dash and a sending unit in the tank, there are some differences in how each one functions, depending on its design. And they need to be compatible with each other. Getting the right pair isn’t just a matter of just picking up any old parts off the shelf at the local auto parts store.

Here’s a look at how these systems work, how to determine exactly which parts are at fault and what you need to replace them with.

The gauges

In older machines and vehicles there are two common types of gauges: thermostatic and balancing coil. (Newer equipment and vehicles may have gauges controlled by a microprocessor.)

Current flowing through a thermostatic gauge heats a bi-metallic strip, causing it to bend. That bending motion is linked to the gauge pointer and causes it to move a distance equal to how much fuel is in the tank.

Balancing coil gauges, on the other hand, use competing magnetic pull from two small electrical coils to move the needle one way or the other. The difference in current flow, and therefore magnetic pull, between the two coils is determined by how much resistance is created by the sending unit in the fuel tank. Resistance from a sending unit also determines the amount of heat created in a thermostatic gauge.

The only practical difference between these gauges is the balancing coil gauge needs an additional wire run from it to a chassis ground. No big deal.

The sending unit

As the sending unit float moves up and down riding on top of the fuel in the tank, the resistance to current flow between the gauge and chassis ground increases or decreases.

But how much resistance a sending unit creates and whether it creates more or less when the tank is empty varies. Sending units in some older GM vehicles, for example, create maximum resistance when the tank is full. Older Ford and Chrysler vehicles are exactly the opposite. Even if a sending unit creates resistance in the correct way for a gauge to work, it needs to operate within the same range the gauge is calibrated for.

The good news is either type of gauge, thermostatic or balancing coil, will work with a sending unit as long as the resistance range is compatible.

Diagnosing the fault

A couple of simple tests will determine whether the gauge, the sending unit or both are faulty.

To test the gauge, disconnect the wire leading to the sending unit and connect it to a good chassis ground. Or disconnect it at the gauge and run a separate wire from the gauge terminal to a ground. (The sending unit wire terminal on the back of the gauge will likely be marked with an “S.”) Turn on the ignition key and ensure voltage is available at the other gauge terminal (likely marked with an “I”). For gauges designed to work with low resistance when the tank is full, which is the most common arrangement, the needle should move all the way to the full position. If it doesn’t move, the gauge needs to be replaced.

The best way to test a sending unit is to remove it from the tank. Using a multimeter set to read ohms (resistance to current flow), place the positive lead on the connection terminal and the negative lead on the body of the sender. Move the float up and down to see if there is continuity (current can flow) and note what the resistance range is. If the sending unit is working, knowing the resistance range measured by the multimeter will be important in order to match it to any new aftermarket gauge.

It is possible to do a limited test on a sending unit without removing it from the tank. Just remove the wire leading to the gauge and attach the leads from the multimeter the same way as before. If the sender is working there will be a resistance reading. And if you know approximately how much fuel is in the tank, you can make a good guess at the resistance range.

For example, if the tank is half full, you get a reading of 45 ohms and you’re working on an older GM truck, you probably have a 0-90 ohm sending unit.

If the multimeter reading shows the sending unit has no continuity (won’t let current flow through it), the sender is faulty.

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The metal shrinker-stretcher basically does exactly what its name implies: shrink or stretch metal. It does this with two sets of jaws. As each one clamps down it also either compresses the metal or pulls it apart, which creates the shrinking or stretching action. In this way, the tool actually changes the thickness of the steel, which makes part of the piece longer or shorter depending on which jaw set is used. The dimensions of the unworked section remain the same, so the lengthened or shortened sections created by the tool cause the metal to deform into a curved shape. It’s useful for creating curved corner pieces of sheet metal.

The pictures demonstrate what the shrinker-stretcher does to a flat piece of steel with a single 90-degree bend in it. By using the “stretch” jaws, the outside of the flange is lengthened, creating a curved shape. Using the shrinker side of the tool would cause the metal to “shrink,” forming an arc in the opposite direction.

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The trouble is, removing and reinstalling some tightly-fitted replacement parts can sometimes be difficult. But there is an easy way to help many parts easily slide into position: simply heat or cool them. An extreme change in temperature causes a part’s dimensions to change. A lot of heat makes things expand. Conversely, as you’d expect, cooling will cause them to contract. Often that subtle change in dimension is enough to allow a part to simply drop into place, which prevents having to beat on it with a hammer and possibly damage it.

In this Shop Class example, cooling new suspension bushings that had to be installed in a set of replacement leaf springs makes for a good demonstration of that effect. The bushings are designed to stay firmly in position in the end of each leaf spring and not move, which prevents any wear to the leaf spring.

Before attempting to install the bushings, we placed them in a deep freeze for a couple of hours. It was then possible to tap them part way into position with a soft-faced hammer. Next, a small hydraulic press was used to push them all the way home. All the bushings were easily seated this way, without damage.

The press probably could have pushed the bushings into position without cooling them down, but it would have taken much more force and increased the risk of damage to the thin metal sides of the bushings, which have a rubber centre.

But if a part needs to expand in order to get it to seat in position, such as the ring gear on an engine flywheel, be careful to apply the heat as evenly as possible to avoid distortion.

If a hand-held torch is used, keep it moving around the part to avoid uneven heat build up in one area. That will reduce the distortion risk. An old mechanic’s trick is to use a portable propane space heater to do the job. Putting the part in front of the heater will allow a broader flow of heat than a small torch supplies to blow over the entire part at once (unless the part is really large), heating it evenly. Putting the part in an oven also achieves the same result, applying heat evenly.

Heating one part while cooling its mate magnifies the affect and makes installation even easier, although it’s often impractical to do that because one component is frequently part of a large housing.

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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.

• Read more: How to pick a replacement carb
• Read more: Carburetion: It’s all about the air to fuel ratio

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.

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.

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.

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.

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.

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