How much frustration and arm fatigue from pulling on starter ropes those small engines dish out this spring depends a lot on how well they’ve been maintained. And importantly, how they were put to bed last fall.

The instability of modern gasoline is often the cause of small engine woes after long periods of inactivity. Leave some in the tank and carburetor for a few months and that engine will likely refuse to start. So if your small engine refuses to cooperate this spring, here’s a look at some of the basic troubleshooting steps necessary to get it running again.

It’s often said the eyes are the windows to the soul of a person. When it comes to a small engine, the spark plug might be the window to its soul — figuratively speaking. If a small engine won’t fire, the best first step is to pull out the plug and take a look at it. Its condition will often suggest the source of the problem and the overall condition of the engine as well.

If the engine has been cranked over several times and the plug is bone dry, that suggests no fuel is reaching the cylinder, which is the most common problem for small engines that have been sitting for a while.

Of course, always check for the simplest things first. Is there fresh fuel in the tank? Disconnect the fuel line at the carb to make sure fuel is getting to it. Pulling off the carburetor float bowl will reveal if there is fuel inside it. Some float bowls have a drain. If so, just open that. If fuel is making it that far but not into the engine, the main jet in the carb is probably gummed up from old, evaporated fuel.

Diaphragm carbs on things like weed trimmers and chains saws are different, though, and don’t have a conventional float bowl. If there is fuel at the inlet, it’s clear the carb isn’t functioning properly.

But in either case pulling off the carb and cleaning it with carb or brake cleaner and some gentle blasts of compressed air will often solve the problem. If not, the carb will need to be fully disassembled and put into a cleaning tank.

Having a carb kit containing new gaskets on hand will sometimes be necessary to get it properly buttoned back up.

A very wet spark plug indicates the engine is probably flooded. Wipe the plug dry, or better yet replace it, turn the engine over a few times to purge any excess fuel, reinstall the plug and you could be back in business. Before torqueing the plug back in, though, it’s easy to double check that the ignition system is supplying spark. Just rest the plug along the cylinder head to ground it and turn the engine over. (Make sure it’s well away from the opening. Fuel being expelled from the cylinder could be ignited by the plug and turn your lawn trimmer or other engine into a momentary flamethrower.)

If there is no spark, try replacing the plug. If that doesn’t help, check that the air gap between the magnet on the flywheel and the ignition and trigger coils is correct, as a rule of thumb that’s going to be somewhere around 12/1000 of an inch. If you don’t have a feeler gauge handy, that’s about the thickness of a standard business card. Coil positions can be adjusted with a couple of screws to correct the gap.

If there is still no spark, the coil pack may be at fault.

Technology hasn’t overlooked the small engine industry either, and a solid-state electronic module that houses a capacitor controls some ignition systems. If there is voltage going into the module but not coming out, replacing it is the only alternative.

To keep two-cycle engines running properly, it’s important to get the fuel-oil mixture right. If the manual calls for a 50:1 mixture ratio, make sure that’s what you blend. In this case if a little is good, a lot isn’t a lot better. Too much oil in the fuel will form excessive carbon deposits inside the cylinder. And that can raise the compression due to the amount of build up on the piston, which reduces the effective size of the combustion chamber.

Eventually, that can kill an engine. Basically what happens is the excessive heat from too much compression ignites some of the fuel prematurely. Eventually the spark plug also ignites some of the fuel, and as the two flame fronts collide it creates a detonation that can damage the engine over time.

If the engine is going to be stored unused for several months, adding fuel stabilizer is a must — or drain the tank and run the engine until the carb empties out and it stops. But for small two-cycle engines with a diaphragm carburetor, it might be best to leave stabilized fuel in it. That can help prevent the delicate diaphragms from drying and cracking over time.

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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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There are companies that manufacture new carbs, designed to work for a variety of applications (think Edelbrock or Holley for example); however, you’ll need to know your engine’s CFM (cubic feet per minute) requirements in order to pick the right one.

Just because an alternative carb will bolt onto the manifold and the diameter of the air horn looks right, it doesn’t mean it’s right for the job. It needs to be rated for the correct CFM of air intake and have the right size jets to supply a correct air-fuel mixture for the entire operating range of the engine.

• Read more: How to overhaul your own carburetor
• Read more: Carburetion. It’s all about the air to fuel ratio

To determine the engine’s CFM requirements, you’ll need to do a little math. This will determine the size of carb adequate to provide enough air flow for 100 per cent volumetric engine efficiency. Here is the formula to use when figuring that out.

CFM = (cubic inch displacement x maximum r.p.m.) divided by 3456

For example, here is how that equation works for a 350 cubic inch engine with a maximum r.p.m. of 4,500:

(350 x 4,500) divided by 3456 = 455.72 CFM

If you only know the displacement of an engine in litres, multiply that number by 1,000 to get cubic centimetres, then divide by 16.39 to convert to cubic inches. For example, a five litre (5,000 CC) engine converts to cubic inches this way:

5,000 CCs divided by 16.39 = 305 cubic inches.

The post How to pick a replacement carb appeared first on Grainews.

No matter which way air flows through it, a carb must blend that air with fuel to create an explosive vapour. The ratio of atomized fuel to air is essential in creating the correct blend, allowing the engine to run properly. Liquid gasoline won’t ignite properly or at all in the cylinders. If there is too little air, the mixture is said to be “lean.” Too much fuel creates a “rich” mixture.

• Read more: How to overhaul your own carburetor
• Read more: How to pick a replacement carb

Strictly speaking, a “Lambda” fuel ratio (14.7:1 air to fuel) is required for a complete fuel burn. But engines run best on varying ratios depending on their operating speeds. For example, a richer mixture is required for starting, because fuel doesn’t vaporize well in a cold engine and the speed of air moving into the cylinders is relatively slow. Closing the choke valve on a carb creates that rich condition and allows the fuel-air blend to lean back out as the engine reaches operating temperature.

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