Small Engine Carb Basics
Article by Mark Trotta
The function of a carburetor is to mix a small amount of gasoline with the right amount of air so that an engine runs properly. The vacuum that a running engine makes pulls air in from carburetor, which mixes with gas drawn from the fuel tank.
A five-horsepower or less small engine will have one of two different style carburetors, a diaphragm-type or a float-bowl type.
One of the simplest small engine carburetors is the diaphragm carb. Mounted above the gas tank, a diaphragm type of carburetor has no fuel bowl or float.
Pulse Diaphragm Carburetors
The "pulse" style diaphragm carburetor has been around for nearly 50 years, and relies on vacuum to pull gas into the engine. The most common style has a single tube drawing fuel up from the gas tank below it. A second type has two down-tubes on the carb.
Read: Briggs Tank-Mounted Carb Cleaning And Repair
As the name implies, a float type of carburetor has a float inside a fuel bowl. Reserve gas is held in the bowl. When throttled, gas in the float bowl drops, as does the float, which allows more fuel into the bowl. When the bowl is full, the float rises to the top, preventing any more fuel from entering.
Float-type carbs don't pump fuel, they only regulate it, so they depend on gravity for fuel. For this reason, a float-bowl carburetor will usually be mounted below the gas tank.
Larger engines, like those found on lawn tractors, are often equipped with a fuel pump. This allows the gas tank to be mounted away from the motor.
Small Engine Carb Problems
The majority of small engine carb problems is due to bad gas. Quite often, a simple carburetor disassembly and cleaning gets the motor running again. Other problems can be more serious, and may require a carburetor replacement.
Start With Basic Troubleshooting
After six months of non-use, my Briggs-powered pressure washer wouldn't start. I checked for spark, which was good, and it started and ran OK before it sat, so I knew it was a fuel problem.
Because of inactivity, the gas went bad and clogged the carburetor. This is a common problem on small engines. And it probably wouldn't have happened had I been running non-ethanol gas.
Engine surging is when the motor is at idle speed, then decelerates but doesn't stall, and idles up again. The surging just repeats over and over the entire time the engine is running. This is very common on lawn mowers.
When a small engine surges, it's telling you something. The culprit is usually a small piece of debris stuck in the carburetor. Engine surging is most always a carburetor problem. The solution is to remove the carb, clean all passages, and reassemble.
Keep in mind that a gas engine is basically an air pump, so anywhere there is a chance for an air leak, it's a potential trouble area. Loose intake manifold bolts or a worn-out carb to manifold gasket will allow unwanted air to be sucked into the engine. This disrupts the vacuum necessary to pull the air/fuel mixture from the carburetor, plus the excess air throws the air/fuel mixture out of balance.
To check where a vacuum leak may be, spray carburetor cleaner (or penetrating fluid like WD40) on the suspected area while the engine is running. If there is a leak, the liquid spray will displace the leaking air, and the engine idle will go up noticeably.
Rebuilding A Carburetor
Before you start, buy a carburetor kit for your make and model engine.
Take the carburetor off the engine, but before disassembly, spray it down with carburetor cleaner if dirty. All that's usually really required is cleaning and reassembling, and using new gaskets from the carb kit. Float carburetors will require the additional step of checking and adjusting the float height.
NOTE: small engine carburetors have small parts that easily get lost. Clean an area to work on, where parts can be found if accidentally dropped.
Soak the Carb (And Parts)
If you need to rebuild a really old and dirty carburetor, invest in a gallon can of Berryman's Chem-Dip. This stuff will dissolve the old dried-up remnants of fuel and remove any sludge. Be careful though, it will also eat plastics, puff up gaskets, and remove paint.
Shop: Berryman Parts Cleaner with Basket
If you have a float-style carb, you can soak the body and float bowl, but not the float.
After soaking, a little brushing from a nylon brush will help clean things up. Rinse the carburetor and parts with water and dry with compressed air.
On really dirty carburetors, you'll need to run something through the small passages to clean out the stubborn junk. You'll be surprised at how much stuff comes out of the ports.
Shop: Carburetor Cleaner Brushes
To get a good cleaning without soaking the carb in Chem-Dip, spray some carb cleaner (like Gumout) down into the fuel wells enough to make it puddle, then let it sit overnight. Rinse the carburetor and parts with water and then dry with compressed air before reassembling.
Most carburetors will have an adjustable air-mixture screw with a slotted head (some have a Phillips head). With the motor running, turn the screw in (clockwise) until the engine almost stalls. From there, turn the air mixture screw out (counter-clockwise) about 1-1/4 turns. With a little experience, you can hear when the mixture adjustment is correct.
Is Foam Supposed To Be In My Gas Tank?
While rebuilding an old 5-hp Briggs engine, I noticed the gas tank had foam in it. I didn't think anything of it, so I went ahead and re-used it.
After the motor was together and running, the engine died after several minutes of running. Turns out that old pieces of foam in the tank were breaking off and getting sucked into the carburetor.
Remove Or Replace Gas Tank Foam
The foam on the inside of the tank is designed to keep gas from sloshing around. It was supposed to be a safety feature, but isn't really needed.
To prevent this from happening again, I removed all the foam from the gas tank, mostly by using a pair of needle-nose pliers and pulling it out in chunks. Then I had to disassemble, clean, and reassemble the carb.
Should I Remove The Governor On My Small Engine?
A properly adjusted governor maintains a steady engine speed regardless of changes in the terrain, and any other conditions that would increase the work load on the engine. Unless you're building a race kart motor, I would recommend leaving it in place.
All gas engines need some type of air filter to keep dirt out of the carburetor, and small engines are no different. With the exception of initial break-in and troubleshooting, always run your engine with an air filter.
Air filters should be checked periodically, and cleaned or replaced when dirty. This will help keep your small engine running it's best. If so equipped, fuel filters should also be replaced.
On float-style carbs, the most popular upgrade is swapping the standard jet with a larger jet. This goes hand-in-hand with a less restrictive exhaust and air filter.
Read: Small Engine Performance
Replace your gas filter at least once a year.
Briggs Tank-Mounted Carb Cleaning And Repair
Tecumseh Carburetor Replacement
Nikki Carb Rebuild
Best Fuel For Small Engines
Small engine carburetors have small parts that easily get lost. Clean an area to work on, where parts can be found if accidentally dropped.
Using a standard work bench or heavy duty work bench makes it easy to disassemble a carburetor and organize all the small parts.