Category: small-engines

The A to Z of Small Engine Carburetors

Your mower’s engine won’t start. You decide to take a look around online for a possible solution and conclude that there may be a problem with your carburetor.

Then, you realize you have another problem. You don’t understand the terms and parts associated with a carburetor. How are you going to explain or try to fix your problem?

It’s important to be familiar with the details of your equipment for the purposes of both maintenance and operation. Below, you will find a list of some of the basic terms and parts related to small engine carburetors. But first, what exactly does a carburetor do?

Put simply, a carburetor is a device that blends fuel and air into a mixture that is delivered to an internal combustion engine for energy. Such a task requires a complex series of parts and processes. To learn more about the actual operation of a carburetor, check out this article.

Without further ado, the A to Z of small engine carburetors:


  • Accelerator Pump – Liquid gasoline is denser than air and as a result, reacts more slowly to force than air does. When the throttle is opened, airflow is faster than the flow of fuel which leads to an oversupply of air, causing an engine misfire. The accelerator pump forces extra gasoline through a jet to balance the amount of air and fuel.
  • Airflow - The flow of air entering the carburetor to be mixed with fuel.
  • Air-Fuel Mixture - The final product of the carburetor: a precise mixture of fuel and air that is delivered to the engine to be used for combustion.
  • B

  • Butterfly Valve – A type of valve that regulates and controls the airflow coming into the carburetor. It can be rotated to a position that almost completely restricts all airflow, or to a position that barely blocks any airflow at all.
  • C

  • Choke - If the engine is cold, more fuel than air is needed in the mixture to start the engine and run it until it is warm. A choke restricts the flow of air at the opening of the carburetor. This creates a stronger vacuum effect, which will pull extra fuel into the mixture.
  • Cold Start - An attempt to start an engine that is cold. Cold temperatures cause fuel to vaporize less easily and condense on the walls of the intake manifold. The carburetor needs to compensate by creating a richer mixture with more fuel.
  • Combustion - Burning an air-fuel mixture to create heat energy that is used to apply force to a component of the engine.
  • D

  • Detonation - Premature ignition and explosion of the air/fuel mixture. This will interfere with the cycle of the engine and possibly even damage the engine. Also known as knocking.
  • E

  • Engine - Machine that converts energy into mechanical energy that can power something. Internal combustion engines convert fuel into heat energy.
  • Emissions - Air pollutants that introduce harmful chemicals into the environment. Many modern carburetors are required to maintain low amounts of emissions.
  • F

  • Flooded Engine - This occurs when the air-fuel mixture given to the engine is too rich and cannot be ignited.
  • Float Chamber - Also known as a bowl, this component contains an amount of fuel that is ready to be used by the carburetor.
  • Fuel Pump - Replenishes the float chamber with fuel when necessary.
  • G

  • Gas Tank - When the float chamber is reduced to a certain level of fuel, it is refilled with fuel from the main gas tank.
  • Gasoline - The most common liquid fuel that is mixed with air for use in an engine.
  • Gummed Up - Old gas or deteriorated metal/rubber that blocks up or obstructs the carburetor's processes.
  • H

  • Heat Soak - Engine problems brought on by the fuel in the float chamber being exposed to excessive heat.
  • Heat Deflector - These help to minimize the effect known as “heat soak”.
  • I

  • Intake Manifold - The part of the engine that supplies the air-fuel mixture created by the carburetor to the cylinders of the engine.
  • Idling - When an engine is running only to stay running, not to perform any actions.
  • J

  • Jet - A very precisely sized opening in the fuel path that helps adjust the flow of fuel by introducing more fuel from the float chamber into the air-fuel mixture.
  • K

  • Knocking - Also known as detonation. Premature ignition and explosion of the air/fuel mixture. This will interfere with the cycle of the engine and possibly even damage the engine.
  • L

  • Liquid Fuel - Most commonly gasoline, this is what is stored in the float chamber and mixed with air for use in the engine.
  • Lean Mixture - An imbalanced air-fuel mixture caused by an oversupply of air, which makes the engine misfire.
  • M

  • Metering System - Ensures that an accurate amount of fuel is brought into the mixture for combustion.
  • N

  • Needle Valve - Needle Valve – A small valve that allows flow to be precisely regulated.
  • O

  • Open Throttle Circuit - As the throttle valve is opened more, air speed is increased in the venturi. This creates lower pressure and brings fuel into the airstream.
  • P

  • Priming - Getting the engine ready to start.
  • Piston Pump - Used to either move liquids or compress gases.
  • Q

  • Quantity of Air-Fuel Mixture - The amount of air that enters the engine is controlled by the throttle valve.
  • R

  • Rich Mixture - An air-fuel mixture with an abundance of fuel. This can be easier to ignite and is often necessary for cold starting. Can cause flooding.
  • S

  • Spark Plug - A device that delivers an electric current from the ignition system to ignite the compressed mixture of fuel and air.
  • Scrutinizing Spark Plugs - By removing the spark plugs, you can identify which type of mixture you have. Black and sooty plugs mean your mixture is too rich. White and light gray plugs mean you have a lean mixture.
  • T

  • Throttle Valve - A valve at the bottom of the carburetor that, when open, allows more air and fuel to flow into the engine, and when closed, allows less air and fuel into the engine.
  • U

  • Updraft Carburetor - A type of carburetor in which air enters from the bottom and exits out of the top, typically used with older engines.
  • V

  • Venturi - A narrow kink in the pipe of the carburetor that causes the air flow to speed up and, as a result, a drop in pressure.
  • Valve - A device that regulates and controls the flow of a liquid.
  • Viscosity - How “thick” a liquid is.

Now that you know some of the basic terms associated with carburetors and engines, you’re ready to better understand carburetor maintenance and operation. If you want to know more about carburetors, check out the following:

Carburetor Rebuild or Replace? – How to Choose

Dirt, bad gas, varnish, and corrosion are popular evils that can clog and wear down the carburetor on your lawn mowers, chainsaws, and other small engine equipment.

If you’ve diagnosed your equipment and found carburetor problems to be the root of your engine troubles, you have two choices – Repair or Rebuild.

Your choice whether to rebuild or replace the carburetor depends on your technical skills, budget, time, and the extent of damage to the carburetor.

Always check the price of a new carburetor. Sometimes, just a few more bucks spent on a new carb assembly can save you the workload of overhauling and cleaning your current carburetor.

Rebuilding the Carb

A carburetor rebuild/overhaul kit contains replacement gaskets and other necessary components to rejuvenate your carburetor.

Cost $10 – $60 (Do-It-Yourself)
$70/hr plus $10-$60 in parts (Shop Repair Bill)
Time 1.5 – 2 or more Hours
Difficulty Mechanically Skilled
Task Summary     Remove, Disassemble, Clean, Replace Components, Rebuild, Reinstall

Replacing the Carb

Replace the carburetor if there is extensive corrosion or significant damage that simple cleaning or rebuilding would not fix.

Cost $20 – $200 (Do-It-Yourself)
$70/hr plus $20-$200 in parts (Shop Repair Bill)
Time 30 – 45 minutes
Difficulty Moderate
Task Summary     Remove Old, Install New Carburetor Assembly

*Tip: Locate your equipment’s model number and serial number, plus the carburetor model number (typically stamped onto the carburetor body). You will need this information to find a correct replacement carburetor or rebuild kit for your engine. Locate Your Model Number »

Find a Replacement Carburetor or Rebuild Kit »

How to Properly Use Ethanol Fuel in Power Equipment

Ethanol Fuel (E10) is everywhere! Now almost all gasoline contains this blend of 90% gas and 10% ethanol.

The positive side of Ethanol fuel is that it reduces exhaust emissions and helps reduce the dependence on foreign oil.

For automobiles like cars, E10 won't do much harm, except reduce your gas mileage by a few mpgs (as some claim).

But for Outdoor Power Equipment, ethanol fuel can be a damaging and costly issue, if NOT Used Properly!

What are the Effects of Ethanol?

  • Over time, Ethanol will start to absorb water, which is never good to have in your fuel tank. This will lead to poor engine performance.
  • Ethanol is also a destructive solvent. It will slowly dissolve the parts of your fuel system, creating gummy/varnish deposits that clog the carburetor and internal engine parts.

Ethanol (E10) PROPER USE Guidelines:

  1. Always add a Fuel Stabilizer as soon as you get fresh gas.
  2. Shake the fuel mixture in your gas can to stir up any small amounts of water that may have collected at the bottom.
  3. Store fuel in an air tight gas container (no vent caps).
  4. Don't store fuel/oil mixtures with a stabilizer longer than 90 days, just to play it safe.
  5. Keep gas caps secured on tanks at all times between fill ups.
  6. Long-Term Storage: Completely drain the fuel tank and run the unit dry. Also drain the carburetor bowl where some left over gas will collect.

*The Ethanol Golden Rule:

Only buy a 30 Day Supply. Keep your Fuel Fresh! And never run stale gas in your equipment.

Jack's Safety Tips: Before servicing or repairing any power equipment, disconnect the spark plug and battery cables. Remember to wear appropriate safety glasses and gloves to protect against harmful chemicals and debris. View our Disclaimer.

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How to Service Your Spark Plugs

Spark plugs are a very important part of the internal combustion engine and need to be kept in good condition to allow for optimal performance. The electrodes of the spark plug need to be kept clean and sharp to create the powerful spark necessary for ignition.

A damaged or dirty spark plug can lead to inconsistent firing which can result in reduced or inconsistent performance. It can also cause an excess of fuel consumption and result in deposits being left on the cylinder head.

The good thing is, spark plugs are easy to repair, maintain, and if necessary, replace. We will explore the steps to inspect and service your spark plugs. With this information, you can then decide whether or not you want to replace your spark plugs.

Inspecting and Cleaning a Spark Plug

  1. Start by disconnecting the spark plug lead from the spark plug. Remove any debris around the spark plug before you remove the plug to keep the debris from getting into the combustion chamber.
  2. Remove the spark plug by using a spark plug socket wrench.
  3. Using a wire brush and spray on plug cleaner, remove light deposits from the plug. If there are still difficult deposits, try using a strong knife to scrape them off. DO NOT use a shot blaster or abrasives to clean your spark plug.
  4. Inspect your spark plug for damage and heavy deposits. If there is any damage, you should replace your spark plug. Refer to our guide on How to Read a Spark Plug to help you determine any issues with your spark plug.
  5. Next, check the gap between the electrodes at the tip of the spark plug by using a spark plug gauge. A typical gap for small engine plugs is 0.030". But, you should always check the specifications for your particular model to be sure.

    Note: Too small of a gap could result in a smaller and weaker spark, leading to an increase in fuel consumption. Too large of a gap could result in misfiring or premature electrode wear.

  6. Finally, reinstall your spark plug and attach the spark plug lead. Be sure not to overtighten your spark plug when you reinstall it.

If you've done all this and still have problems you may want to use a spark plug tester to diagnose whether you have a problem with your ignition system or you are having a spark plug misfire. If you have a spark plug that is misfiring, you should replace your spark plug. Check out our selection of spark plugs here at Jack's.

Jack's Safety Tips: Before servicing or repairing any power equipment, disconnect the spark plug and battery cables. Remember to wear appropriate safety glasses and gloves to protect against harmful chemicals and debris. View our Disclaimer.

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Small Engine Troubleshooting

Your Engine just started acting up and you don't know what to do.

Below is a chart of common Engine problems and some easy troubleshooting to fix the problem you may be experiencing.

This chart is to be used as a guideline for troubleshooting engine problems. For specific engine tuning or recommendations consult your owner's manual.

  • Engine Will Not Crank
  • Cranks - Will Not Start
  • Runs Rough On Low & Will Not Accelerate
  • Backfires on Full Throttle
  • Electrical system problem
  • Engine not running (After Hot Test)
  • Stops on High Idle
  • Hot - Will Not Restart
  • Cannot Reach High Idle RPM (No Load)
  • Starting Hints - Command Retractable Start Engine

  • Battery lead connections are loose.
  • Battery charge low or discharged.
  • Battery amperage too low.
  • Starting leads reversed.
  • Fuse in wiring harness blown.
  • Keyswitch wired wrong.
  • Safety switch malfunctioning.
  • Wiring harness incorrect.
  • Wiring harness wired wrong.
  • Starting cable connectors loose.
  • Connectors corroded or worn.
  • Starter/Solenoid malfunctioning.
  • Electric clutch air gap wrong.
  • Drive pulley against engine PTO face.
  • Drive belt wedged between idler pulley.
  • Drive belt wedged between belt guide.
  • Drive belt tension adjusted too tight.
  • Starting torque too high for starter.
  • Automatic compression release inoperative.
  • Combustion chamber flooded with oil/gas.
  • No oil in crankcase causing seized rod.
  • Drive shaft coupling forcing engine shaft against internal thrust face eliminating crankshaft end play.
  • Transmission brakes too tight.

  • Fuel tank empty.
  • Fuel hose kinked, pinched.
  • Fuel filter clogged.
  • Fuel valve shut off
  • Fuel solenoid inoperative.
  • LPG Regulator not opening.
  • Water in fuel, stale fuel.
  • Wrong type fuel (Diesel).
  • Throttle control in stop position.
  • Choke not fully closed.
  • Choke on, flooding hot engine.
  • Power take off clutch on.
  • Safety interlocks inoperative.
  • Spark plug wire disconnected.
  • Spark plug improperly gapped.
  • Wrong type of spark plug.
  • Wiring harness not connected.
  • Wiring broken, loose or wrong.
  • Transmission not in neutral.
  • Electrical clutch not adjusted.
  • Cranking speed too slow to start.
  • Spark plug loose.
  • Low or no compression.


  • Spark plug gap incorrect.
  • Engine not properly warmed up.
  • Running out of fuel.
  • Low idle set too low (Below 1000 RPM).
  • Low idle fuel setting too lean.
  • Electric clutch dragging.
  • Parasitic load on PTO shafts.
  • Crankshaft end play zeroed drive.


  • Water contaminated fuel.
  • Running out of fuel.
  • Fuel mixture too rich or too lean.
  • Carburetor fuel solenoid shorting.
  • Throttle or choke improperly set.
  • Dirty carburetor needs cleaning.
  • Kill switch contacting intermittently.
  • Keyswitch malfunctioning.
  • Safety interlock(s) malfunctioning.
  • Faulty fuse in unit control box.
  • Leaking, worn valves in engines.


Charges and gradual discharges

  • Improper ground causing battery to lose charge when accessories activated.
  • Faulty battery will not hold charge.

Will not charge

  • Regulator-Rectifier failed.
  • Regulator-Rectifier not grounded.
  • Flywheel magnet not charged.
  • Fuse blown or circuit breaker open.
Blowing fuses

  • Regulator-Rectifier burned out.
  • Leads pinched causing shorting.
  • Alternator stator shorted.

Electric clutch will not engage

  • Wrong Regulator-Rectifier used.
  • Wiring wrong or not grounded.

Engine runs on when switched off

  • Ignition grounding terminal or ground lead not connected at ignition module.
  • Faulty switch.


Smoke rises out of top of horizontal shaft engine or flywheel screen on vertical shaft engine.
  • Paint or oil burn off.
  • Battery cables reversed causing charging stator to short out.
  • Defective regulator-rectifier causing charging stator to short out.
  • Electric clutch - No running clearance.


  • Running out of fuel.
  • Running out of oil in crankcase.
  • Carburetor solenoid malfunctioning.
  • Keyswitch malfunctioning.
  • Safety interlocks malfunctioning.
  • Spark plug lead loose.
  • Fuse blown in control box.
  • Carburetor out of adjustment.
  • LPG Regulator malfunctioning.
  • Loss of vacuum to LPG Regulator.
  • Tight drive belt causing crankshaft bearing to seize.


  • Overheated - Clogged in cooling fins.
  • Overheated - Blocked shrouding.
  • Closed choke flooding engine.
  • Throttle not in midway position.
  • Throttle left in stop position.
  • Carburetor solenoid malfunctioning.
  • Fouled spark plug.
  • Kill lead shoring out.
  • Ran out of fuel.
  • Lost compression.
  • LPG Regulator not functioning.
  • Vacuum too low to open LPG Regulator.


  • Choke not completely opened.
  • Throttle control cable clip loose.
  • Throttle control improperly installed.
  • Fuel filter or line clogged.
  • Fuel inlet line too small.
  • Fuel running low.
  • Fuel tank too far from engine.
  • Fuel pump malfunctioning.
  • Gravity feed tank below carburetor.
  • Foreign material in carburetor.
  • Carburetor out of adjustment.
  • Spark plug gapped wrong causing intermittent firing.
  • Drive system binding.


Dual control cable instructions

  1. Close choke.
  2. Set throttle to full.
  3. Pull starter rope to set engine on compression.
  4. Return starter rope, then give a full, steady pull.
  5. When engine starts, return choke to full open immediately.
  6. Position throttle control to desired setting..
Single control cable instructions

  1. Move throttle/choke lever to choke position.
  2. Pull starter rope to set engine on compression.
  3. Return starter rope, then give a full, steady pull.
  4. When engine starts, move the control lever to the desired setting.

Jack's Safety Tips: Before servicing or repairing any power equipment, disconnect the spark plug and battery cables. Remember to wear appropriate safety glasses and gloves to protect against harmful chemicals and debris. View our Disclaimer.

Featured Parts and Products: