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How to adjust the carburetor on a Briggs and Stratton lawnmower, step by step

There can be various reasons why you want to adjust your carburetor on your Briggs and Stratton lawnmower. It is typically bolted to the side or top of a lawnmower engine, depending on the type you have. The carburetor ensures a proper mixture of fuel and air for the engine cylinder where combustion takes place. Over time the carburetor can start working less optimal, and the air-fuel ratio may get too lean or too rich. The timing may also get retarded or advanced. All these problems require that you adjust the carburetor to its peak capability. The following article will take you through the necessary steps to do this.

How to adjust the carburetor on a Briggs and Stratton lawnmower, step by step:

  • Step 1. Gather the necessary tools
  • Step 2. Prepare the Lawnmower
  • Step 3. Adjust the idle speed mixture
  • Step 4. Adjust the high-speed mixture
  • Step 5. Adjust carburetor choke linkages
  • Step 6. Check the carburetor connections and engine speed
(CC BY 2.0) by ndrwfgg

The carburetor can be adjusted by tightening or loosening some screws. Also, checking the inlet and outlet lines is necessary.

The factory adjusts the carburetor on your lawnmower such that it gives optimal performance. With time, the engine RPMs, mixture settings, and speeds may get disturbed. Hence, the carburetor that supplies the mixture has to be adjusted. Whether the mower is idle or working under heavy load, the mixture composition and the engine RPM vary. When adjusting the carburetor, one has to set all these mixture settings and the condition of fuel lines. The latest mowers are mostly equipped with a limiter cap around the idle adjustment screws to adjust carburetor mixtures’ settings. Adjusting the screws will fix your carburetor.

If you are new to this particular technical task, follow the article step-by-step as we go through the technicalities.

General Briggs and Stratton carburetor troubleshooting guide

Suppose your engine runs roughly at high speed or idle. In that case, there is a need for a carburetor’s adjustment on a Briggs & Stratton lawnmower to improve the rough-running condition. This blog post will provide you with the troubleshooting guide for the task.

● Step 1. Gather the necessary tools.

All sorts of works need a lot less effort if you use the right tools for the job. Adjusting the carburetor is no exception. It would be best if you have the following tools:

  • A pair of safety gloves
  • Set of Pliers
  • Screwdrivers
  • Wrench
  • RPM gauge or tachometer
  • carburetor cleaner

● Step 2. Prepare the Lawnmower.

Start the riding mower engine and let the engine warm-up for five to seven minutes. By pulling the choke lever to the slow position, turn your mower in a turtle shape bent form. Note the engine sound and noise at this idle position. Please give it a run and again note the sound and speed of the engine. Now, turn the ignition system off and let it cool down. Then grab the tools required for the operation. Set the parking brakes in position and set the drive gear to a neutral position so that mower does not roll.

Open the casing of your riding mower and locate the carburetor adjusting screws. The carburetor is near the air filter assembly. There you have the two idle adjustment screws that may be situated on top or at the carburetor side.

● Step 3. Adjust the idle speed mixture.

Few steps are required to fix your carburetor’s idle speed mixture. Idle speed mixture is the air-fuel mixture the carburetor supplies when there is no load on the machine. In this position, the engine works only to keep the machine running. Follow the following steps:

– Step1: Locate the carburetor:

Locate the air filter casing and air filters mainly on the top of the riding lawnmower engine. Unbolt the nut on the air filter cover to reach the filter. Remove the casing and then set all of these components like nuts and screws to the side for re-installation later on. Then, spray the carburetor cleaner onto the carburetor to wipe out any debris, dirt, and residual gunk that may have gathered.

– Step 2: Find the idle adjustment screw on your carburetor:

The idle screw is a simple screw located on the carburetor’s side. The 2 screws are located opposite each other and mostly labeled. Now, rotate the idle screw in a clockwise direction by a screwdriver in such a way that the needle on the end of the screw hardly touches the casing of the carburetor. Then turn the screw 1.5 turns counterclockwise. This is the ideal required tightening.

– Step 3: Locate the main jet adjustment screw:

Locate the main jet adjustment screen. Often it is located at the bottom of the float bowl, but not all carburetors have it. To ensure a good mixture, some carburetors have a “float chamber or float bowl” containing a quantity of fuel at near-atmospheric pressure. As fuel is drawn for the bowl area, the float drops, opening the float valve. Fuel pump pressure causes the bowl to refill, floating the valve closed. It serves as a constant-level reservoir of fuel that is metered into the passing flow of air.

– Step 4: Adjust the main jet screws:

Turn the main jet screw clockwise until the needle on the end of the screw touches the base and then turns the screw counterclockwise 1.5 turns to a max of 2 turns. This will first give the lean mixture and then start getting rich. The RPMs are best suited for this position.

– Step 5: Start your engine:

Start your mower engine and let it run for about 5 to 7 minutes at half throttle. For now, do not cover the engine with a filter casing.

– Step 6: Check if you have made the right carburetor adjustment:

Now, you need to check the engine sound. Turn the idle screw a bit clockwise and a bit anti-clockwise till you find the prime spot. The engine sound is quite relaxed when it is in an idle setting.

– Step 7: Attach the casing:

Place the air filter casing and tighten the connections.

● Step 4. Adjust the high-speed mixture.

The high-speed mixture is the air-fuel mixture provided by the carburetor when the lawnmower is under load. In this condition, engine RPMs are increased. The steps required to adjust the high-speed mixture are as follows:

– Step 1: Locate the high-speed adjustment screw:

They are mostly located on the opposite side of the idle adjustment screw. Not all carburetors have this adjustment option. It is based on the manufacturer.

– Step 2: Remove the filter covering:

Eradicate the air filter casing and the filter to locate the carburetor’s main body.

– Step 3: Adjust the high-speed screws:

Turn the high-speed screw clockwise with a screwdriver until the needle on the end of the screw touches the seat. Turn the screw counterclockwise 1.25 turns.

– Step 4: Set the throttle speed:

Set the throttle to a fast position and start the mower engine. Turn the high-speed screw first clockwise until the engine slows down, then in a counterclockwise direction until it slows. Figure out the center point where the engine sounds best if your mower does not have an RPM gauge. Move the throttle from slow to fast position, then back to slow to test your adjustment. The adjustment point is mostly around that 1.25 turns vicinity.

– Step 5: Attach the casing:

Place the filter casing and filter on the mower and tighten it with the nut.

● Step 5. Adjust carburetor choke linkages.

Few steps are required to adjust the carburetor choke linkages:

– Step 1: Remove the outer casing of air filters:

Remove the air filter cover, and locate the carburetor’s choke linkage. Move the throttle lever to the fast position.

– Step 2: Loosen the bracket:

Release the bracket holding the casing. Use a flathead screwdriver to loosen the screw on the bracket and release the cable. Do not take the screw entirely out.

– Step 3: Adjust the choke linkages properly:

The choke generally increases fuel flow to the carburetor to start the engine in cold weather. The richer mixture ignites fast to help with starting up. So, you need to adjust the choke setting in terms of how rich the mixture should be. Move the choke cables with your fingers until the choke plate on the carburetor closes. Grip the cables in this position and tighten up the bracket down alongside the cable casing to secure it in place. Move the throttle from fast to slow position, then fast again, and see choke plate movement in the carburetor, i.e., opens and closes. Adjust it in a medium position.

Check the spring connector because it may be why the engine was not giving higher RPMs. If it’s loose due to overuse, try to tighten it using the same screw or a shorter screw. It will generally help you to improve and adjust the carburetor RPMs without any high-profile technical skill.

● Step 6. Check the carburetor connections and engine speed.

When you are done with mixture adjustment, clearing the carburetor flow lines is a key step. Therefore, check the fuel line, air intake, and exit for any dirt, clogs, and debris. Wipe them clean by giving them a petrol wash and drying in the sun. Keep any water away. Now, you can close the components in the same way you opened them. Adjust the tubes back in the carburetor. Turn the ignition on and let it run.

Check the engine RPM with a tachometer. You can further fine-tune the idle if you have a handheld tachometer. Attach your tachometer lead to the spark plug wire. Twist the idle screw until the engine achieves 1700 rpm for an aluminum engine or 1200 rpm for a cast-iron cylinder engine. These are the preferred values. In case you don’t have a tachometer, the steps till now were enough.

Frequently asked questions

1. What causes a lawnmower engine to rev up and down?

If the carburetor is not adjusted correctly, the engine revolutions fluctuate.  A carburetor that is not adjusted correctly is a common cause of poor engine idling resulting in surging problems. Luckily, most of the mowers have two screws that let you fine-tune the carburetor yourself without calling in a professional. One of the screws controls the idle speed, and the other adjusts the idle mixture for your carburetor.

2. Why does my mower engine’s RPM seem to fluctuate, which in return causes rough running of my lawnmower?

Engine hunting and surging issues at idle are usually caused by fuel intake problems, air leaks, or governor system problems. Cleaning and overhauling the carburetor, adjusting the governor, and changing the air filter will solve the fluctuation issue. This will also keep the engine smooth.

3. What are the symptoms if my carburetor is running lean?

When the engine runs slightly lean, the rpm will drop, and the engine will begin to run rough. Your engine will likely still run, but it lacks power and makes a weaker dying sound while running. It will result in jerking motions within the combustion engine’s mechanics, which leads to damage to the engine, such as burned valves, pop and sputter noise produced by the engine. This indicates that there is a higher concentration of air to fuel than there should be.

Final remarks

The carburetor may get out of adjustment with everyday use, resulting in less than an optimal engine and mower performance. Most Briggs and Stratton riding lawn mowers use a gas engine to drive the mower and rotate the cutting blades for grass cutting. A carburetor on the engine provides the gas flow from the fuel tank to the engine. One way to achieve the highest fuel efficiency from your lawnmower is to set the idle to optimal performance. I hope this article offers you a better understanding of adjusting the carburetor on your Briggs and Stratton Lawnmower for an excellent performance.