How to Properly Adjust a Gas Trimmer Carburetor

Despite the high productivity, mobility and unpretentiousness in work, modern lawn mowers are difficult to attribute to complex devices. Over time, any mechanism undergoes wear, and therefore parts and assemblies require special attention of the owner. One of the elements of the trimmer is a carburetor, designed to form a fuel mixture and its supply to the combustion chamber. Increased consumption with the correct proportions of gasoline and oil or a drop in engine power indicate the need to adjust the carburetor of the gas trimmer.

Trimmer carb setting

The initial step in restoring the correct operation of the fuel system is to clean the air filter.

How to Properly Adjust a Gas Trimmer Carburetor

Screw trimmer carburetor

Experts recommend rinsing it in a solution of water with detergents after every 10-15 hours. Direct adjustment of the trimmer carburetor is carried out by three adjusting screws:

  • Fuel feed adjustment screw (“L”);
  • Adjustment screw at high speeds ("H");
  • The screw responsible for idling the motor (“T” or “AT”).

Adjustment of fuel supply at the minimum turns

Before starting the carburetor adjustment of the gas trimmer, it is necessary to warm the engine to operating temperature.

Fuel Adjustment Screws

To do this, start the engine and at idle let him work 10-15 minutes. At the first stage, with the trimmer working, slowly tighten the screw “L” until the beginning of intermittent operation or until it dies. Next, unscrew the screw by ¼ turn and check the speed of maximum speed. If the lawn mower easily and quickly reaches high speeds, this will be the most economical mode. In most models on the market, the fuel screw “L” is turned clockwise, however, there are trim tabs where the decrease and increase in the mixture flow occurs in the reverse order.

Trimmer idle adjustment

Location of carburetor adjustment screws

The screw "T" ("AT") serves as a limiter of the reciprocating movement of the fuel shutter and when it is twisted, the engine rpm increases, and turning it off leads to a decrease in the crankshaft speed. Professionals advise adjusting the idling screw of the lawn mowers so that the ICE works a little faster at minimum speed, but the head with the fishing line or knife remains motionless. If it is difficult to start the trimmer “cold” after the manipulations, then it is enough to tighten the “T” screw a quarter of a turn and try to start the engine again.

Adjustment of the screw of the maximum turns of a lawn-mowing

After restoring the fuel supply at minimum speed and adjusting the idle speed, it is necessary to check the stability of the mechanism at an increased speed.

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Setting at maximum speed

Adjustment of the motor at high speeds is carried out in order to protect against overheating and reduce hand fatigue when pressing the accelerator key. If the internal combustion engine is unstable at maximum speed, then screw “H” must be slowly turned in to ensure smooth and clear operation.

During operation of the lawn mowers, it may be necessary to adjust the fuel feed screw (“L”) for an optimal set of revolutions.

Tips for repairing a gas trimmer carburetor

  1. Regardless of the manufacturer, there is a factory defect of the carburetor parts of the lawn mowers.

Adjusting screw spring

For example, the adjustment procedure described above may not give positive results with an excessively large number of turns at the spring of the mixture feed screw and, as a result, the impossibility of optimally reducing the level of gasoline supplied. To eliminate this, you can independently shorten the spring yourself or purchase a new one in accordance with the instruction manual.

Many owners of lawn mowers when buying oil follow the recommendations indicated there for the preparation of the fuel mixture.

Oil for gas trimmer

So, often the information on mixing gasoline and oil indicated on the bank differs from the instructions in the trimmer’s passport. As a result, a black deposit can quickly form on the spark plug, which leads to a drop in power and an increase in fuel consumption. Therefore, when preparing the working mixture, the recommendations specified in the instruction manual should be followed. In most cases, the proportion for a run-in ICE is 1:25. It is worth noting that many tool owners prefer to cook fuel “by eye”, which will inevitably lead to rapid wear of the piston spit in the event of a decrease in the portion of oil and the formation of soot on the spark plug with an increased amount of lubricating fluid.

Video: How to Properly Adjust a Gas Trimmer Carburetor


  • The correct tips for setting up the trimmer carburetor correctly performed above will help to avoid not only engine overheating and increased fuel consumption, but also significantly increase the life of other parts and components of the tool. Pay attention to the operation of the lawn mowers in all modes and timely carry out regulated maintenance and repair of the tool.
  • The increased fuel consumption and the rapid formation of soot on the lawn mowing candles when using a correctly prepared fuel mixture indicate the need for carburetor adjustment. This describes how to set trimmers with carburetors that have one screw for adjusting the fuel supply.

    Carb adjustment

    Flush the air filter before adjusting the carburetor. It is recommended to rinse it after every 10 hours of operation. After that, you can begin to adjust. The example photograph shows the carburetor of the gasoline trimmer PRORAB-8406. The numbers indicate the adjusting screws that we will use.

    PRORAB-8406 gas trimmer carburetor adjusting screws

    Purpose of adjusting screws:

    1. Fuel mixture adjusting screw.
    2. Idle adjustment screw.
    3. The screw limits the maximum speed.
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    Start the lawn mower and wait until it warms up.

    Fuel adjustment

    Start slowly tightening the fuel mixture adjusting screw until the engine stalls or stalls unless you catch the moment. Then unscrew the screw a quarter of a turn if the engine is not dead, or half a turn if it is dead. Start the stalled engine. Check, by opening the throttle, how the engine accelerates. Continue to loosen screw # 1 a quarter of a turn until the engine starts to confidently gain high revs when progazovka. This will be the moment of the most economical stable engine operation. At the PRORAB-8406 gas trimmer, the adjusting screws are turned clockwise, unscrewed – against, for some other brands the lawn mowing may be vice versa.

    Idle adjustment

    We proceed to adjust the low speed. Screw number 2 restricts the return movement of the throttle lever – by wrapping it, we increase idle speed, unscrewing – reduce it. Adjust so that the engine runs confidently at low speeds and even slightly faster (for a better start), but so that the reel with fishing line or knife does not rotate, and before starting to rotate, there is a sufficient margin of speed. If the engine starts poorly, still increase idle speed.

    Maximum speed limit

    After adjusting the flow of the fuel mixture and low revs, check how the lawn mowing engine will operate at maximum speed. If necessary, limit the maximum speed to the required level using the No. 3 hollow screw. Loosen the locking nut and screw in to decrease the maximum speed or unscrew to increase. After adjustment, tighten the nut. Setting the maximum speed is necessary to protect the trimmer motor from overheating and the ability to use the release key lock to reduce fatigue of the right hand.

    In the future, it may be necessary to adjust the trimmer carburetor. For example, if the engine, when operating at full load, starts to pick up revs poorly, loosen screw No. 1 another quarter of a turn. If it is difficult to start, increase the idle speed. Weather conditions and the quality of the fuel mixture can affect the operation of lawn mowers. The vibration can also make the adjusting screws turn themselves.

    The spring may interfere

    My first attempt to adjust the carburetor of the lawn mowers did not work, as the spring of the fuel mixture adjustment screw prevented it. The factory spring turned out to be too thick in a compressed state and did not give the opportunity to reduce the fuel supply to the required level. I had to pick up another spring and continue adjusting after replacing it.

    Spring mixture adjustment screw

    In the photo above, the factory spring, below the adjustment screw with a selected spring. I am writing about this, because I’m not sure that this is an isolated case, and someone else may encounter the same situation.

    From personal experience

    Lawn mowing or trimmer

    A trimmer is a broader concept than a lawn mow (lawn mow), since it can also be electric. A gas trimmer and a lawn mower are one and the same.

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    Some authors claim that the trimmer is a weaker tool, in contrast to the more powerful lawn mowers, which can be used to mow shrubs. Personally, I have a tool with a maximum power of 1.7 kW and capable of mowing a small shrub up to 10 mm thick, as stated in the instructions. The user manual is called "Petrol trimmer …".

    Fuel mix ratio

    I read somewhere that if in the instruction manual for lawn mowers and on the canister with oil different proportions of the preparation of the fuel mixture are indicated, the proportion indicated in the manual should be used.

    It did so for a long time – excess oil flowed from the engine to the gas tank and soot was constantly forming on the candle, since the manual indicated the proportion for the engine being run in 1:25, and on the canister – 1:50. After the transition to a proportion of 1:50, the appearance of oil drips stopped. The candle continued to burn, but with less intensity (the carburetor was not yet adjusted).

    Until now, some users of gasoline tools determine the proportion of the fuel mixture “by eye”. If during the preparation of the mixture the amount of oil is underestimated, after some time an extraneous sound may appear immediately after the engine starts, gradually disappearing during warming up, and the engine stops with a sharp jerk. Continued operation of the trimmer in this mode will lead to its failure.

    Candle cleaning

    In the worst days of working with a trimmer with an unregulated carburetor, the engine stopped starting after using each fuel tank. And only after cleaning the candles from soot could we continue to work.

    To clean the electrodes, fine sandpaper was used, and to clean the space between the insulator and the body, a thin clerical clip with a bent end was used. First, the electrodes were wiped off with a lot of dirt with a rag, then they were cleaned with sandpaper. After that, the candle was lowered into diesel fuel and the moistened soot was scraped off with a paper clip from the surfaces of the insulator, the inside of the housing and between them. Further, the candle was lowered several times in diesel fuel, which was then shaken off from it along with impurities. Sometimes the cleaning procedure with a paper clip and washing are repeated. After that, the candle was wiped dry and was ready for further use.

    Incredibly, one tank of the fuel mixture with an unregulated carburetor can be consumed in less than 10 minutes – it was tested in practice when the trimmer was started and operated at high speeds with maximum fuel supply (the adjustment screw was unscrewed much more than normal).