How to Set Fuel Delivery On a Stihl Trimmer

The increased fuel consumption and the rapid formation of carbon deposits on the lawn mowing candles when using a properly 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.

How to Set Fuel Delivery On a Stihl Trimmer

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.

Start the lawn mower and wait until it warms up.

Fuel adjustment

Start slowly turning the fuel mixture adjusting screw until the engine stalls or stalls unless you catch the moment. After that, 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 unscrew screw No. 1 a quarter of a turn until the engine starts to confidently gain increased speed when it is progazovaniya. 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. Counterclockwise, 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.

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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 adjusting, 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 poorly, loosen screw No. 1 by 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 even make the adjusting screws rotate.

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.

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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), as it can also be electric. A gas trimmer and a lawn mower are one and the same.

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.

This has been the case 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).

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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 times 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 case, a thin clerical clip with a bent end was used. First, the electrodes were wiped from a large dirt with rags, 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. Next, the candle was lowered several times in diesel fuel, which was then shaken off with the dirt. Sometimes the cleaning procedure with a paper clip and rinsing were 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 adjusting screw was unscrewed much more than normal).