Disassembly And Assembly Of The Trimmer Carburetor

Build your Home! This article is about DIY brushcutter carburetor repair, we will show you how easy it is!

Step 11

Disassembly And Assembly Of The Trimmer Carburetor

Preparing for assembly

Flush the jets, carburetor body, internal passages. Once the carburetor is disassembled, blow it out with compressed air. Attention! Blowing out the carburetor with compressed air can damage the diaphragms. Internal components can also be blown out, especially if the carburetor cover is removed. If you blow through the channels with a cleaning emulsion, you can often wash out stuck valve balls, etc.

Carefully inspect and clean the inner passages of the top of the carburetor (cover). Trace the channels along their entire length and, pouring the solvent into one hole, make sure that the solvent flows out of the outlet.

When reassembling, replace all gaskets with new ones. Also replace the needle valve, float shaft and diaphragm. After checking the condition, if necessary replace the quality screw, main fuel jets, idle jets, air jets and accelerator pump spray. Replace damaged rods, screws, springs and vacuum hoses.

Screw all jets into place properly, but do not overtighten. An untwisted jet can cause a lean or rich mixture.

Before installing new gaskets, clean all joint surfaces from traces of old ones. Do not use any type of sealant when assembling the carburetor and installing it on the engine. Often times a carburetor with channels clogged with sealant is easier to throw away and replace with a new one. Make sure all fittings and hoses are routed correctly.

Assembling the carburetor

Install the claim device to the carburetor body with the required new gaskets and seals. The throttle valve of the secondary chamber must be fully closed. Its adjusting screw (stop) usually cannot be adjusted. However, if necessary, adjust this screw so that the speaker is covered as much as possible, but does not get stuck in the closed position. If there is a measuring template, the damper can be adjusted according to it.

If the carburetor is equipped with an autonomous idle system, the primary throttle adjusting (stop) screw is also usually left untouched. However, if necessary, its position can be adjusted so that the damper is as close as possible, but does not get stuck in this position. If there is a measuring template, the angle is adjusted with it.

Install all diaphragms, their bodies and covers, and tighten the mounting screws. Make sure all springs are correctly installed. Wrap all jets in place (do not mix up). Install the solenoid valve (if equipped). Install the accelerator pump sprayer with valve.

Install the autonomous idle adjustment screws. Screw them in carefully until they stop, then back off three full turns. This will ensure they are approximately correctly set and allow the engine to start. Please note that all threads are very delicate in a carburetor made of aluminum alloy, do not mix up the threads when installing the screws. Damage to the threads will require replacing the carburetor.

Clean or replace the inlet filter, then install the inlet.
Place a new float chamber gasket on the carburetor cover (upper body).

Install a new needle valve using a new sealing washer. Do not overtighten the threads.

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Install the float and secure its shaft. Adjust the level in the float chamber. Bend only the tongue, not the lever. In some carburetors, the level in the float chamber is not regulated; the float is replaced. The correct weight of the float ensures the correct fuel level in the chamber.

Install the carburetor cover and secure with the retaining screws. Tighten the screws gradually and evenly to avoid bending surfaces. If so, secure the ground wire securely.

Connect the trigger vacuum hose (if equipped). Connect the choke control cable and check its operation. Adjust the valve travel at the starting speed.

Install the carburetor to the engine.

Always adjust the idle speed and mixture after assembling / installing the carburetor, with the engine warm and, if possible, using a gas analyzer.

Disassembly and assembly of the carburetor

Required special tools:

  • Special screwdrivers.

When disassembling the carburetor, to screw out the fuel jets of the main metering systems and air jets of the idle systems, in order to avoid damage to the threads of the channels and slot of the jets, use the special screwdrivers shown in the figure and in the table.

Table. The main dimensions of special screwdrivers for disassembling the K-126P carburetor

Carburetor part name Screwdriver dimensions, mm
d l s
Idle air jet 6.5 6 0.8
Main fuel jet 8.5 8.2 1
Additional system valve ten nine 1.6

Sequence of operations:

  1. Unscrew the plug of the strainer and remove the filter.
  2. Disconnect the link connecting the levers on the axles of the air and throttle valves, for which remove the cotter pin from the upper bent end of the link.
  3. Unscrew the screws securing the upper and middle parts of the carburetor; remove the top and the gasket.
  4. Remove the float shaft and remove the float.
  5. Remove the needle valve together with the damping spring. Unscrew the needle valve housing.
  6. Unscrew the idle fuel jets.
  7. Unscrew the plugs covering the channels of the idle air jets and unscrew the jets.
  8. Unscrew the air jets of the main metering systems.
  9. Unscrew the caps of the emulsion tube wells and remove the emulsion tubes.
  10. Unscrew the plugs accessing the main fuel jets and unscrew the jets.
  11. Unscrew the fuel-conducting screw and the screw that secures the accelerator pump sprayer housing and the additional system; remove the nozzle body and gasket. Carefully turning the carburetor body over, remove the accelerator pump discharge valve from the channel.
  12. Remove the four screws securing the bottom of the carburetor to the middle. Separate the lower part of the carburetor while simultaneously pressing on the accelerator pump drive bar and remove the shackle connecting the throttle shaft lever with the accelerator pump drive rod.
  13. Remove the accelerator pump drive rod together with the bar and the piston and remove the rod return spring. Remove the safety ring of the inlet valve from the well of the accelerating pump and, turning the body of the float chamber, remove the inlet valve. Unscrew the auxiliary system valve.
  14. Unscrew the idle speed adjustment screw from the carburetor mixing chamber.
  15. Bend the edge of the lock washer on the throttle valve shaft and unscrew the nut that secures the throttle valve drive fork; remove the fork and intermediate lever from the axle. Remove the two screws securing the secondary throttle rocker mechanism cover and remove the cover.
  16. In case of emergency (for example, when replacing the throttle valve return springs), the throttle valve shaft is removed in the following sequence:
    • A) unscrew the nut securing the accelerating pump drive lever on the throttle valve shaft, remove the washer and the lever;
    • B) unscrew the two screws securing the throttle valve on the axle, remove the flaps and remove the axle with a spring from the housing of the mixing chambers.
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    The secondary chamber throttle shaft is removed in the same way. Subsequent installation of throttle valves with axles is carried out in the reverse order. After installing and fixing the throttle valves on their axles, the screws must be reamed.

    All disassembled carburetor parts must be thoroughly rinsed in gasoline.

    If there are abundant resinous deposits on the parts, it is allowed to wash them with acetone or a solvent for nitro paints.

    It is completely unacceptable to use a wire, however soft, to clean the jets. With this method of cleaning, the calibration of the jets is inevitable. The consequence of this will be a deterioration in the traction qualities of the vehicle and an increase in the operating consumption of gasoline. The washed carburetor parts and passages must then be blown out with compressed air.

    The carburetor is assembled in the reverse order. In this case, it is necessary:

    1. Inspect the gaskets installed between the top and middle and between the middle and bottom of the carburetor. Replace damaged gaskets with new ones.
    2. Check the secondary system valve for leaks and not sticking in the seat guide. Check if the accelerator pump piston moves easily in the cylinder. It should move in the cylinder without jamming.
    3. Check, at full throttle, the clearance between the adjusting nuts (auxiliary valve actuator stem and accelerator pump piston rod) and the actuator rail. On the piston rod of the accelerating pump, this gap should be 1 mm, on the valve drive rod, 15 mm.
    4. When installing large diffusers, fuel and air jets, both main metering systems and idle systems, in both carburetor chambers, the technical data of the carburetor should be strictly followed.
    5. When tightening the plugs that close the carburetor channels (drain, etc.), pay attention to the presence and quality of their fiber pads. Replace damaged gaskets.
    6. Check the tightness of the fuel supply needle valve and make sure that in the absence of a load, the damping spring 2 of the needle valve 3 protrudes 0.71.3 mm above the rod end.
    7. Check the float for leaks before installing it in the float chamber cover. If it is necessary to solder the float, take appropriate precautions to avoid an explosion of gasoline vapors. After soldering, maintain the weight of the float within 13.3 ± 0.7 g.
    8. Check the correct position of the float relative to the needle valve. To do this, turn the cover 1 of the float chamber 180 and, slightly pressing the float 5 with your finger, measure the distance from the upper surface of the float to the plane of the cover with the cardboard gasket removed. This distance should be 42 ± 0.4 mm and controlled with a special template. If necessary, the correct position of the float can be restored by correspondingly bending the tongue 4 of the float lever away from the lever.

    Fuel ratio

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

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    For a long time, it did so from the engine to the gas tank, excess oil flowed down and carbon deposits constantly formed on the candle, since the manual indicated the proportion for the run-in engine 1:25, and on the canister 1:50. After switching to a 1:50 ratio, the appearance of oil drips stopped. The plug 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 a while an extraneous sound may appear immediately after starting the engine, gradually disappearing during warming up, and the engine stops with a sharp jerk. Continued use of the trimmer in this mode will lead to its failure.

    Idle speed adjustment

    We start adjusting low speed. Screw number 2 limits the return movement of the throttle lever by turning it, we increase the idle speed, unscrewing it, we decrease it. Adjust so that the engine runs confidently at low revs and even a little accelerated (for better starting), but so that the line reel or knife does not rotate, and there is a sufficient reserve of revolutions before they start rotating. If the engine starts poorly, increase the idle speed even more.

    Gas cutter or trimmer

    A trimmer is a broader concept than a brushcutter (brushcutter), since it can also be electric. A petrol trimmer and a petrol cutter are one and the same.

    Some authors claim that the trimmer is a weaker tool in contrast to the more powerful petrol cutter, which can be used for cutting bushes. Personally, I have a tool with a maximum power of 1.7 kW and is capable, as stated in the instructions, to mow small bushes up to 10 mm thick. The user manual is called Petrol Trimmer.

    Carburetor 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 start adjusting. The graph shows an example of a PRORAB-8406 petrol trimmer carburetor. The numbers indicate the adjusting screws that we will use.

    PRORAB-8406 petrol trimmer carburetor adjusting screws

    Purpose of adjusting screws:

    1. Fuel mixture adjustment screw.
    2. Idling adjustment screw.
    3. Maximum speed limiting screw.

    Start a petrol cutter and wait until it warms up.

    Spring may interfere

    My first attempt to adjust the carburetor of the brushcutters did not work, since the spring of the screw for adjusting the fuel mixture interfered with. The factory spring turned out to be too thick when compressed and did not make it possible to reduce the fuel supply to the required level. I had to pick up another spring and continue adjusting after replacing it.

    Fuel mixture adjusting screw with spring

    On the graph above the factory spring, below the adjusting screw with the selected spring. I am writing about this because I’m not sure if this is an isolated case, and someone else may face the same situation.