How to troubleshoot a two-stroke carburetor

Written by tom lutzenberger
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How to troubleshoot a two-stroke carburetor
Motocross motorcycles commonly use two-stroke carburettors and engines. (motocross image by Jerome Dancette from

Figuring out why a carburettor is not working properly tends to involve a process of elimination. This is due to combustion engines requiring a mixture of fuel and air to work and produce energy. The carburettor's function is to produce this mix properly for the engine, given known settings about the engine size, speed of propulsion, and the jetting of the carburettor. Thus, problems in a carburettor are typically the result of a setting being incorrect for the conditions demanded at the time.

Skill level:

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Things you need

  • Socket wrench and sockets
  • Screwdriver
  • Shop rag
  • Carb cleaner

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  1. 1

    Pull the spark plug cap off the spark plug. Remove the spark plug with a socket wrench and socket. Examine the spark plug tip to see what colour it is (black or white -- bad jetting, brown -- OK). Reinstall the spark plug and replace the cap. Start the engine and let it warm up.

  2. 2

    Pull the throttle handle to check for correct and immediate carburettor response and see if it bogs out (blocked fuel line). Take the motorcycle for a ride to see if the carburettor fails to function correctly at low speeds, mid-range speed, and high-speed. Keep track of any performance issues that occur during the ride (bogging out -- no fuel, racing engine or soft seize -- not enough fuel, dying with dirty spark plug -- too much fuel).

  3. 3

    Park the vehicle and turn it off. Turn off the fuel flow. Use a screwdriver to loosen the banjo bolt holding the carburettor to the intake manifold hose. Use a crescent wrench to disconnect the fuel line. Pull the carburettor unit out and use a screwdriver to loosen the throttle slide cap on the carburettor. Pull out the throttle slide and place it next to the engine for now.

  4. 4

    Open up the float bottom section of the carburettor with a crescent wrench to expose the carburettor jets. Remove the jets with a screwdriver. Clean out the jets with carb cleaner or a solvent spray. Replace the existing jets with bigger jets or smaller ones, depending on your performance results in Step 2, indicating what is happening with fuel flow.

  5. 5

    Re-assemble the motorcycle and reinstall the carburettor back into the engine, connecting it to the throttle slide and intake manifold by reversing the actions in Step 3. Restart the engine and take the motorcycle for another ride, repeating the performance test in Step 2 and checking the spark plug again as described in Step 1.

  6. 6

    Repeat the testing and changing of the jets until the motorcycle runs properly in all speed ranges and the spark plug when pulled shows a chocolate brown colour on the tip.

Tips and warnings

  • By using the stock jetting recommendations from the manufacturer, you can develop a baseline for what your carburettor should be using for jetting and make minor changes from that point depending on what other equipment changes are involved.
  • Err on the side of jetting more fuel than needed. The worst that will happen is that you will spoil a spark plug which only costs a few dollars to replace. Running a two-stroke engine with jetting that is too small for it could cause the engine to have a hard piston seizure at high speed which would damage multiple expensive parts of the engine.

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